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# bst_headmodeler

## (Toolbox/bst_headmodeler.m in BrainStorm 2.0 (Alpha))

### Function Synopsis

    [varargout] = bst_headmodeler(varargin);

### Help Text

    BST_HEADMODELER - Solution to the MEG/EEG forward problem
     function [varargout] = bst_headmodeler(varargin);
     Authorized syntax:
     [G, OPTIONS] = bst_headmodeler(StudyFile, OPTIONS);
     [G, OPTIONS] = bst_headmodeler(OPTIONS);
     [OPTIONS] = bst_headmodeler;

     --------------------------------- INPUTS -------------------------------------
     INPUTS

          [OPTIONS] = bst_headmodeler; Returns the default values for the OPTIONS
          parameter structure

          StudyFile: the name of a BrainStorm study file. We suppose the
          corresponding BrainStorm channel file  is available in the same folder
          with the conventional file name (e.g., for a study file calles
          meg_brainstormstudy.mat, BST_HEADMODELER expects to find the
          corresponding channel file under the name meg_channel.mat). If the
          channel file were not sitting in the same folder as  the study file,
          OPTIONS.ChannelFile enforces computation of the forward model with the
          channel information contained in OPTIONS.ChannelFile.

          If no further input arguments are specified, forward modeling is
          completed with the default parameters specified below

          OPTIONS a structure where optional parameters may be specified using the
          following fields  Note: if no options are specified, BST_HEADMODELER will
          proceed to the computation of the foward problem on a 3D grid of source
          locations that cover the entire head volume See
          OPTIONS.VolumeSourceGridSpacing for default settings

          Important notice: there is no need to define all the following fields
          when using the OPTIONS argument. The undefined field(s) will be assigned
          default values.

          *
          * Fields Related to forward approach
          *

          .Method: is either a character string or a cell array of two strings that
                   specifies the kind of approach to be applied to the compuation
                   of the foward model In case Method is a cell array, it should
                   contain 2 strings, one to specifiy the method to be used for MEG
                   and the other for EEG. If only a single string is specified, the
                   foward computation will be completed on the set of corresponding
                   CHANndx only (i.e. MEG or MEG) Available forward modeling
                   methods and corresponding authorized strings for Method:
                   - MEG
                        'meg_sphere' (DEFAULT) : Spherical head model designed
                           following the Sarvas analytical formulation (i.e.
                           considering the true orientation
                           of the magnetic field sensors) (see OPTIONS.HeadCenter)
                        'meg_os' : MEG overlapping sphere forward model
                        'meg_bem' : Apply BEM computation (see OPTIONS.BEM for details)
                   - EEG
                        'eeg_sphere' : Single-sphere forward modeling (see
                           OPTIONS.HeadCenter, OPTIONS.Radii, OPTIONS.Conductivity)
                        'eeg_3sphere' : EEG forward modeling with a set of 3
                           concentric spheres (Scalp, Skull, Brain/CSF) (see
                           OPTIONS.HeadCenter, OPTIONS.Radii, OPTIONS.Conductivity)
                        'eeg_3sphereBerg' (DEFAULT) : Same as eeg_3sphere with
                           correction for possible dipoles outside the sphere
                        'eeg_os' : EEG overlapping sphere head model (see
                           OPTIONS.HeadCenter, OPTIONS.Radii, OPTIONS.Conductivity)
                        'eeg_bem' : Apply BEM computation (see OPIONS.BEM for details)

                    Default is {'meg_sphere','eeg_3sphereBerg'};

          .HeadModelName : a character string that specifies the name of the
                           headmodel represented by this file, e.g "Spherical",
                           "Overlapping Spheres", "Constant Collocation BEM", etc.
                           Default is "Default", meaning it will include the the
                           name(s) of the method(s) used in the MEG and/or EEG
                           forward models

          *
          * Fields Related to function's I/O
          *

          .HeadModelFile : Specifies the name of the head model file where to store
                           the forward model.  If set to 'default', the default
                           nomenclature for BrainStorm's head model file name is
                           used and BST_HEADMODELER creates a file in StudyFile's
                           folder.
                           Default is empty.
          .ImageGridFile : Specifies the name of the file where to store the full
                           cortical gain matrix file  If set to 'default', the
                           default nomenclature for BrainStorm's head model file
                           name is used and BST_HEADMODELER creates a file in
                           StudyFile's folder.
                           Default is empty.
          .ImageGridBlockSize : Number of sources for which to compute the forward
                                model at a time in a block computation routines
                                (saves memory space). This option is relevant only
                                when some forward modeling on cortical surface is
                                requested (i.e. when .Cortex is specified)
                                Default is 2000
          .FileNamePrefix : A string that specifies the prefix for all file names
                           (Channel, HeadModel, Gain Matrices) when .HeadModelFile
                           is set to 'default' and .ChannelFile is empty.
                            Default is 'bst_'
          .Verbose     : Toggles verbose mode on/off;
                         Default is 1 (on)

          *
          * Fields Related to Head Geometry *
          *

          .Scalp     : A structure specifying the Scalp surface envelope to serve
                       for parameter adjustment of best-fitting sphere, with
                       following fields:
                       .FileName : A string specifying the name of the BrainStorm
                                   tessellation file containing the Scalp
                                   tessellation (default is 1);
                       .iGrid    : An integer for the index of the Scalp surface in
                                   the Faces, Vertices and Comments cell arrays in
                                   the tessellation file
                                   Default is empty (Best-fitting sphere is
                                   computed from the sensor array).
          .HeadCenter: a 3-element vector specifying the coordinates, in the
                       sensors coordinate system, of the center of the spheres that
                       might be used in the head model.
                       Default is estimated from the center of the best-fitting
                       sphere to the sensor locations
          .Radii     : a 3-element vector containing the radii of the single or 3
                       concentric spheres, when needed;
                       Order must be the following : [Rcsf, Routerskull, Rscalp];
                       Default is estimated from the best-fitting sphere to the
                       sensor locations and  OPTIONS.Radii is set to: Rscalp [.88
                       .93 1]. Rscalp is estimated from the radius of the
                       best-fitting sphere;
          .Conductivity : a 3-element vector containing the values for the
                          conductivity of the tissues in the following order:
                          [Ccsf, Cskull, Cscalp];
                          Default is set to [.33 .0042 .33];
          .EEGRef    : the NAME (not index of the channel file) of the electrode
                       that acts as the reference channel for the EEG. If data is
                       referenced to instantaneous average (i.e. so called
                       average-reference recording) value is 'AVERAGE REF';
                       IMPORTANT NOTICE: When user calls bst_headmodeler with the
                       .ChannelLoc option and .ChannelType = 'EEG'and wants the EEG
                       reference to be e.g. channel 26, then .EEGRef should be set
                       to 'EEG 26'
                       Default is 'AVERAGE REF'.

          .OS_ComputeParam : if 1, force computation of all sphere parameters when
                             choosing a method based on either the MEG or EEG
                             overlapping-sphere technique,  if 0 and when
                             .HeadModelFile is specified, sphere parameters are
                             loaded from the pre-computed HeadModel file.
                             Default is 1.

          .BEM       : Structure that specifies the necessary BEM parameters
                       .Interpolative : Flag indicating whether exact or
                                        interpolative approach is used to compute
                                        the forward solution using BEM.
                                        if set to 1, exact computation is run on a
                                        set of points distributed wihtin the inner
                                        head volume and any subsequent request for
                                        a forward gain vector (e.g. during a
                                        volumic source scan using RAP-MUSIC) is
                                        computed using an interpolation of the
                                        forward gain vectors of the closest volumic
                                        grid points. This allows faster computation
                                        of the BEM solution during source search.
                                        if set to 0, exact computation is required
                                        at every source location.
                                        We recommend to set it to 0 (default) when
                                        sources have fixed location, e.g.
                                        constrained on the cortical surface.
                       .EnvelopeNames : a cell array of strutures that specifies
                                        the ORDERED tessellated surfaces to be
                                        included in the BEM computation.
                                        .EnvelopeNames{k}.TessFile : A string for
                                        the name of the tessellation file
                                        containing the kth surface
                                        .EnvelopeNames{k}.TessName : A string for
                                        the name of the surface within the
                                        tessellation file  This string should match
                                        one of the Comment strings in the
                                        tessellation file. The chosen surfaces must
                                        be ordered starting by the the innermost
                                        surface  (e.g. brain or inner skull
                                        surface) and finishing with the outermost
                                        layer (e.g. the scalp)
                       .Basis         : set to either 'constant' or 'linear' (default)
                       .Test          : set to either 'Galerkin' or 'Collocation' (default)
                       .ISA           : Isolated-skull approach set to 0 or 1 (default is 1)
                       .NVertMax      : Maximum number of vertices per envelope,
                                        therefore leading to decimation of orginal
                                        surfaces if necessary
                                        (default is 1000)
                       .ForceXferComputation: if set to 1, force recomputation of
                                              existing transfer matrices in current
                                              study folder (replace existing
                                              files);
                                              Default is 1

          *
          * Fields Related to Sensor Information *
          *

          .ChannelFile : Specifies the name of the file containing the channel
                         information (needs to be a BrainStorm channel file). If
                         file does not exists and sensor information is provided in
                         .ChannelLoc, a BrainStorm Channl file with name
                         .ChannelFile is created
                         Default is left blank as this information is extracted
                         from the channel file associated to the chosen BrainStorm
                         studyfile.
          .Channel     : A full BrainStorm channel structure if no channel file is
                         specified;
                         Default is empty
          .ChannelType : A string specifying the type of channel in ChannelLoc. Can
                         be either 'MEG' or 'EEG'. Note that the same channel type
                         is assumed for every channel.
                         Default is empty
          .ChannelLoc  : Specifies the location of the channels where to compute
                         the forward model Can be either a 3xNsens (for EEG or
                         MEG-magnetometer) or 6xNsens matrix (for the
                         MEG-gradiometer case). (for magnetometer or gradiometer
                         MEG - channel weights are set to -1 and 1 for each
                         magnetometer in the gradiometer respectively)  Note that
                         in the MEG-gradiometer case, the 3 first (res. last) rows
                         of .ChannelLoc stand for each of the magnetometers of the
                         gradiometer set. In the case of a mixture of MEG magneto
                         and MEG gradio-meters, .ChannelLoc needs to be a 6xNsens
                         matrix where the last 3 rows are filled with NaN for
                         MEG-magnetometers If ones wants to handle both EEG and MEG
                         sensors, please create a full ChannelFile and use the
                         .ChannelFile option.
                         Default is empty (Information extracted from the ChannelFile).
          .ChannelOrient : Specifies the orientation of the channels where to
                           compute the EEG or MEG forward model   Can be either a
                           3xNsens (for EEG and magnetometer MEG) or 6xNsens (for
                           gradiometer MEG) matrix or a cell array of such matrices
                           (one cell per type of method selected)
                           Default is empty (Information extracted from the
                           ChannelFile or assume radial orientation when
                           .ChannelLoc is filled).

          *
          * Fields Related to Source Models *
          *
          .SourceModel : A vector indicating the type of source models to be
                         computed; The following code is enfoced:
                         -1 : Compute the forward fields of Current Dipole sources
                              (available for all forward approaches)
                          1 : ----------------------------- Fisrt-order Current
                              Multipole Sources (available for sphere-based MEG
                              approaches only)
                         User can set OPTIONS.SOurceMOdel tp e.g., [-1 1] to
                         compute forward models from both source models.
                         Default is -1

          *
          * Fields Related to Source Localization *
          *
          .Cortex                  :  A structure specifying the Cortex surface
                                      envelope to serve as an image support with
                                      following fields.
                                      .FileName : A string specifying the name of
                                                  the BrainStorm tessellation file
                                                  containing the Cortex
                                                  tessellation;
                                      .iGrid    : An integer for the index of the
                                                  Cortex surface in the Faces,
                                                  Vertices and Comments cell arrays
                                                  in the tessellation file (default
                                                  is 1)
                                                  Default is empty.
          .GridLoc                 :  A 3xNsources matrix that contains the
                                      locations of the sources at which the forward
                                      model will be computed
                                      Default is empty (Information taken from
                                      OPTIONS.Cortex or OPTIONS.VolumeSourceGrid);
          .GridOrient              :  a 3xNsources matrix that forces the source
                                      orientation at every vertex of the .ImageGrid
                                      cortical surface;
                                      Defaults is empty; this information being
                                      extracted from the corresponding tessellated
                                      surface.
          .ApplyGridOrient         : if set to 1, force computation of the forward
                                     fields by considering the local orientation of
                                     the cortical surface;
                                     If set to 0, a set of 3 orthogonal dipoles are
                                     considered at each vertex location on the
                                     tessellated surface.
                                     Default is 1.
          .VolumeSourceGrid        : if set to 1, a 3D source grid is designed to
                                     fit inside the head volume and will serve as a
                                     source space for scannig techniques such as
                                     RAP-MUSIC;
                                     if set to 0, this grid will be computed at the
                                     first call of e.g. RAP-MUSIC);
                                     Default is 1
          .VolumeSourceGridSpacing : Spacing in centimeters between two consecutive
                                     sources in the 3D source grid described above;
                                     Default is 2 cm.
          .VolumeSourceGridLoc     : a 3xN matrix specifying the locations of the
                                     grid points that will be used to design the
                                     volumic search grid (see .VolumicSourceGrid)
                                     Default is empty (locations are estimated
                                     automatically to cover the estimated inner
                                     head volume)
          .SourceLoc               : a 3xNsources matrix that contains the
                                     locations of the sources at which the forward
                                     model will be computed
                                     Default is empty (Information taken from
                                     OPTIONS.ImageGrid or
                                     OPTIONS.VolumeSourceGrid);
          .SourceOrient            : a 3xNsources matrix that contains the
                                     orientations of the sources at which the
                                     forward model will be computed
                                     Default is empty.

     --------------------------------- OUTPUTS ------------------------------------
     OUTPUT
          G        if the number of sources (Nsources) if less than
                   .ImageGridBlockSize then G is a gain matrix of dimension
                   Nsensors x Nsources: Each column of G is the forward field
                   created by a dipolar source of unit amplitude. Otherwise, G is
                   the name of the binary file containing the gain matrix. This
                   file can be read using the READ_GAIN function.
          OPTIONS  Returns the OPTIONS structure with updated fields following the
                   call to BST_HEADMODELER.  Can be useful to obtain a full
                   BrainStorm Channel structure when only the .ChannelLoc and
                   possibly .ChannelOrient fields were provided.

### Cross-Reference Information

This function calls

    bem_gain                           C:\BrainStorm_2001\Toolbox\bem_gain.m
    bem_xfer                           C:\BrainStorm_2001\Toolbox\bem_xfer.m
    berg                               C:\BrainStorm_2001\Toolbox\berg.m
    bst_message_window                 C:\BrainStorm_2001\Toolbox\bst_message_window.m
    colnorm                            C:\BrainStorm_2001\Toolbox\colnorm.m
    dist_sph                           C:\BrainStorm_2001\Toolbox\dist_sph.m
    eeg_bem                            C:\BrainStorm_2001\Toolbox\eeg_bem.m
    eeg_sph                            C:\BrainStorm_2001\Toolbox\eeg_sph.m
    get_channel                        C:\BrainStorm_2001\Toolbox\get_channel.m
    get_user_directory                 C:\BrainStorm_2001\Toolbox\get_user_directory.m
    good_channel                       C:\BrainStorm_2001\Toolbox\good_channel.m
    gridmaker                          C:\BrainStorm_2001\Toolbox\gridmaker.m
    inorcol                            C:\BrainStorm_2001\Toolbox\inorcol.m
    meg_bem                            C:\BrainStorm_2001\Toolbox\meg_bem.m
    norlig                             C:\BrainStorm_2001\Toolbox\norlig.m
    offset                             C:\BrainStorm_2001\Toolbox\offset.m
    os_meg                             C:\BrainStorm_2001\Toolbox\os_meg.m
    overlapping_sphere                 C:\BrainStorm_2001\Toolbox\overlapping_sphere.m
    rownorm                            C:\BrainStorm_2001\Toolbox\rownorm.m
    save_fieldnames                    C:\BrainStorm_2001\Toolbox\save_fieldnames.m
    source_grids                       C:\BrainStorm_2001\Toolbox\source_grids.m
    view_surface                       C:\BrainStorm_2001\Toolbox\view_surface.m

This function is called by

    bst_static_taskbar                 C:\BrainStorm_2001\Toolbox\bst_static_taskbar.m
    bstscript_ForwardInverse_basic     C:\BrainStorm_2001\Toolbox\bstscript_ForwardInverse_basic.m
    data_manager                       C:\BrainStorm_2001\Toolbox\data_manager.m
    headmodeler_gui                    C:\BrainStorm_2001\Toolbox\headmodeler_gui.m
    make_default_study                 C:\BrainStorm_2001\Toolbox\make_default_study.m

### Listing of function C:\BrainStorm_2001\Toolbox\bst_headmodeler.m

    function [varargout] = bst_headmodeler(varargin);
    %BST_HEADMODELER - Solution to the MEG/EEG forward problem
    % function [varargout] = bst_headmodeler(varargin);
    % Authorized syntax:
    % [G, OPTIONS] = bst_headmodeler(StudyFile, OPTIONS);
    % [G, OPTIONS] = bst_headmodeler(OPTIONS);
    % [OPTIONS] = bst_headmodeler;
    %
    % --------------------------------- INPUTS -------------------------------------
    % INPUTS
    %
    %      [OPTIONS] = bst_headmodeler; Returns the default values for the OPTIONS
    %      parameter structure
    %
    %      StudyFile: the name of a BrainStorm study file. We suppose the
    %      corresponding BrainStorm channel file  is available in the same folder
    %      with the conventional file name (e.g., for a study file calles
    %      meg_brainstormstudy.mat, BST_HEADMODELER expects to find the
    %      corresponding channel file under the name meg_channel.mat). If the
    %      channel file were not sitting in the same folder as  the study file,
    %      OPTIONS.ChannelFile enforces computation of the forward model with the
    %      channel information contained in OPTIONS.ChannelFile.
    %
    %      If no further input arguments are specified, forward modeling is
    %      completed with the default parameters specified below
    %
    %      OPTIONS a structure where optional parameters may be specified using the
    %      following fields  Note: if no options are specified, BST_HEADMODELER will
    %      proceed to the computation of the foward problem on a 3D grid of source
    %      locations that cover the entire head volume See
    %      OPTIONS.VolumeSourceGridSpacing for default settings
    %
    %      Important notice: there is no need to define all the following fields
    %      when using the OPTIONS argument. The undefined field(s) will be assigned
    %      default values.
    %
    %      *
    %      * Fields Related to forward approach
    %      *
    %
    %      .Method: is either a character string or a cell array of two strings that
    %               specifies the kind of approach to be applied to the compuation
    %               of the foward model In case Method is a cell array, it should
    %               contain 2 strings, one to specifiy the method to be used for MEG
    %               and the other for EEG. If only a single string is specified, the
    %               foward computation will be completed on the set of corresponding
    %               CHANndx only (i.e. MEG or MEG) Available forward modeling
    %               methods and corresponding authorized strings for Method:
    %               - MEG
    %                    'meg_sphere' (DEFAULT) : Spherical head model designed
    %                       following the Sarvas analytical formulation (i.e.
    %                       considering the true orientation
    %                       of the magnetic field sensors) (see OPTIONS.HeadCenter)
    %                    'meg_os' : MEG overlapping sphere forward model
    %                    'meg_bem' : Apply BEM computation (see OPTIONS.BEM for details)
    %               - EEG
    %                    'eeg_sphere' : Single-sphere forward modeling (see
    %                       OPTIONS.HeadCenter, OPTIONS.Radii, OPTIONS.Conductivity)
    %                    'eeg_3sphere' : EEG forward modeling with a set of 3
    %                       concentric spheres (Scalp, Skull, Brain/CSF) (see
    %                       OPTIONS.HeadCenter, OPTIONS.Radii, OPTIONS.Conductivity)
    %                    'eeg_3sphereBerg' (DEFAULT) : Same as eeg_3sphere with
    %                       correction for possible dipoles outside the sphere
    %                    'eeg_os' : EEG overlapping sphere head model (see
    %                       OPTIONS.HeadCenter, OPTIONS.Radii, OPTIONS.Conductivity)
    %                    'eeg_bem' : Apply BEM computation (see OPIONS.BEM for details)
    %
    %                Default is {'meg_sphere','eeg_3sphereBerg'};
    %
    %      .HeadModelName : a character string that specifies the name of the
    %                       headmodel represented by this file, e.g "Spherical",
    %                       "Overlapping Spheres", "Constant Collocation BEM", etc.
    %                       Default is "Default", meaning it will include the the
    %                       name(s) of the method(s) used in the MEG and/or EEG
    %                       forward models
    %
    %      *
    %      * Fields Related to function's I/O
    %      *
    %
    %      .HeadModelFile : Specifies the name of the head model file where to store
    %                       the forward model.  If set to 'default', the default
    %                       nomenclature for BrainStorm's head model file name is
    %                       used and BST_HEADMODELER creates a file in StudyFile's
    %                       folder.
    %                       Default is empty.
    %      .ImageGridFile : Specifies the name of the file where to store the full
    %                       cortical gain matrix file  If set to 'default', the
    %                       default nomenclature for BrainStorm's head model file
    %                       name is used and BST_HEADMODELER creates a file in
    %                       StudyFile's folder.
    %                       Default is empty.
    %      .ImageGridBlockSize : Number of sources for which to compute the forward
    %                            model at a time in a block computation routines
    %                            (saves memory space). This option is relevant only
    %                            when some forward modeling on cortical surface is
    %                            requested (i.e. when .Cortex is specified)
    %                            Default is 2000
    %      .FileNamePrefix : A string that specifies the prefix for all file names
    %                       (Channel, HeadModel, Gain Matrices) when .HeadModelFile
    %                       is set to 'default' and .ChannelFile is empty.
    %                        Default is 'bst_'
    %      .Verbose     : Toggles verbose mode on/off;
    %                     Default is 1 (on)
    %
    %      *
    %      * Fields Related to Head Geometry *
    %      *
    %
    %      .Scalp     : A structure specifying the Scalp surface envelope to serve
    %                   for parameter adjustment of best-fitting sphere, with
    %                   following fields:
    %                   .FileName : A string specifying the name of the BrainStorm
    %                               tessellation file containing the Scalp
    %                               tessellation (default is 1);
    %                   .iGrid    : An integer for the index of the Scalp surface in
    %                               the Faces, Vertices and Comments cell arrays in
    %                               the tessellation file
    %                               Default is empty (Best-fitting sphere is
    %                               computed from the sensor array).
    %      .HeadCenter: a 3-element vector specifying the coordinates, in the
    %                   sensors coordinate system, of the center of the spheres that
    %                   might be used in the head model.
    %                   Default is estimated from the center of the best-fitting
    %                   sphere to the sensor locations
    %      .Radii     : a 3-element vector containing the radii of the single or 3
    %                   concentric spheres, when needed;
    %                   Order must be the following : [Rcsf, Routerskull, Rscalp];
    %                   Default is estimated from the best-fitting sphere to the
    %                   sensor locations and  OPTIONS.Radii is set to: Rscalp [.88
    %                   .93 1]. Rscalp is estimated from the radius of the
    %                   best-fitting sphere;
    %      .Conductivity : a 3-element vector containing the values for the
    %                      conductivity of the tissues in the following order:
    %                      [Ccsf, Cskull, Cscalp];
    %                      Default is set to [.33 .0042 .33];
    %      .EEGRef    : the NAME (not index of the channel file) of the electrode
    %                   that acts as the reference channel for the EEG. If data is
    %                   referenced to instantaneous average (i.e. so called
    %                   average-reference recording) value is 'AVERAGE REF';
    %                   IMPORTANT NOTICE: When user calls bst_headmodeler with the
    %                   .ChannelLoc option and .ChannelType = 'EEG'and wants the EEG
    %                   reference to be e.g. channel 26, then .EEGRef should be set
    %                   to 'EEG 26'
    %                   Default is 'AVERAGE REF'.
    %
    %      .OS_ComputeParam : if 1, force computation of all sphere parameters when
    %                         choosing a method based on either the MEG or EEG
    %                         overlapping-sphere technique,  if 0 and when
    %                         .HeadModelFile is specified, sphere parameters are
    %                         loaded from the pre-computed HeadModel file.
    %                         Default is 1.
    %
    %      .BEM       : Structure that specifies the necessary BEM parameters
    %                   .Interpolative : Flag indicating whether exact or
    %                                    interpolative approach is used to compute
    %                                    the forward solution using BEM.
    %                                    if set to 1, exact computation is run on a
    %                                    set of points distributed wihtin the inner
    %                                    head volume and any subsequent request for
    %                                    a forward gain vector (e.g. during a
    %                                    volumic source scan using RAP-MUSIC) is
    %                                    computed using an interpolation of the
    %                                    forward gain vectors of the closest volumic
    %                                    grid points. This allows faster computation
    %                                    of the BEM solution during source search.
    %                                    if set to 0, exact computation is required
    %                                    at every source location.
    %                                    We recommend to set it to 0 (default) when
    %                                    sources have fixed location, e.g.
    %                                    constrained on the cortical surface.
    %                   .EnvelopeNames : a cell array of strutures that specifies
    %                                    the ORDERED tessellated surfaces to be
    %                                    included in the BEM computation.
    %                                    .EnvelopeNames{k}.TessFile : A string for
    %                                    the name of the tessellation file
    %                                    containing the kth surface
    %                                    .EnvelopeNames{k}.TessName : A string for
    %                                    the name of the surface within the
    %                                    tessellation file  This string should match
    %                                    one of the Comment strings in the
    %                                    tessellation file. The chosen surfaces must
    %                                    be ordered starting by the the innermost
    %                                    surface  (e.g. brain or inner skull
    %                                    surface) and finishing with the outermost
    %                                    layer (e.g. the scalp)
    %                   .Basis         : set to either 'constant' or 'linear' (default)
    %                   .Test          : set to either 'Galerkin' or 'Collocation' (default)
    %                   .ISA           : Isolated-skull approach set to 0 or 1 (default is 1)
    %                   .NVertMax      : Maximum number of vertices per envelope,
    %                                    therefore leading to decimation of orginal
    %                                    surfaces if necessary
    %                                    (default is 1000)
    %                   .ForceXferComputation: if set to 1, force recomputation of
    %                                          existing transfer matrices in current
    %                                          study folder (replace existing
    %                                          files);
    %                                          Default is 1
    %
    %      *
    %      * Fields Related to Sensor Information *
    %      *
    %
    %      .ChannelFile : Specifies the name of the file containing the channel
    %                     information (needs to be a BrainStorm channel file). If
    %                     file does not exists and sensor information is provided in
    %                     .ChannelLoc, a BrainStorm Channl file with name
    %                     .ChannelFile is created
    %                     Default is left blank as this information is extracted
    %                     from the channel file associated to the chosen BrainStorm
    %                     studyfile.
    %      .Channel     : A full BrainStorm channel structure if no channel file is
    %                     specified;
    %                     Default is empty
    %      .ChannelType : A string specifying the type of channel in ChannelLoc. Can
    %                     be either 'MEG' or 'EEG'. Note that the same channel type
    %                     is assumed for every channel.
    %                     Default is empty
    %      .ChannelLoc  : Specifies the location of the channels where to compute
    %                     the forward model Can be either a 3xNsens (for EEG or
    %                     MEG-magnetometer) or 6xNsens matrix (for the
    %                     MEG-gradiometer case). (for magnetometer or gradiometer
    %                     MEG - channel weights are set to -1 and 1 for each
    %                     magnetometer in the gradiometer respectively)  Note that
    %                     in the MEG-gradiometer case, the 3 first (res. last) rows
    %                     of .ChannelLoc stand for each of the magnetometers of the
    %                     gradiometer set. In the case of a mixture of MEG magneto
    %                     and MEG gradio-meters, .ChannelLoc needs to be a 6xNsens
    %                     matrix where the last 3 rows are filled with NaN for
    %                     MEG-magnetometers If ones wants to handle both EEG and MEG
    %                     sensors, please create a full ChannelFile and use the
    %                     .ChannelFile option.
    %                     Default is empty (Information extracted from the ChannelFile).
    %      .ChannelOrient : Specifies the orientation of the channels where to
    %                       compute the EEG or MEG forward model   Can be either a
    %                       3xNsens (for EEG and magnetometer MEG) or 6xNsens (for
    %                       gradiometer MEG) matrix or a cell array of such matrices
    %                       (one cell per type of method selected)
    %                       Default is empty (Information extracted from the
    %                       ChannelFile or assume radial orientation when
    %                       .ChannelLoc is filled).
    %
    %      *
    %      * Fields Related to Source Models *
    %      *
    %      .SourceModel : A vector indicating the type of source models to be
    %                     computed; The following code is enfoced:
    %                     -1 : Compute the forward fields of Current Dipole sources
    %                          (available for all forward approaches)
    %                      1 : ----------------------------- Fisrt-order Current
    %                          Multipole Sources (available for sphere-based MEG
    %                          approaches only)
    %                     User can set OPTIONS.SOurceMOdel tp e.g., [-1 1] to
    %                     compute forward models from both source models.
    %                     Default is -1
    %
    %
    %      *
    %      * Fields Related to Source Localization *
    %      *
    %      .Cortex                  :  A structure specifying the Cortex surface
    %                                  envelope to serve as an image support with
    %                                  following fields.
    %                                  .FileName : A string specifying the name of
    %                                              the BrainStorm tessellation file
    %                                              containing the Cortex
    %                                              tessellation;
    %                                  .iGrid    : An integer for the index of the
    %                                              Cortex surface in the Faces,
    %                                              Vertices and Comments cell arrays
    %                                              in the tessellation file (default
    %                                              is 1)
    %                                              Default is empty.
    %      .GridLoc                 :  A 3xNsources matrix that contains the
    %                                  locations of the sources at which the forward
    %                                  model will be computed
    %                                  Default is empty (Information taken from
    %                                  OPTIONS.Cortex or OPTIONS.VolumeSourceGrid);
    %      .GridOrient              :  a 3xNsources matrix that forces the source
    %                                  orientation at every vertex of the .ImageGrid
    %                                  cortical surface;
    %                                  Defaults is empty; this information being
    %                                  extracted from the corresponding tessellated
    %                                  surface.
    %      .ApplyGridOrient         : if set to 1, force computation of the forward
    %                                 fields by considering the local orientation of
    %                                 the cortical surface;
    %                                 If set to 0, a set of 3 orthogonal dipoles are
    %                                 considered at each vertex location on the
    %                                 tessellated surface.
    %                                 Default is 1.
    %      .VolumeSourceGrid        : if set to 1, a 3D source grid is designed to
    %                                 fit inside the head volume and will serve as a
    %                                 source space for scannig techniques such as
    %                                 RAP-MUSIC;
    %                                 if set to 0, this grid will be computed at the
    %                                 first call of e.g. RAP-MUSIC);
    %                                 Default is 1
    %      .VolumeSourceGridSpacing : Spacing in centimeters between two consecutive
    %                                 sources in the 3D source grid described above;
    %                                 Default is 2 cm.
    %      .VolumeSourceGridLoc     : a 3xN matrix specifying the locations of the
    %                                 grid points that will be used to design the
    %                                 volumic search grid (see .VolumicSourceGrid)
    %                                 Default is empty (locations are estimated
    %                                 automatically to cover the estimated inner
    %                                 head volume)
    %      .SourceLoc               : a 3xNsources matrix that contains the
    %                                 locations of the sources at which the forward
    %                                 model will be computed
    %                                 Default is empty (Information taken from
    %                                 OPTIONS.ImageGrid or
    %                                 OPTIONS.VolumeSourceGrid);
    %      .SourceOrient            : a 3xNsources matrix that contains the
    %                                 orientations of the sources at which the
    %                                 forward model will be computed
    %                                 Default is empty.
    %
    %
    % --------------------------------- OUTPUTS ------------------------------------
    % OUTPUT
    %      G        if the number of sources (Nsources) if less than
    %               .ImageGridBlockSize then G is a gain matrix of dimension
    %               Nsensors x Nsources: Each column of G is the forward field
    %               created by a dipolar source of unit amplitude. Otherwise, G is
    %               the name of the binary file containing the gain matrix. This
    %               file can be read using the READ_GAIN function.
    %      OPTIONS  Returns the OPTIONS structure with updated fields following the
    %               call to BST_HEADMODELER.  Can be useful to obtain a full
    %               BrainStorm Channel structure when only the .ChannelLoc and
    %               possibly .ChannelOrient fields were provided.

    %<autobegin> ---------------------- 12-Oct-2004 12:00:59 -----------------------
    % --------- Automatically Generated Comments Block Using AUTO_COMMENTS ---------
    %
    % CATEGORY: Forward Modeling
    %
    % Alphabetical list of external functions (non-Matlab):
    %   toolbox\bem_gain.m
    %   toolbox\bem_xfer.m
    %   toolbox\berg.m
    %   toolbox\bst_message_window.m
    %   toolbox\colnorm.m
    %   toolbox\get_channel.m
    %   toolbox\get_user_directory.m
    %   toolbox\good_channel.m
    %   toolbox\gridmaker.m
    %   toolbox\inorcol.m
    %   toolbox\norlig.m
    %   toolbox\overlapping_sphere.m
    %   toolbox\rownorm.m
    %   toolbox\save_fieldnames.m
    %   toolbox\source_grids.m
    %   toolbox\view_surface.m
    %
    % Subfunctions in this file, in order of occurrence in file:
    %   BEMGaingridFname = bem_GainGrid(DataType,OPTIONS,BEMChanNdx)
    %   g = gterm_constant(r,rq)
    %
    % At Check-in: $Author: Mosher $  $Revision: 54 $  $Date: 10/12/04 10:23a $
    %
    % This software is part of BrainStorm Toolbox Version 2.0 (Alpha) 12-Oct-2004
    %
    % Principal Investigators and Developers:
    % ** Richard M. Leahy, PhD, Signal & Image Processing Institute,
    %    University of Southern California, Los Angeles, CA
    % ** John C. Mosher, PhD, Biophysics Group,
    %    Los Alamos National Laboratory, Los Alamos, NM
    % ** Sylvain Baillet, PhD, Cognitive Neuroscience & Brain Imaging Laboratory,
    %    CNRS, Hopital de la Salpetriere, Paris, France
    %
    % See BrainStorm website at http://neuroimage.usc.edu for further information.
    %
    % Copyright (c) 2004 BrainStorm by the University of Southern California
    % This software distributed  under the terms of the GNU General Public License
    % as published by the Free Software Foundation. Further details on the GPL
    % license can be found at http://www.gnu.org/copyleft/gpl.html .
    %
    % FOR RESEARCH PURPOSES ONLY. THE SOFTWARE IS PROVIDED "AS IS," AND THE
    % UNIVERSITY OF SOUTHERN CALIFORNIA AND ITS COLLABORATORS DO NOT MAKE ANY
    % WARRANTY, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO WARRANTIES OF
    % MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE, NOR DO THEY ASSUME ANY
    % LIABILITY OR RESPONSIBILITY FOR THE USE OF THIS SOFTWARE.
    %<autoend> ------------------------ 12-Oct-2004 12:00:59 -----------------------

    % /---Script Author--------------------------------------\
    % |                                                      |
    % | *** Sylvain Baillet Ph.D.                            |
    % | Cognitive Neuroscience & Brain Imaging Laboratory    |
    % | CNRS UPR640 - LENA                                   |
    % | Hopital de la Salpetriere, Paris, France             |
    % | sylvain.baillet@chups.jussieu.fr                     |
    % |                                                      |
    % \------------------------------------------------------/
    %
    % Date of creation: March 2002

    % ----------------------------- Script History ---------------------------------
    %
    % SB 07-Aug-2002  Fixed GridLoc saving issues - now GridLoc
    %                 is a cell array of length one which indicates
    %                 the name of the tessellation file that was
    %                 used as an imaging support.
    % SB 03-Sep-2002  Fixed minor bugs when function is called from command line
    %                 Now bst_headmodeler can be called even when Brainstorm is not running
    % SB 05-Sep-2002  BEM can now be called from command line as well.
    %                 Updated script header
    % SB 06-Sep-2002  Updated EEG BEM : added option for arbitrary reference of the
    %                 potentials (takes now OPTIONS.EEGRef into account).
    % SB 18-Nov-2002  Updated EEG reference management.
    % SB 21-Nov-2002  Minor fixes for command line calls.
    % SB 21-Nov-2002bis Fixed bug from the get(*,'VertexNormals') command that
    %                 sometimes returns [0 0 0] at some vertex
    % SB 23-Dec-2002  Correct HeadModelFile name is passed to OPTIONS output argument
    % JCM 27-May-2004 Cleaning comments
    %                 Found Matlab bug and temp solution.
    %                 Matlab 6.5.0 bug, Technical Solution Number: 1-19NOK
    %                 http://www.mathworks.com/support/solutions/data/1-19NOK.html?solution=1-19NOK
    %                 Must REWIND first before fseek.
    %                 Apparently fixed in 6.5.1
    %                 Example:
    %                 status = fseek(fdest,0,'bof');
    %                 status = fseek(fdest,offset,'bof');
    %
    % ----------------------------- Script History ---------------------------------

    % Default options settings--------------------------------------------------------------------------------------------------
    DefaultMethod = {'meg_sphere','eeg_3sphereBerg'};

    ReducePatchScalpNVerts = 500; % Number of vertices the scalp tessellation will be reduced to in OS computations
    BEM_defaults = struct(...
        'Basis','linear',...
        'Test','galerkin',...
        'Interpolative',0,...
        'ISA',1,...
        'NVertMax',1000,...
        'ForceXferComputation', 1 ...
        );

    Def_OPTIONS = struct(...
        'ApplyGridOrient',1,...
        'BEM', BEM_defaults,...
        'Channel', [],...
        'ChannelFile', '',...
        'ChannelLoc', '',...
        'ChannelOrient', '',...
        'ChannelType', '',...
        'Conductivity', [.33 .0042 .33],...
        'Cortex',[],...
        'EEGRef','',...
        'HeadCenter',[],...
        'HeadModelFile', '',...
        'HeadModelName','Default',...
        'ImageGridBlockSize', 2000,...
        'ImageGridFile', '',...
        'GridOrient',[],...
        'Method', {DefaultMethod},...
        'OS_ComputeParam', 1,...
        'PrefixFileName','bst_',...
        'Radii', [],...
        'Scalp',[],...
        'SourceLoc',[],...
        'SourceModel', [-1],...
        'SourceOrient',[],...
        'StudyFile','',...
        'TessellationFile','',...
        'VolumeSourceGrid',1,...
        'VolumeSourceGridSpacing', 2,...
        'VolumeSourceGridLoc', [],...
        'Verbose', 1 ...
        );

    SourceOrderString = {'Current Dipole',...
            'Unvalid Order (was Magnetic Dipole Moment)',...
            'Current Multipole Expansion',...
            'Current Dipole Pairs',...
            'Unvalid Order (was Magnetic Dipole Moment PAIRS)',...
            'Current Multipole Expansion Pairs'};

    if nargin == 0
        if nargout > 1
            varargout{1} = Def_OPTIONS;
            varargout{2} = Def_OPTIONS;
        else
            varargout{1} = Def_OPTIONS;
        end
        return
    elseif nargin == 1
        if ischar(varargin{1}) % User enters only the studyfile name
            StudyFile = varargin{1};
            OPTIONS = Def_OPTIONS;
        elseif isstruct(varargin{1}) % User enters only the OPTION field
            OPTIONS = varargin{1};
        else
            errordlg('Uncorrect input parameter type, please check the help file'), varargout = cell(nargout,1);
            return,
        end
    elseif nargin == 2 % No options were specified, apply default
        StudyFile = varargin{1};
        OPTIONS = varargin{2};
    else
        errordlg('Wrong number of arguments when calling head modeler')
        varargout = cell(nargout,1);
        return
    end

    % Check field names of passed OPTIONS and fill missing ones with default values
    DefFieldNames = fieldnames(Def_OPTIONS);
    for k = 1:length(DefFieldNames)
        if ~isfield(OPTIONS,DefFieldNames{k}) | strcmp(DefFieldNames{k},'BEM')
            if ~isfield(OPTIONS,DefFieldNames{k})

                OPTIONS = setfield(OPTIONS,DefFieldNames{k},getfield(Def_OPTIONS,DefFieldNames{k}));

            elseif strcmp(DefFieldNames{k},'BEM')

                BEM_DefFieldNames = fieldnames(BEM_defaults);
                for kk = 1:length(BEM_DefFieldNames)
                    if ~isfield(OPTIONS.BEM,BEM_DefFieldNames{kk})
                        OPTIONS.BEM = setfield(OPTIONS.BEM,BEM_DefFieldNames{kk},getfield(BEM_defaults,BEM_DefFieldNames{kk}));
                    end
                end
            end
        end
    end

    if isempty(OPTIONS.Conductivity)
        OPTIONS.Conductivity = Def_OPTIONS.Conductivity;
    end

    clear Def_OPTIONS

    if isempty(OPTIONS.HeadModelFile) & ~isempty(OPTIONS.ImageGridFile)
        % Force creation of a headmodel file
        OPTIONS.HeadModelFile = 'default';
    end

    OPTIONS.HeadModelFileOld = OPTIONS.HeadModelFile;

    % What type of forward model (MEG and/or EEG) ?
    DataType.MEG = strmatch('meg',OPTIONS.Method); % Rank of the respective forward method in cell array Method (ie could be Method = {'meg_bem','eeg_sphere'} or vice-versa)
    DataType.EEG = strmatch('eeg',OPTIONS.Method);

    if ~iscell(OPTIONS.Method)
        OPTIONS.Method = {OPTIONS.Method};
    end

    MegMethod = [];
    if ~isempty(DataType.MEG)
        MegMethod = OPTIONS.Method{DataType.MEG}; % String indicating the forward method selected for MEG (res. EEG)
    end
    EegMethod = [];
    if ~isempty(DataType.EEG)
        EegMethod = OPTIONS.Method{DataType.EEG};
    end

    % Check inputs integrity

    %Source models
    if ~isempty(find(OPTIONS.SourceModel == 0)) | ~isempty(find(abs(OPTIONS.SourceModel) > 1)) % unValid source models
        if ~isempty(DataType.MEG)
            errordlg('Valid source model orders for MEG are: -1 (Current Dipole) and 1 (fist-order Current Multipole Expansion)')
        end
        if ~isempty(DataType.EEG)
            errordlg('Valid source model order for EEG is: -1 (Current Dipole) ')
        end
        varargout = cell(nargout,1);
        return
    end

    % Source locations
    if isempty(OPTIONS.SourceLoc) & ~OPTIONS.VolumeSourceGrid & isempty(OPTIONS.Cortex) % No source locations were specified
        errordlg('No source locations are specified. Please fill either one of the following fields of OPTIONS: .SourceLoc / .VolumeSourceGrid / .Cortex')
        varargout = cell(nargout,1);
        return
    end

    %--------------------------------------------------------------------------------------------------------------------------------------------
    %
    % HEAD MODELING BEGINS
    %
    %--------------------------------------------------------------------------------------------------------------------------------------------

    if OPTIONS.Verbose,

        clockk = fix(clock);
        time0 = clock;

        bst_message_window({...
                ' ',...
                '__________________________________________',...
                sprintf(...
                'Head modeling begins (%s - %dH %dmin %ds)',date,clockk(4:6))...
            })

    end

    User = get_user_directory;
    if isempty(User) % Function is called from command line: BrainStorm is not running
        % Use default folders
        User.SUBJECTS = pwd;
        User.STUDIES = pwd;
    end

    cd(User.STUDIES)

    % Get all Subject and Study information------------------------------------------------------------------------------------------------------
    if exist('StudyFile','var')
        if OPTIONS.Verbose, bst_message_window('Loading Study and Subject Information...'), end
        try
            load(StudyFile) % Load study information
            [StudyPath,tmp,ext] = fileparts(StudyFile); clear tmp
        catch
            errordlg(['Could not find ',StudyFile,' in current Matlab path'])
            varargout = cell(nargout,1);
            return
        end

        if isempty(OPTIONS.StudyFile)
            OPTIONS.StudyFile = StudyFile;
        end

        SubjectFile = BrainStormSubject; clear BrainStormSubject

        OPTIONS.rooot = findstr(StudyFile,'brainstormstudy.mat');
        if isempty(OPTIONS.rooot)
            errordlg('Study file name should be of the form ''*brainstormstudy.mat''')
            varargout = cell(nargout,1);
            return
        end
        OPTIONS.rooot = strrep(StudyFile,'brainstormstudy.mat','');

        Study = load(StudyFile);
        %User = get_user_directory;
        try
            cd(User.SUBJECTS)
            Subject = load(Study.BrainStormSubject);
        catch
            errordlg(sprintf('Please make sure the subject''s information in %s is available on this plateform',Study.BrainStormSubject)); return
        end

        if isempty(OPTIONS.TessellationFile) % No Tessellation file was passed to the headmodeler
            if isfield(Subject,'Tesselation')
                OPTIONS.TessellationFile = Subject.Tesselation; % Take default (OBSOLETE as for BsT MMII because we now consider all tessellation files in Subject's folder)
            else
                OPTIONS.TessellationFile = '';
                %[OPTIONS.TessellationFile,DataPopup, Leader] = find_brainstorm_files('tess',fullfile(Users.SUBJECTS,OPTIONS.subjectpath));
            end
        end
    end

    % Get Channel Information ------------------------------------------------------------------------------------------------------------------
    if ~isempty(OPTIONS.Channel)
        Channel = OPTIONS.Channel;
    else
        if isempty(OPTIONS.ChannelFile) & isempty(OPTIONS.ChannelLoc) % Load Channel file in current folder

            [Channel, ChannelFile] = get_channel(fullfile(User.STUDIES,OPTIONS.StudyFile));
            OPTIONS.ChannelFile = fullfile(fileparts(fullfile(User.STUDIES,OPTIONS.StudyFile)),ChannelFile);

           if isempty(ChannelFile)
               errordlg(sprintf('Channel file %s is missing. Please have it available it the current study folder.',OPTIONS.ChannelFile), 'Missing Channel File')
               return
           end

        elseif isempty(OPTIONS.ChannelLoc) & exist(OPTIONS.ChannelFile,'file') % If no specific channel locations are given and channel file exists, load the proper channel file

            OPTIONS.rooot = strrep(lower(OPTIONS.ChannelFile),'channel.mat','');
            try
                load(OPTIONS.ChannelFile)
            catch
                cd(User.STUDIES)
                load(OPTIONS.ChannelFile)
            end

        else % Create a dummy Channel structure  with Channel Locations (res. Orientations) specified in OPTIONS.ChannelLoc (res .ChannelOrient)

            if OPTIONS.Verbose, bst_message_window('Creating the channel structure from information in the OPTIONS fields. . .'), end

            % Get Channel Locations
            nchan = size(OPTIONS.ChannelLoc,2); % Number of channels
            Channel = struct('Loc',[],'Orient',[],'Comment','','Weight',[],'Type','','Name','');
            Channel(1:nchan) = deal(Channel);

            ChanType = upper(OPTIONS.ChannelType);
            if isempty(ChanType)
                errordlg('Please specify a channel type (i.e. MEG or EEG) in the ChannelType field of OPTIONS'),
                bst_message_window('Please specify a channel type (i.e. MEG or EEG) in the ChannelType field of OPTIONS'),
                varargout = cell(nargout,1);
                return
            end
            [Channel(:).Type] = deal(ChanType);
            if size(OPTIONS.ChannelLoc,1) == 6 % MEG Gradiometers or mixture gradio/magneto meters
                OPTIONS.ChannelLoc = reshape(OPTIONS.ChannelLoc,3,nchan*2);
                iGradFlag = 1; % Flag - gradiometer-type sensor set
            else
                iGradFlag = 0; % Flag - EEG or magnetometer-type sensor set
            end
            ichan = 1;
            for k = 1:nchan
                if iGradFlag
                    Channel(k).Loc = OPTIONS.ChannelLoc(:,ichan:ichan+1);
                    ichan = ichan+2;
                else
                    if strcmp(ChanType,'MEG')
                        Channel(k).Loc = OPTIONS.ChannelLoc(:,ichan);
                    elseif strcmp(ChanType,'EEG') % Artificially add a dummy column full of zeros to each Channel(k).Loc
                        Channel(k).Loc = [OPTIONS.ChannelLoc(:,ichan) [0 0 0]'];;
                    end
                    ichan = ichan+1;
                end
                Channel(k).Name = sprintf('%s %d',ChanType,k);
                Channel(k).Comment = int2str(k);
            end
            clear ichan k

            % Get Channel Orientations
            if isempty(OPTIONS.ChannelOrient) & strcmp(ChanType,'MEG') % No channel orientation were specified: use radial sensors
                if OPTIONS.Verbose, bst_message_window('Assign radial orientation to all Channels. . .'), end
                if isempty(OPTIONS.HeadCenter)
                    if iGradFlag
                        Vertices = OPTIONS.ChannelLoc(:,1:2:end)';
                    else
                        Vertices = OPTIONS.ChannelLoc';
                    end

                    nscalp = size(Vertices,1);
                    if nscalp > 500 % 500 points is more than enough to compute scalp's best fitting sphere
                        Vertices = Vertices(unique(round(linspace(1,nscalp,500))),:);
                        nscalp = size(Vertices,1);
                    end
                    nmes = size(Vertices,1);

                    % Run parameters fit --------------------------------------------------------------------------------------------------------------
                    mass = mean(Vertices); % center of mass of the scalp vertex locations
                    R0 = mean(norlig(Vertices - ones(nscalp,1)*mass)); % Average distance between the center of mass and the scalp points
                    vec0 = [mass,R0];

                    [minn,brp] = fminsearch('dist_sph',vec0,[],Vertices);
                    OPTIONS.HeadCenter = minn(1:end-1);
                    if isempty(OPTIONS.Radii)
                        OPTIONS.Radii = minn(end);
                        OPTIONS.Radii = minn(end)*[1/1.14 1/1.08 1];
                    end

                    if OPTIONS.Verbose, bst_message_window({...
                                sprintf('Center of the Sphere : %3.1f %3.1f %3.1f (cm)',100*OPTIONS.HeadCenter),...
                                sprintf('Radius : %3.1f (cm)',100*OPTIONS.Radii(3)),...
                                '-> DONE',' '})
                    end

                    clear minn brp mass R0 Vertices vec0
                end

                tmp = [Channel.Loc] - repmat(OPTIONS.HeadCenter',1,nchan*(1+(iGradFlag==1)));
                OPTIONS.ChannelOrient = tmp*inorcol(tmp); % Radial orientation for every channel
                clear tmp
            elseif ~isempty(OPTIONS.ChannelOrient) & strcmp(ChanType,'MEG') & iGradFlag
                OPTIONS.ChannelOrient = reshape(OPTIONS.ChannelOrient,3,nchan*2);
            end

            if strcmp(ChanType,'MEG')
                for k=1:nchan
                    Channel(k).Orient = OPTIONS.ChannelOrient(:,((1+(iGradFlag==1))*k-1):(1+(iGradFlag==1))*k);
                end
            end

            clear k

            % Define Weights
            if iGradFlag
                [Channel(:).Weight] = deal([1 -1]);
            else
                [Channel(:).Weight] = deal([1]);
            end

            if strcmp(ChanType,'MEG') % Force no reference channels
                Channel(1).irefsens = [];
            end

            % New Channel stbemructure completed: save as a new channel file
            if ~isempty(OPTIONS.ChannelFile)
                %OPTIONS.ChannelFile = [OPTIONS.rooot,'channel.mat'];
                save(OPTIONS.ChannelFile,'Channel')
                if OPTIONS.Verbose, bst_message_window({...
                            sprintf('Channel file created in : %s',OPTIONS.ChannelFile),...
                            '-> DONE',' '}), end
            end

        end

        %load(OPTIONS.ChannelFile)
        OPTIONS.Channel = Channel;
    end

    % Find MEG and EEG channel indices
    MEGndx = good_channel(Channel,[],'MEG');
    EEGndx = good_channel(Channel,[],'EEG');
    EEGREFndx = good_channel(Channel,[],'EEG REF');

    MEG = ~isempty(MEGndx) & ~isempty(DataType.MEG); %  == 1 if MEG is requested and is available
    EEG = ~isempty(EEGndx) & ~isempty(DataType.EEG);

    if ~EEG & ~MEG
        errordlg('Please check that data (MEG or EEG) and channel types are compatible.')
        return
    end

    % Test whether CME models are requested for EEG
    if ~isempty(find(OPTIONS.SourceModel == 1)) & EEG
        if MEG
            if OPTIONS.Verbose
                bst_message_window('wrap',...
                    {'',...
                        'CME model not available for EEG - Skipping. . .',...
                        'Keeping it for MEG forward modeling only',...
                        ''...
                    })
            end
        else % EEG only is requested
            errordlg(...
                {'CME model not available for EEG - Skipping. . .',...
                    'Keeping it for MEG forward modeling only'...
                })
            return
        end

    end

    % Detect EEG reference - if none is specified in the .Comment or Type fields,
    % and if none was passed through OPTIONS.EEGRef
    % -> Apply average-referencing of the potentials by default.

    if EEG
        if isempty(OPTIONS.EEGRef)

            % EEG Reference Channel
            EEGREFndx = good_channel(Channel,[],'EEG REF');
            if isempty(EEGREFndx) % Average EEG reference anyway
                [Channel(EEGndx).Comment] = deal('AVERAGE REF');
            end

        else % EEG Reference is specified

            switch(OPTIONS.EEGRef)
            case 'AVERAGE REF'
                [Channel(:).Comment] = deal('AVERAGE REF');

            otherwise

                EEGREFndx = strmatch(OPTIONS.EEGRef,char(Channel(:).Name));
                if isempty(EEGREFndx)
                    errordlg(sprintf(...
                        'No channel named ''%s'' was found amongst available EEG channels. Cannot use it as a reference for EEG.',OPTIONS.EEGRef...
                        ))
                    return
                end

            end

        end

        %     if isempty(EEGREFndx) & ~MEG % No reference electrode was defined for the EEG stop if no MEG to compute...
        %         errordlg('Please make sure you have defined a reference for the EEG')
        %         return
        %     elseif isempty(EEGREFndx) & MEG %  Skip EEG forward modeling and proceed to MEG
        %         EEG = 0;
        %         if OPTIONS.Verbose
        %             bst_message_window({...
        %                     'No reference was defined for the EEG',...
        %                     'Skipping EEG modeling and completing MEG only...',...
        %                 })
        %         end
        %     end

    end

    if OPTIONS.Verbose,
        if EEG & MEG
            if ~isempty(EEGREFndx)
                bst_message_window({'Channel count for current computation:',[int2str(length(MEGndx)), ' MEG Channels'],...
                        sprintf('%d EEG Channels with Reference: %s',length(EEGndx),Channel(EEGREFndx).Name),' '})
            else
                bst_message_window({'Channel count for current computation:',[int2str(length(MEGndx)), ' MEG Channels'],...
                        sprintf('%d EEG Channels with Average Reference',length(EEGndx)),' '})
            end

        elseif MEG
            bst_message_window({'Channel count for current computation:',[int2str(length(MEGndx)), ' MEG Channels'],...
                    ' '})
        elseif EEG
            if ~isempty(EEGREFndx)
                bst_message_window({'Channel count for current computation:',...
                        sprintf('%d EEG Channels with Reference: %s',length(EEGndx),Channel(EEGREFndx).Name),' '})
            else
                bst_message_window({'Channel count for current computation:',...
                        sprintf('%d EEG Channels with Average Reference',length(EEGndx)),' '})
            end

        end
    end

    if MEG & isempty(MEGndx) % User wants to compute MEG but no MEG data is available
        errordlg('Sorry - No MEG data is available'), return
    end

    if EEG & isempty(EEGndx) % User wants to compute EEG but no EEG data is available
        errordlg('Sorry - No EEG data is available'), return
    end

    % Computation of parameters of the best-fitting sphere --------------------------------------------------------------------------------------------------------------
    if isempty(findstr('bem',[OPTIONS.Method{:}])) % Only if sphere-based head models are requested

        % Best-fitting sphere parameters --------------------------------------------------------------------------------------------------------------
        if isempty(OPTIONS.HeadCenter) | isempty(OPTIONS.Radii)

            if OPTIONS.Verbose, bst_message_window('Estimating Center of the Head. . .'), end

            if isempty(OPTIONS.Scalp) % Best-fitting sphere is derived from the sensor array
                %             ans = questdlg('No scalp surface was selected - Do you want to use a spherical approximation of the head derived from the sensor locations instead ?',...
                %                 '','Yes','No','Yes');
                if EEG & length(EEGndx) > 9 % If EEG is available but a reasonable number of electrodes, use it to compute the sphere parameters
                    ndx = [EEGndx]; % Compute Sphere parameters from EEG sensors only
                else
                    ndx = [MEGndx];
                end

                Vertices = zeros(length(ndx),3);
                for k=1:length(ndx)
                    Vertices(k,:) = Channel(ndx(k)).Loc(:,1)';
                end

            else % Best-fitting sphere is computed from the set of vertices of the scalp surface enveloppe

                try
                    load(fullfile(User.SUBJECTS,OPTIONS.Scalp.FileName),'Vertices')
                catch
                    load(OPTIONS.Scalp.FileName,'Vertices')
                end

                if ~isfield(OPTIONS.Scalp,'iGrid') % Apply Default
                    OPTIONS.Scalp.iGrid = 1;
                end

                try
                    Vertices = Vertices{OPTIONS.Scalp.iGrid}';
                catch
                    errordlg(sprintf(...
                        'Tessellation file %s does not contain %d enveloppes',...
                        OPTIONS.Scalp.FileName,OPTIONS.Scalp.iGrid))
                end
            end

            nscalp = size(Vertices,1);
            nmes = size(Vertices,1);

            % Run parameters fit --------------------------------------------------------------------------------------------------------------
            mass = mean(Vertices); % center of mass of the scalp vertex locations
            R0 = mean(norlig(Vertices - ones(nscalp,1)*mass)); % Average distance between the center of mass and the scalp points
            vec0 = [mass,R0];

            [SphereParams,brp] = fminsearch('dist_sph',vec0,[],Vertices);

            if OPTIONS.Verbose
                bst_message_window({...
                        sprintf('Center of the Sphere : %3.1f %3.1f %3.1f (cm)',100*SphereParams(1:end-1)'),...
                        sprintf('Radius : %3.1f (cm)',100*SphereParams(end)),...
                        '-> DONE',' '})

            end

        end % no head center and sphere radii were specified by user

        % Assign default values for sphere parameters
        % if none were specified before
        if isempty(OPTIONS.Radii)
            OPTIONS.Radii = SphereParams(end)*[1/1.14 1/1.08 1];
        end

        if isempty(OPTIONS.HeadCenter)
            OPTIONS.HeadCenter = SphereParams(1:end-1)';
        end

        if isempty(OPTIONS.Conductivity)
            OPTIONS.Conductivity = [.33 .0042 .33];
        end

    end % Use BEM Approach, so proceed to the selection of the envelopes

    % ---------------------------------------------------------------------------------------------------------
    %Create HeadModel Param structure
    Param(1:length(Channel)) = deal(struct('Center',[],'Radii',[],'Conductivity',[],'Berg',[]));

    if ~isempty(findstr('os',[OPTIONS.Method{:}])) % Overlapping-Sphere EEG or MEG is requested: load the scalp's tessellation

        if isempty(OPTIONS.Scalp)
            errordlg('Please specify a subject tessellation file for Scalp enveloppe in OPTIONS.Scalp when using Overlapping-Sphere forward approach');
            return
        end

        % load(fullfile(User.SUBJECTS,BrainStorm.SubjectTess),'Faces');
        load(fullfile(User.SUBJECTS,OPTIONS.Scalp.FileName),'Faces','Vertices');

        Faces = Faces{OPTIONS.Scalp.iGrid};
        Vertices = Vertices{OPTIONS.Scalp.iGrid}';

        if size(Vertices,1) > ReducePatchScalpNVerts % Reducepatch the scalp tessellation for fastest OS computation
            nfv = reducepatch(Faces,Vertices,2*ReducePatchScalpNVerts);
            if OPTIONS.Verbose, bst_message_window({...
                        sprintf('Decimated scalp tessellation from %d to %d vertices.',size(Vertices,1),size(nfv.vertices,1)),...
                        ' '});
            end
            clear Faces Vertices
            SubjectFV.vertices = nfv.vertices';
            SubjectFV.faces = nfv.faces;
            clear nfv
        else
            SubjectFV.vertices = Vertices; % Available from the computation of the head center above
            clear Vertices
            SubjectFV.faces = Faces; clear Faces
        end

        if length(findstr('os',[OPTIONS.Method{:}])) == 2 % OS approach requested fro both MEG and EEG
            ndx = [MEGndx,EEGndx];

        else
            if MEG
                if strcmpi('meg_os',OPTIONS.Method{DataType.MEG}) % OS for MEG only
                    ndx = MEGndx;
                end
            end

            if EEG
                if strcmpi('eeg_os',OPTIONS.Method{DataType.EEG}) % OS for EEG only
                    ndx = [EEGndx, EEGREFndx];
                end
            end
        end
        if OPTIONS.Verbose
            bst_message_window({...
                    ' ', 'Computing Overlapping-sphere model. . .'})
        end

        if isempty(OPTIONS.HeadModelFile) | OPTIONS.OS_ComputeParam

            Sphere = overlapping_sphere(Channel(ndx),SubjectFV,OPTIONS.Verbose,OPTIONS.Verbose); % Compute all spheres parameters
            if OPTIONS.Verbose
                bst_message_window({...
                        'Computing Overlapping-sphere model -> DONE',' '})
            end
            [Param(ndx).Center] = deal(Sphere.Center);
            [Param(ndx).Radii] = deal(Sphere.Radius);
            [Param(ndx).Conductivity] = deal(OPTIONS.Conductivity);

        elseif exist(OPTIONS.HeadModelFile,'file')

            load(OPTIONS.HeadModelFile,'Param') % or use precomputed
            if OPTIONS.Verbose
                bst_message_window({...
                        sprintf('Sphere parameters loaded from : %s -> DONE',OPTIONS.HeadModelFile),' '})
            end

        else

            errordlg(sprintf('Headmodel file %s does not exist in Matlab''s search path',OPTIONS.HeadModelFile))
            return

        end

    end

    % Saving HeadModel's Param structure in the headmodel file
    if strcmp(lower(OPTIONS.HeadModelFile),'default')
        if ~isfield(OPTIONS,'rooot')
            OPTIONS.rooot = OPTIONS.PrefixFileName;
        end
        OPTIONS.HeadModelFile = [OPTIONS.rooot,'headmodel.mat'];
        ifile = 0;
        while exist(OPTIONS.HeadModelFile,'file') % Do not overwrite headmodel files
            ifile = ifile + 1;
            OPTIONS.HeadModelFile = [OPTIONS.rooot,'headmodel_',int2str(ifile),'.mat'];
        end

    elseif ~isfield(OPTIONS,'rooot') & ~isempty(OPTIONS.HeadModelFile)
        OPTIONS.rooot = strrep(OPTIONS.HeadModelFile,'headmodel.mat','');
    end
    if  ~isfield(OPTIONS,'rooot') % Last Chance
        OPTIONS.rooot = 'bst_';
    end

    if ~isempty(findstr('sphere',[OPTIONS.Method{:}])) % Single or nested-sphere approaches

        [Param([MEGndx, EEGndx, EEGREFndx]).Center] = deal(OPTIONS.HeadCenter);
        [Param([MEGndx, EEGndx, EEGREFndx]).Radii] = deal(OPTIONS.Radii);
        [Param([MEGndx, EEGndx, EEGREFndx]).Conductivity] = deal(OPTIONS.Conductivity);

        if EEG & strcmpi('eeg_3sphereberg',lower(OPTIONS.Method{DataType.EEG})) % BERG APPROACH

            if ~isempty(OPTIONS.HeadModelFile) & exist(OPTIONS.HeadModelFile,'file')
                if OPTIONS.Verbose, bst_message_window('Checking for previous EEG "BERG" parameters'), end
                ParamOld = load(OPTIONS.HeadModelFile,'Param');
                iFlag = 0; % Flag
                if isfield(ParamOld.Param,'Berg') & ~isempty(ParamOld.Param(1).Radii)
                    % Check if these older parameters were computed with the same as current radii and conductivity values
                    if ParamOld.Param(1).Radii ~= Param(1).Radii;
                        iFlag = 1;
                    end
                    if ParamOld.Param(1).Conductivity ~= Param(1).Conductivity;
                        iFlag = 1;
                    end
                else
                    iFlag = 1;
                end
            else
                iFlag = 1;
            end

            if iFlag == 1
                if OPTIONS.Verbose , bst_message_window('Computing EEG "BERG" Parameters. . .'), end
                [mu_berg_tmp,lam_berg_tmp] = berg(OPTIONS.Radii,OPTIONS.Conductivity);
                Param(EEGndx(1)).Berg.mu = mu_berg_tmp; clear mu_berg_tmp
                Param(EEGndx(1)).Berg.lam = lam_berg_tmp; clear lam_berg_tmp
                if OPTIONS.Verbose, bst_message_window({'Computing EEG "BERG" Parameters -> DONE',' '}), end
            else
                if OPTIONS.Verbose , bst_message_window('Using Previous EEG "BERG" Parameters'), end
                Param(EEGndx(1)).Berg.mu = ParamOld.Param(1).Berg.mu;
                Param(EEGndx(1)).Berg.lam = ParamOld.Param(1).Berg.lam; clear ParamOld
            end
            [Param.Berg]= deal(Param(EEGndx(1)).Berg);

        end

    end

    if ~isempty(findstr('bem',[OPTIONS.Method{:}])) % BEM approaches - Compute transfer matrices

        if OPTIONS.BEM.Interpolative==0 & OPTIONS.VolumeSourceGrid %& isempty(OPTIONS.Cortex) % User wants volumic grid : force Interpolative approach
            OPTIONS.BEM.Interpolative = 1;    % CBB (SB, 07-May-2004)| Should work also for volumic grid
            hwarn = warndlg('Volumic Source Grid BEM is only available for interpolative BEM. BEM computation will be now forced to interpolative. If you want a non-interpolative BEM on a cortical image grid, first uncheck the Volumic Grid box from headmodeler gui.','Limitation from current BrainStorm version');
            drawnow
            waitfor(hwarn)
        end

        if MEG
            if ~isempty(findstr('bem',OPTIONS.Method{DataType.MEG})) % BEM is requested for MEG
                BEMChanNdx{DataType.MEG} = MEGndx;
            end
        end

        if EEG
            if ~isempty(findstr('bem',OPTIONS.Method{DataType.EEG})) % BEM is requested for EEG
                BEMChanNdx{DataType.EEG} = sort([EEGndx,EEGREFndx]); % EEGREFndx = [] is average ref
            end
        end

        OPTIONS.Param = Param;

        if OPTIONS.BEM.Interpolative % Computation of gain matrix over 3D interpolative grid
            if OPTIONS.BEM.ForceXferComputation
                BEMGaingridFileName = bem_GainGrid(DataType, OPTIONS, BEMChanNdx); % Computation of the BEM gain matrix on the 3D interpolative grid for MEG and/or EEG data
            else
                try
                    BEMGaingridFileName = OPTIONS.BEM.GaingridFileName;
                catch
                    cd(User.STUDIES)
                    BEMGaingridFileName = bem_GainGrid(DataType, OPTIONS, BEMChanNdx); % Computation of the BEM gain matrix on the 3D interpolative grid for MEG and/or EEG data
                end
            end

            if MEG & EEG
                [Param(MEGndx).bem_gaingrid_mfname] = deal(BEMGaingridFileName.MEG);
                [Param([EEGndx]).bem_gaingrid_mfname] = deal(BEMGaingridFileName.EEG);
            else
                [Param(:).bem_gaingrid_mfname] = deal(BEMGaingridFileName);
            end

        else % BEM gain matrix computation over cortical surface

            BEMGaingridFileName = bem_GainGrid(DataType, OPTIONS, BEMChanNdx);

            [Param(:).bem_gaingrid_mfname] = deal('');

        end

        OPTIONS = rmfield(OPTIONS,'Param');

        ndx = sort([BEMChanNdx{:}]);

        [Param(ndx).Center] = deal([]);

        if OPTIONS.VolumeSourceGrid
            % Now define the outer scalp envelope for the source volume gridding if requested

            if OPTIONS.BEM.ForceXferComputation | ~test
                global nfv
                SubjectFV.vertices = nfv(end).vertices';
                SubjectFV.faces = nfv(end).faces; clear Faces
            else
                load(fullfile(User.SUBJECTS,fileparts(OPTIONS.Subject),OPTIONS.BEM.EnvelopeNames{end}.TessFile))

                idScalp = find(strcmpi(OPTIONS.BEM.EnvelopeNames{end}.TessName,Comment)); clear Comment
                if isempty(idScalp)
                    errodlg(sprintf(...
                        'Scalp tessellation %s was not found in %s.',OPTIONS.BEM.EnvelopeNames{end}.TessName, OPTIONS.BEM.EnvelopeNames{end}.TessFile),...
                        'Error during BEM computation')
                    return
                end

                SubjectFV.vertices = Vertices{idScalp}'; clear Vertices
                SubjectFV.faces = Faces{idScalp}; clear Faces
            end

        end

    end % if BEM

    if EEG

        switch(lower(OPTIONS.Method{DataType.EEG}))
        case 'eeg_sphere'
            [Param(EEGndx).EEGType] = deal('EEG_SINGLE');
        case 'eeg_3sphere'
            [Param(EEGndx).EEGType] = deal('EEG_3SHELL');
        case 'eeg_3sphereberg'
            [Param(EEGndx).EEGType] = deal('EEG_BERG');
        case 'eeg_os'
            [Param(EEGndx).EEGType] = deal('EEG_OS');
        case 'eeg_bem'
            [Param(EEGndx).EEGType] = deal('BEM');
            if EEG & ~MEG
                [Param(EEGndx).bem_gaingrid_mfname] = deal(BEMGaingridFileName);
            end
        end

    else

        tmp = [];
        [Param.Conductivity] = deal(tmp);
        [Param.Berg] = deal(tmp);
        [Param.EEGType] = deal(tmp);

    end

    %_________________________________________________________________________________________________________________________________________________________________________
    %
    %  The following part is an adaptation of the former HEADMODEL_MAKE script
    %
    % ________________________________________________________________________________________________________________________________________________________________________

    % Allocate function names depending on the selected forward approaches specified in the Method argument
    Function = cell(1,length(Channel)); %allocate function names

    if MEG
        if ~isempty(findstr('bem',OPTIONS.Method{DataType.MEG})) % MEG BEM
            Function(MEGndx) = deal({'meg_bem'});
            if isfield(Channel(MEGndx(1)),'irefsens')
                Function(Channel(MEGndx(1)).irefsens) = deal({'meg_bem'});
            end

        else
            Function(MEGndx) = deal({'os_meg'});
            if isfield(Channel(MEGndx(1)),'irefsens')
                Function(Channel(MEGndx(1)).irefsens) = deal({'os_meg'});
            end
            if isfield(OPTIONS,'BEM')
                OPTIONS = rmfield(OPTIONS,'BEM');
            end

        end
    end

    if EEG
        if ~isempty(findstr('bem',OPTIONS.Method{DataType.EEG})) % MEG BEM
            Function(EEGndx) = deal({'eeg_bem'});
        else
            Function(EEGndx) = deal({'eeg_sph'});
            if isfield(OPTIONS,'BEM')
                OPTIONS = rmfield(OPTIONS,'BEM');
            end
        end

    end
    % --------------------------------------------------------------------------------------------------------

    DIMS = [3 12]; % number of columns for each parametric source model: Current Dipole / Current Multipole
    HeadModel.Param = Param;
    HeadModel.Function = Function;

    if OPTIONS.VolumeSourceGrid % if SearchGain matrices are requested

        SearchGain = cell(1,6); % One cell for each source order + pairs of sync. sources - keep 2 'phantom' cells for former Magnetic Dipole model

        %------------------------------ Computing SearchGridLoc and SearchGain-------------------------------------------

        if ~isempty(findstr(MegMethod,'bem')) | (EEG & ~MEG)
            if OPTIONS.Verbose, bst_message_window(...
                    {'Forward modeling not available for EEG''s Current Mulipole models or the BEM approach','Computing forward fields for Current Dipole models. . .'...
                        ,' '}), end
            %return
            OPTIONS.SourceModel = [-1]; % Force current dipole source model when MEG BEM or any EEG is requested
        end

        if OPTIONS.Verbose, bst_message_window(...
                'Computing the Gain Matrices from the Volumic Search Grid. . .'), end

        % Prepare call to source_grids

        GUI.VALIDORDER = OPTIONS.SourceModel;
        GUI.SPACING = 10*OPTIONS.VolumeSourceGridSpacing; % Pass it to source_grids in millimeters
        GUI.VERBOSE = OPTIONS.Verbose;
        GUI.MEG = MEG;
        GUI.EEG = EEG;

        if ~exist('SubjectFV','var')
            % Create a spherical tessellation of the INNER SKULL surface boundary for source gridding

            [x,y,z] = sphere(30);
            %Scale to Radius BrainStorm.R
            [TH,PHI,R] = cart2sph(x,y,z);
            if OPTIONS.Radii(1) == 0
                error('The sphere radius you have specified is ZERO - please change it to a non null positive value')
            end
            R = OPTIONS.Radii(1) * ones(size(R));
            [x,y,z] = sph2cart(TH,PHI,R);

            % Tessellate both hemispheres
            Im = find(z>=0);
            tri = delaunay(x(Im),y(Im));

            %Gather everything into the same patch
            SubjectFV.vertices = [x(Im),y(Im),z(Im);...
                    x(Im),y(Im),-z(Im)];

            % Translate about the head center
            SubjectFV.vertices = SubjectFV.vertices + repmat(OPTIONS.HeadCenter',size(SubjectFV.vertices,1),1);

            SubjectFV.faces = [tri;tri(:,[3 2 1])+max(tri(:))];

            clear x y z R TH PHI Im tri

        end

        HeadModel = source_grids(HeadModel,Channel,SubjectFV,GUI);
        clear SubjectFV

        if OPTIONS.Verbose, bst_message_window({...
                    '-> DONE',...
                    'Now saving HeadModel file. . .'...
                })
        end

        % Now save VolumeSourceGrid headmodel(s)
        SaveHeadModel.Param = HeadModel.Param;
        SaveHeadModel.Function = HeadModel.Function;

        if strcmpi(OPTIONS.HeadModelName,'Default') % Specify default HeadModelName
            if MEG & EEG
                SaveHeadModel.HeadModelName = sprintf('%s | %s', OPTIONS.Method{DataType.MEG},OPTIONS.Method{DataType.EEG});
            elseif MEG
                SaveHeadModel.HeadModelName = sprintf('%s', OPTIONS.Method{DataType.MEG});
            elseif EEG
                SaveHeadModel.HeadModelName = sprintf('%s', OPTIONS.Method{DataType.EEG});
            end
        else
            SaveHeadModel.HeadModelName = OPTIONS.HeadModelName;
        end

        if ~isempty(OPTIONS.HeadModelFile),
            [HeadModelFilePath,HeadModelFile,ext] = fileparts(OPTIONS.HeadModelFile);
        end

        cd(User.STUDIES)

        for k = 1:length(GUI.VALIDORDER) % One file per source order

            % Specify Source Model Name and Other Fields
            if GUI.VALIDORDER(k) == -1
                SourceOrderString = 'CD'; % Current Dipole
            elseif GUI.VALIDORDER(k) == 0
                SourceOrderString = 'MD'; % Magnetic Dipole
            elseif GUI.VALIDORDER(k) == 1
                SourceOrderString = 'CME'; % Current Multipole
            end

            [SaveHeadModel.Param.Order] = deal(GUI.VALIDORDER(k));
            SaveHeadModel.SourceOrder = GUI.VALIDORDER(k); % Alternative to previous line
            SaveHeadModel.HeadModelType = 'SearchGrid';
            if MEG
                SaveHeadModel.MEGMethod = OPTIONS.Method{DataType.MEG};
            end
            if EEG
                SaveHeadModel.EEGMethod = OPTIONS.Method{DataType.EEG};
            end

            SaveHeadModel.GridName = {sprintf('%s Volumic Grid : %3.1f cm spacing',SourceOrderString, OPTIONS.VolumeSourceGridSpacing)};
            SaveHeadModel.GridLoc  = HeadModel.GridLoc(GUI.VALIDORDER(k)+2);
            SaveHeadModel.Gain = {single(HeadModel.Gain{GUI.VALIDORDER(k)+2})}; % Convert to single precision gain matrix as in BST MMII convention

            % now collect together and save
            if ~isempty(OPTIONS.HeadModelFile), % User has provided a headmodel file name
                SaveHeadModelFile = fullfile(HeadModelFilePath,[HeadModelFile,...
                        sprintf('VolGrid_%s',SourceOrderString),...
                        ext]);
                ifile = 0;
                while exist(SaveHeadModelFile,'file') % Do not overwrite headmodel files
                    ifile = ifile + 1;
                    SaveHeadModelFile = fullfile(HeadModelFilePath,[HeadModelFile,...
                            sprintf('VolGrid_%s',SourceOrderString),...
                            '_',int2str(ifile),ext]);
                end

                if OPTIONS.Verbose, bst_message_window({...
                        sprintf('Writing HeadModel file:'),...
                        sprintf('%s', SaveHeadModelFile)...
                    })
                end

                if strcmp(lower(OPTIONS.HeadModelFileOld),'default')
                    OPTIONS.HeadModelFile = SaveHeadModelFile;
                end

                save_fieldnames(SaveHeadModel, SaveHeadModelFile);

                if OPTIONS.Verbose, bst_message_window(...
                        {'-> DONE',' '}), end
            end

        end

        OPTIONS.HeadModelName = SaveHeadModel.HeadModelName;

        if OPTIONS.Verbose, bst_message_window(...
                {'Grid gain matrices are saved','RAP-MUSIC & Least-Squares Fits Approaches are now available',' '})
        end

    end

    %% -------------------------------------------------------------------------
    %
    %  Now proceed to forward modeling of cortical grids or at some other specific source locations
    %
    %% -------------------------------------------------------------------------

    if ~isempty(OPTIONS.Cortex), % subject has cortical vertices as source supports
        % First make room in memory
        HeadModel.SearchGain = [];
        clear SearchGridLoc SearchGain G

        if OPTIONS.Verbose, bst_message_window({...
                    'Computing the Image Gain Matrices (this may take a while). . .'})
        end

        % Find the cortical grid where to compute the forward model in the tessellation file
        try
            ImageGrid = load(fullfile(User.SUBJECTS,OPTIONS.Cortex.FileName),'Comment','Vertices');
        catch
            ImageGrid = load(OPTIONS.Cortex.FileName,'Comment','Vertices');
        end

        if ~isfield(OPTIONS.Cortex,'iGrid')
            OPTIONS.Cortex.iGrid = 1;
        end

        if OPTIONS.Cortex.iGrid > length(ImageGrid.Comment) % Corresponding cortical surface not found
            errordlg(sprintf('Cortical Tessellation file "%s" does not contain %d surfaces',...
                OPTIONS.Cortex.FileName,OPTIONS.Cortex.iGrid))
            return
        end

        GridLoc = ImageGrid.Vertices{OPTIONS.Cortex.iGrid}; % keep only the desired cell
        ImageGrid = rmfield(ImageGrid,'Vertices');

    elseif ~isempty(OPTIONS.SourceLoc) % Specific source locations are provided
        if size(OPTIONS.SourceLoc,1) ~= 3
            OPTIONS.SourceLoc = OPTIONS.SourceLoc';
        end
        GridLoc = OPTIONS.SourceLoc;

    else
        rmfield(OPTIONS,'rooot');
        if nargout == 1
            varargout{1} = OPTIONS;
        else
            varargout{1} = HeadModel;
            varargout{2} = OPTIONS;
        end
        return % No ImageGrid requested

    end

    if ~isempty(OPTIONS.Cortex)
        GridName{OPTIONS.Cortex.iGrid} = ImageGrid.Comment{OPTIONS.Cortex.iGrid};
        OPTIONS.Cortex.Name  = ImageGrid.Comment{OPTIONS.Cortex.iGrid};
    end

    %for i = 1 % CHEAT - Dipoles only here
    for Order = OPTIONS.SourceModel % Compute gain matrices for each requested source models    (-1 0 1)

        i = 1; % Index to cell in headmodel cell arrays (MMII convention)

        switch(Order)
        case -1
            SourceOrderString = 'CD'; % Current Dipole
            Dims = DIMS(1);% number of columns per source
        case 0
            errordlg(sprintf('Unauthorized Source Model Order %d.',iSrcModel),'Wrong HeadModel parameter assignment')
            return
            %SourceOrderString = 'MD'; % Magnetic Dipole  - OBSOLETE
        case 1
            SourceOrderString = 'CME'; % Current Multipole
            Dims = DIMS(2);% number of columns per source
        otherwise
            errordlg(sprintf('Unauthorized Source Model Order %d.',iSrcModel),'Wrong HeadModel parameter assignment')
            return
        end

        if OPTIONS.Verbose,
            bst_message_window(...
                'wrap',...
                sprintf('Computing Gain Matrix for the %s Model',SourceOrderString))
            h = waitbar(0,sprintf('Computing Gain Matrix for the %s Model',SourceOrderString));
            pos = get(h,'position');
        end

        if ~isempty(OPTIONS.Cortex) & OPTIONS.ApplyGridOrient % Use cortical grid
            try
                load(OPTIONS.Cortex.FileName,'Faces');
            catch
                Users = get_user_directory;
                load(fullfile(Users.SUBJECTS,OPTIONS.Cortex.FileName),'Faces');
            end

            Faces = Faces{OPTIONS.Cortex.iGrid};

            ptch = patch('Vertices',GridLoc','Faces',Faces,'Visible','off');
            set(get(ptch,'Parent'),'Visible','off')
            clear Faces
            GridOrient{i} = get(ptch,'VertexNormals')'; % == {1} as of MMII conventions. Most data in HeadModel files are single-cell cell arrays.
            % Cell array structure kept for backward compatibility with BsT2000.
            delete(ptch);

        end

        if OPTIONS.ApplyGridOrient % Take cortex normals into account
            if isempty(OPTIONS.GridOrient) & isempty(OPTIONS.SourceOrient) % Consider the cortical patch's normals

                if isempty(OPTIONS.Cortex)  % Specific source locations in .SourceLoc but nohing in. SourceOrient
                    OPTIONS.ApplyGridOrient = 0; % No source orientation specified: Force computation of full gain matrix
                else
                    [nrm,GridOrient{i}] = colnorm(GridOrient{i});
                    % Now because some orientations may be ill-set to [0 0 0] with the get(*,'VertexNormals' command)
                    % set these orientation to arbitrary [1 1 1]:
                    izero = find(nrm == 0);clear nrm
                    if ~isempty(izero)
                        GridOrient{i}(:,izero) = repmat([1 1 1]'/norm([1 1 1]),1,length(izero));
                    end
                    clear izero

                end

            elseif ~isempty(OPTIONS.GridOrient) % Apply user-defined cortical source orientations

                if size(OPTIONS.GridOrient,2) == size(GridLoc,2) % Check size integrity
                    GridOrient{i} = OPTIONS.GridOrient;
                    [nrm,GridOrient{i}] = colnorm(GridOrient{i});
                    clear nrm
                else
                    errordlg(sprintf('The source orientations you have provided are for %0.f sources. Considered cortical surface has %0.f sources. Computation aborted',...
                        size(OPTIONS.GridOrient,2),size(GridOrient{i},2)));
                    return
                end

            elseif ~isempty(OPTIONS.SourceOrient) % Apply user-defined specific source orientations

                if size(OPTIONS.SourceOrient,2) == size(GridLoc,2) % Check size integrity
                    GridOrient{i} = OPTIONS.SourceOrient;
                    [nrm,GridOrient{i}] = colnorm(GridOrient{i});
                    clear nrm
                else
                    errordlg(sprintf('The source orientations you have provided are for %0.f sources. Computation aborted',...
                        size(OPTIONS.SourceOrient,2)));
                    return
                end

            end
        end

        nv = size(GridLoc,2); % number of grid points

        if ~isempty(OPTIONS.ImageGridFile) % Save cortical gain matrix in a binary file

            [PATH,NAME,EXT,VER] = fileparts(OPTIONS.HeadModelFile);

            if strcmp(lower(OPTIONS.ImageGridFile),'default')
                if exist('GridName','var') % Cortical support was specified
                    destname = [NAME,'_Gain_',strrep(ImageGrid.Comment{OPTIONS.Cortex.iGrid},' ',''),'_',SourceOrderString,'.bin']; % New naming (March, 19 - 2002)
                    k = 1;
                    try
                        while exist(destname,'file') % Don't write over existing .bin gain matrix file
                            destname = [NAME,'_Gain_',strrep(ImageGrid.Comment{OPTIONS.Cortex.iGrid},' ',''),'_',SourceOrderString,'_',int2str(k),'.bin']; % New naming (March, 19 - 2002)
                            k = k+1;
                        end
                    catch
                        while exist(destname,'file') % Don't write over existing .bin gain matrix file
                            destname = [NAME,'_Gain_',strrep(ImageGrid.Comment{OPTIONS.Cortex.iGrid},' ',''),'_',SourceOrderString,'_',int2str(k),'.bin']; % New naming (March, 19 - 2002)
                            k = k+1;
                        end
                    end

                    clear k

                else  % Specific location was provided in .SourceLoc
                    destname = [NAME,'_Gain_SpecLoc_',SourceOrderString,'.bin']; % Specific location was provided in .SourceLoc
                    k = 1;
                    while exist(destname,'file') % Don't write over existing .bin gain matrix file
                        destname = [NAME,'_Gain_SpecLoc_',SourceOrderString,'_',int2str(k),'.bin'];
                        k = k+1;
                    end
                    clear k
                end
                OPTIONS.ImageGridFile = destname;
            else
                [PATH,destname,ext,ver] = fileparts(OPTIONS.ImageGridFile);
                destname = [destname, ext];
            end

            % Check whether this name exists - if yes, don't overwrite
            try
                cd(fullfile(User.STUDIES,PATH))
            catch
                cd(PATH)
            end

            hdml = 1;
            while exist(destname,'file')
                if hdml == 1
                    destname = strrep(destname,'.bin',['_',int2str(hdml),'.bin']);
                else
                    destname = strrep(destname,[int2str(hdml-1),'.bin'],[int2str(hdml),'.bin']);
                end

                hdml = hdml+1;
            end
            clear hdml

            fdest = fopen(destname,'w','ieee-be');
            if fdest < 0
                errordlg('Error creating the file for the cortical image forward model; Please check for disk space availability')
                return
            end
            frewind(fdest);

            fwrite(fdest,length([OPTIONS.Channel]),'uint32');

            %         % BEM and non-interpolative, store gain matrix already computed
            %         if isfield(OPTIONS,'BEM') % BEM computation
            %             if ~OPTIONS.BEM.Interpolative
            %                 global GBEM_grid
            %             end
            %         end

            if fdest < 0 , errordlg('Please Check Write Permissions', ['Cannot create ',destname]), return, end
        else % If no ImageGridFile was specified
            %OPTIONS.ImageGridBlockSize = nv; % Force a one-time computation of all source forward fields
        end

        src_ind = 0;
        jj = 0; % Number of OPTIONS.ImageGridBlockSizeSizes

        for j = 1:(OPTIONS.ImageGridBlockSize):nv,
            jj = jj+1;
            if 1%OPTIONS.ApplyGridOrient
                ndx = [0:OPTIONS.ImageGridBlockSize-1]+j;
            else
                ndx = [0:3*OPTIONS.ImageGridBlockSize-1]+j;
            end

            if 1%OPTIONS.ApplyGridOrient
                if(ndx(end) > nv),  % last OPTIONS.ImageGridOPTIONS.ImageGridBlockSizeSize too long
                    ndx = [ndx(1):nv];
                end
            else
                if(ndx(end) > 3*nv),
                    ndx = [ndx(1):3*nv];
                end
            end

            % Compute MEG
            if MEG & ~isempty(MEGndx)
                Gmeg = NaN*zeros(length(MEGndx),Dims*length(ndx));
                if ~isempty(Function{MEGndx(1)})

                    if MEG & EEG
                        clear('gain_bem_interp2'); % Free persistent variables to avoid confusion
                    end

                    if isfield(OPTIONS,'BEM') % BEM computation
                        if ~OPTIONS.BEM.Interpolative % ~interpolative : retrieve stored gain matrix
                            global GBEM_grid
                            Gmeg = GBEM_grid(MEGndx,[j:min([3*nv,j+3*OPTIONS.ImageGridBlockSize-1])]);
                        end
                    else
                        Gmeg = feval(Function{MEGndx(1)},GridLoc(:,ndx),Channel,Param,Order,OPTIONS.Verbose);
                    end

                end
            else
                Gmeg = [];
            end

            if EEG & ~isempty(EEGndx) & i==1 % % Order -1 only for now in EEG

                Geeg = NaN*zeros(length(EEGndx),Dims*length(ndx));
                if ~isempty(Function{EEGndx(1)})
                    if MEG & EEG
                        clear('gain_bem_interp2'); % Free persistent variables to avoid confusion
                    end

                    if isfield(OPTIONS,'BEM') % BEM computation
                        if ~OPTIONS.BEM.Interpolative % ~interpolative : retrieve stored gain matrix
                            global GBEM_grid
                            Geeg = GBEM_grid(EEGndx,[j:min([3*nv,j+3*OPTIONS.ImageGridBlockSize-1])]);
                        end
                    else
                        Geeg = feval(Function{EEGndx(1)},GridLoc(:,ndx),Channel,Param,Order,OPTIONS.Verbose);
                    end

                end
            else
                Geeg = [];
            end

            if OPTIONS.ApplyGridOrient
                G = NaN*zeros(length(OPTIONS.Channel),length(ndx));
            else
                G = NaN*zeros(length(OPTIONS.Channel),Dims*length(ndx));
            end

            if OPTIONS.Verbose, bst_message_window(...
                    ['Computing Cortical Gain Vectors. . . Block # ',int2str(jj),...
                        ' of ',int2str(length(1:OPTIONS.ImageGridBlockSize:nv))])

                hh = waitbar(0,['Computing Cortical Gain Vectors. . . Block # ',int2str(jj),...
                        ' of ',int2str(length(1:OPTIONS.ImageGridBlockSize:nv))]);
                set(hh,'Position',[pos(1), pos(2)+pos(4),pos(3),pos(4)])
                drawnow
            end

            src = 0;
            % Options on cortical source orientation
            if OPTIONS.ApplyGridOrient % Apply source orientation

                for k = 1:Dims:Dims*length(ndx)-2
                    src = src+1;
                    src_ind = src_ind+1;
                    if ~isempty(Gmeg)
                        G(MEGndx,src) = Gmeg(:,k:k+2) * GridOrient{1}(:,src_ind);
                    end

                    if ~isempty(Geeg)&(i==1)% % Order -1 only
                        G(EEGndx,src) = Geeg(:,k:k+2) * GridOrient{1}(:,src_ind);
                    end
                    if OPTIONS.Verbose,
                        if ~rem(src,5000)
                            waitbar(src/length(1:Dims:Dims*length(ndx)-2),hh)
                        end
                    end

                end

            else % Do not apply cortical orientation. Keep full gain matrix at each cortical location

                if MEG
                    G(MEGndx,:) = Gmeg;
                end
                if EEG
                    G(EEGndx,:) = Geeg;
                end

            end

            if OPTIONS.Verbose,
               if ~rem(src_ind,1000)
                   waitbar(src_ind/nv,h)
               end

                delete(hh)
            end

            clear Gmeg Geeg

            if ~isempty(OPTIONS.ImageGridFile)
                % 4 bytes per element, find starting point
                if OPTIONS.ApplyGridOrient % Apply source orientation
                    offset = 4 + (ndx(1)-1)*length(Channel)*4;
                else
                    offset = 4 + 3*(ndx(1)-1)*length(Channel)*4;
                end

                % Matlab 6.5.0 bug, Technical Solution Number: 1-19NOK
                % http://www.mathworks.com/support/solutions/data/1-19NOK.html?solution=1-19NOK
                % must REWIND first before fseek.
                % Apparently fixed in 6.5.1
                % JCM 27-May-2004
                status = fseek(fdest,0,'bof');
                status = fseek(fdest,offset,'bof');
                if(status == -1),
                    errordlg('Error writing Image Gain Matrix file'); return
                end

                fwrite(fdest,G,'float32');
            end

        end
        if exist('destname','var') & ~isempty(OPTIONS.Cortex)
            Gain{OPTIONS.Cortex.iGrid}{i} = destname;
            OPTIONS.ImageGridFile = destname;
        end

        if OPTIONS.Verbose
            close(h)
        end

        if ~isempty(OPTIONS.ImageGridFile)
            fclose(fdest);
            if OPTIONS.Verbose, bst_message_window({...
                        sprintf('Computing Gain Matrix for the %s Model -> DONE',SourceOrderString),...
                        sprintf('Saved in:'),...
                        sprintf('%s',destname)...
                        }),
            end
        end

        % Now save ImageGrid headmodel(s)-----------------------------------------------------------------------------------
        OPTIONS.HeadModelFile = OPTIONS.HeadModelFileOld; % Use original HeadModelFile entry
        if strcmp(lower(OPTIONS.HeadModelFile),'default')
            if ~isfield(OPTIONS,'rooot')
                OPTIONS.rooot = OPTIONS.PrefixFileName;
            end
            OPTIONS.HeadModelFile = [OPTIONS.rooot,'headmodel.mat'];
            ifile = 0;
            while exist(OPTIONS.HeadModelFile,'file') % Do not overwrite headmodel files
                ifile = ifile + 1;
                OPTIONS.HeadModelFile = [OPTIONS.rooot,'headmodel_',int2str(ifile),'.mat'];
            end

        elseif ~isfield(OPTIONS,'rooot') & ~isempty(OPTIONS.HeadModelFile)
            OPTIONS.rooot = strrep(OPTIONS.HeadModelFile,'headmodel.mat','');
        end

        SaveHeadModel.Param = HeadModel.Param;
        SaveHeadModel.Function = HeadModel.Function;

        if MEG
            SaveHeadModel.MEGMethod = OPTIONS.Method{DataType.MEG};
        end
        if EEG
            SaveHeadModel.EEGMethod = OPTIONS.Method{DataType.EEG};
        end

        if strcmpi(OPTIONS.HeadModelName,'Default') % Specify default HeadModelName
            if MEG & EEG
                SaveHeadModel.HeadModelName = sprintf('%s | %s', OPTIONS.Method{DataType.MEG},OPTIONS.Method{DataType.EEG});
            elseif MEG
                SaveHeadModel.HeadModelName = sprintf('%s', OPTIONS.Method{DataType.MEG});
            elseif EEG
                SaveHeadModel.HeadModelName = sprintf('%s', OPTIONS.Method{DataType.EEG});
            end
        else
            SaveHeadModel.HeadModelName = OPTIONS.HeadModelName;
        end

        if ~isempty(OPTIONS.HeadModelFile), % and is not empty
            [HeadModelFilePath,HeadModelFile,ext] = fileparts(OPTIONS.HeadModelFile);
        end

        % Assign proper GridName
        switch(Order)
        case -1
            SourceOrderString = 'CD'; % Current Dipole
        case 0
            SourceOrderString = 'MD'; % Magnetic Dipole
        case 1
            SourceOrderString = 'CME'; % Current Multipole
        end

        [SaveHeadModel.Param.Order] = deal(Order);
        SaveHeadModel.SourceOrder = Order; % Alternative to previous line
        SaveHeadModel.HeadModelType = 'ImageGrid';

        if ~isempty(OPTIONS.Cortex)
            SaveHeadModel.GridName = {sprintf('%s Surface Grid : %s',SourceOrderString, OPTIONS.Cortex.Name)};
            if ~isempty(OPTIONS.ImageGridFile)
                SaveHeadModel.Gain = {OPTIONS.ImageGridFile}; % Store Image Grid File name in the headmodel.mat file
            end

            SaveHeadModel.GainCovar{1} = cell(1); % May compute it later within headmodel_make
            SaveHeadModel.GainCovarName ='';
            SaveHeadModel.GridLoc{1} = OPTIONS.Cortex.FileName;
            if 1%OPTIONS.Cortex.iGrid > 1 % Meaning that cortical support is not the fisrt surface in tessellation file (default in MMII)
                % Add yet another field to headmodel file to specify this information.
                SaveHeadModel.iGrid = OPTIONS.Cortex.iGrid;
            end
        else
            SaveHeadModel.GainCovar = [];
            SaveHeadModel.GainCovarName = [];
            SaveHeadModel.GridName = [];
            SaveHeadModel.GridLoc = {OPTIONS.SourceLoc};
            SaveHeadModel.Gain = {OPTIONS.ImageGridFile};
        end
        if ~OPTIONS.ApplyGridOrient
            SaveHeadModel.GridOrient = [];
        else
            SaveHeadModel.GridOrient = {OPTIONS.SourceOrient};
        end

        % now collect together and save
        if ~isempty(OPTIONS.HeadModelFile) & exist('destname','var')
            % HeadModel file name
            SaveHeadModelFile = fullfile(HeadModelFilePath,[HeadModelFile,...
                    sprintf('SurfGrid_%s',SourceOrderString),...
                    ext]);
            ifile = 0;
            while exist(SaveHeadModelFile,'file') % Do not overwrite headmodel files
                ifile = ifile + 1;
                SaveHeadModelFile = fullfile(HeadModelFilePath,[HeadModelFile,...
                        sprintf('SurfGrid_%s',SourceOrderString),...
                        '_',int2str(ifile),ext]);
            end
            if OPTIONS.Verbose, bst_message_window({...
                        sprintf('Writing Cortical Image Support HeadModel file:'),...
                        sprintf('%s', SaveHeadModelFile)...
                    })
            end

            if strcmp(lower(OPTIONS.HeadModelFileOld),'default')
                OPTIONS.HeadModelFile = SaveHeadModelFile;
            end

            try
                save_fieldnames(SaveHeadModel, SaveHeadModelFile);
            catch
                cd(User.STUDIES)
                save_fieldnames(SaveHeadModel, SaveHeadModelFile);
            end

            if OPTIONS.Verbose, bst_message_window(...
                    {'-> DONE',' '}), end
        end

        % Save completed -----------------------------------------------------------------------------------

    end % Cortical gain matrix for each source model

    %% -------------------------------------------------------------------------
    if isfield(OPTIONS,'rooot')
        OPTIONS = rmfield(OPTIONS,'rooot');
    end

    if nargout == 0
        clear G SearchGain
    elseif nargout == 1
        varargout{1} = OPTIONS;
    else
        if OPTIONS.ImageGridBlockSize < nv
            varargout{1} = OPTIONS.HeadModelFile;
        else
            varargout{1} = G;
        end

        varargout{2} = OPTIONS;
    end

    fclose('all');

    if OPTIONS.Verbose, bst_message_window(...
            {'The head model has been properly designed and written to disk'})
    end

    if OPTIONS.Verbose, bst_message_window({...
                sprintf(...
                'Head modeling ends (%s - %dH %dmin %ds (took %3.2f seconds))',date,clockk(4:6), etime(clock,time0)),...
                '__________________________________________'...
            })
    end

    %------------------------------------------------------------------------------------------------------------------------------
    %
    %                                         SUB-FUNCTIONS
    %
    %------------------------------------------------------------------------------------------------------------------------------

    function BEMGaingridFname = bem_GainGrid(DataType,OPTIONS,BEMChanNdx)

    % Computation of the BEM gain matrix on the 3D interpolative grid for MEG and/or EEG data
    % DataType : a structure with fields MEG and EEG. DataType.MEG (res. DataType.EEG) is set to 1 if MEG (res. EEG) data is available
    % OPTIONS : the OPTIONS structure, input argument from bst_headmodeler
    % BEMChanNdx :  a cell array of channel indices such that : BEMChanNdx{DataType.EEG} = EEGndx (res. MEG).
    %
    % BEMGaingridFname: a structure with fields:
    %                 .EEG : string with the name of the file containing the gain matrix of the 3D BEM interpolative grid in EEG
    %                 .MEG : same as .EEG respectively to MEG BEM model.

    % Detect the requested BEM computations: MEG and/or EEG___________________________________

    User = get_user_directory;
    if isempty(User)
        User.STUDIES = pwd;
        User.SUBJECTS = pwd;
    end

    MEG = ~isempty(BEMChanNdx(DataType.MEG)); %  == 1 if MEG is requested
    EEG = ~isempty(BEMChanNdx(DataType.EEG)); %  == 1 if EEG is requested
    if MEG
        MEGndx = BEMChanNdx{DataType.MEG};
    end
    if EEG
        EEGndx = BEMChanNdx{DataType.EEG};
    end

    if MEG & EEG
        [Param(:).mode] = deal(3);
    elseif ~MEG & EEG
        [Param(:).mode] = deal(1);
    elseif MEG & ~EEG
        [Param(:).mode] = deal(2);
    else
        errordlg('Please check that the method requested for forward modeling has an authorized name');
        return
    end

    % BEM parameters ________________________________________________________________________
    % Determine what basis functions to use
    constant = ~isempty(strmatch('constant',lower(OPTIONS.BEM.Basis)));
    if constant == 0
        [Param(:).basis_opt] = deal(1);
    else
        [Param(:).basis_opt] = deal(0);
    end

    % Determine what Test to operate
    collocation = ~isempty(strmatch('collocation',lower(OPTIONS.BEM.Test)));
    if collocation == 0
        [Param(:).test_opt] = deal(1);
    else
        [Param(:).test_opt] = deal(0);
    end

    % Insulated-skull approach
    isa = OPTIONS.BEM.ISA;
    if isa == 1
        [Param(:).ISA] = deal(1);
    else
        [Param(:).ISA] = deal(0);
    end

    Param.Ntess_max = OPTIONS.BEM.NVertMax;
    Param.Conductivity = deal(OPTIONS.Conductivity);

    %_________________________________________________________________________________________

    % Load surface envelopes information______________________________________________________

    % Find the indices of the enveloppes selected for BEM computation
    if ~isfield(OPTIONS.BEM,'EnvelopeNames')
        errordlg('Please specify the ordered set of head-tissue envelopes by filling the OPTIONS.BEM.EnvelopeNames field')
        return
    end
    if isempty(OPTIONS.BEM.EnvelopeNames)
        errordlg('Please specify the ordered set of head-tissue envelopes by filling the OPTIONS.BEM.EnvelopeNames field')
        return
    end
    for k = 1:length(OPTIONS.BEM.EnvelopeNames)

        try
            load(fullfile(User.SUBJECTS,OPTIONS.subjectpath,OPTIONS.BEM.EnvelopeNames{k}.TessFile),'Comment');
        catch % Maybe user is using command line call to function with absolute-referenced files OPTIONS.*.TessFile
            try
                OPTIONS.BEM.EnvelopeNames{k}.TessFile = [OPTIONS.BEM.EnvelopeNames{k}.TessFile,'.mat'];
                load(fullfile(User.SUBJECTS,OPTIONS.subjectpath,OPTIONS.BEM.EnvelopeNames{k}.TessFile),'Comment');
            catch
                cd(User.SUBJECTS)
                load(OPTIONS.BEM.EnvelopeNames{k}.TessFile,'Comment');
            end
        end

        Comment = strrep(Comment,' ','');
        % find surface in current tessellation file
        OPTIONS.BEM.EnvelopeNames{k}.SurfId = find(strcmpi(OPTIONS.BEM.EnvelopeNames{k}.TessName,Comment));
        IDs(k) = OPTIONS.BEM.EnvelopeNames{k}.SurfId;
        if isempty(OPTIONS.BEM.EnvelopeNames{k}.SurfId)
            errordlg(...
                sprintf('Surface %s was not found in file %s',...
                OPTIONS.BEM.EnvelopeNames{k}.TessName, OPTIONS.BEM.EnvelopeNames{k}.TessFile))
            return
        end

        % Load vertex locations
        try
            tmp = load(fullfile(User.SUBJECTS,OPTIONS.subjectpath,OPTIONS.BEM.EnvelopeNames{k}.TessFile),'Vertices');
        catch % Maybe user is using command line call to function with absolute-referenced files OPTIONS.*.TessFile
            tmp = load(OPTIONS.BEM.EnvelopeNames{k}.TessFile,'Vertices');
        end
        Vertices{k} = tmp.Vertices{OPTIONS.BEM.EnvelopeNames{k}.SurfId}';

        % Load faces
        try
            tmp = load(fullfile(User.SUBJECTS,OPTIONS.subjectpath,OPTIONS.BEM.EnvelopeNames{k}.TessFile),'Faces');
        catch% Maybe user is using command line call to function with absolute-referenced files OPTIONS.*.TessFile
            tmp = load(OPTIONS.BEM.EnvelopeNames{k}.TessFile,'Faces');
        end

        Faces(k) = tmp.Faces(OPTIONS.BEM.EnvelopeNames{k}.SurfId);
    end
    clear Comment tmp

    cd(User.STUDIES)

    %_________________________________________________________________________________________

    % Channel Parameters______________________________________________________________________
    if MEG
        R_meg1 = zeros(length(MEGndx),3);
        O_meg1 = R_meg1;
        R_meg2 = R_meg1;
        O_meg2 = O_meg1;
        flaggrad = zeros(length(MEGndx),1);% if = 1 - Flag to indicate there are some gradiometers here

        i = 0;
        for k = MEGndx
            i = i+1;
            R_meg1(i,:) = OPTIONS.Channel(k).Loc(:,1)';
            O_meg1(i,:) = OPTIONS.Channel(k).Orient(:,1)';

            if size(OPTIONS.Channel(k).Loc,2) == 2
                if sum(OPTIONS.Channel(k).Loc(:,1)-OPTIONS.Channel(k).Loc(:,2))~=0 % Gradiometer
                    R_meg2(i,:) = OPTIONS.Channel(k).Loc(:,2)';
                    O_meg2(i,:) = OPTIONS.Channel(k).Orient(:,2)';
                    flaggrad(k-min(MEGndx)+1) = 1;
                end
            end

        end
        O_meg1 = (O_meg1' * inorcol(O_meg1'))';
        if exist('O_meg2','var')
            O_meg2 = (O_meg2' * inorcol(O_meg2'))';
        end

        % Handle MEG reference channels if necessary
        if isfield(OPTIONS.Channel(MEGndx(1)),'irefsens')
            irefsens = OPTIONS.Channel(MEGndx(1)).irefsens;
        else
            irefsens = [];
        end

        if ~isempty(irefsens)
            flaggrad_REF = zeros(length(irefsens),1);% if = 1 - Flag to indicate there are some gradiometers here
            R_meg_REF = zeros(length(irefsens),3);
            O_meg_REF = R_meg_REF;
            R_meg_REF = R_meg_REF;
            O_meg_REF = R_meg_REF;

            if ~isempty(irefsens) & ~isempty(OPTIONS.Channel(MEGndx(1)).Gcoef) % Reference Channels are present
                if OPTIONS.Verbose, bst_message_window('Reference Channels have been detected. . .\n'), end
            end
            i = 0;
            for k = irefsens
                i = i+1;
                R_meg_REF(i,:) = OPTIONS.Channel(k).Loc(:,1)';
                O_meg_REF(i,:) = OPTIONS.Channel(k).Orient(:,1)';

                if size(OPTIONS.Channel(k).Loc,2) == 2
                    if sum(OPTIONS.Channel(k).Loc(:,1)-OPTIONS.Channel(k).Loc(:,2))~=0 % Reference Gradiometer
                        R_meg_REF2(i,:) = OPTIONS.Channel(k).Loc(:,2)';
                        O_meg_REF2(i,:) = OPTIONS.Channel(k).Orient(:,2)';
                        flaggrad_REF(k-min(irefsens)+1) = 1;
                    end
                end

            end

        else
            R_meg_REF = [];
            O_meg_REF = R_meg_REF;
            R_meg_REF = R_meg_REF;
            O_meg_REF = R_meg_REF;
        end

        MEGndx_orig = MEGndx;
    else
        R_meg1 = zeros(length(EEGndx),3); % Use dummy channel locations
        O_meg1 = R_meg1;
        R_meg2 = R_meg1;
        O_meg2 = O_meg1;
    end

    if EEG
        R_eeg = zeros(length(EEGndx),3);
        i = 0;
        for k = EEGndx
            i = i+1;
            R_eeg(i,:) = OPTIONS.Channel(k).Loc(:,1)';
        end
        if ~MEG
            flaggrad = [];
            irefsens = [];
        end

    else
        R_eeg = NaN * zeros(size(R_meg1)); % Dummy coordinates
    end

    %_________________________________________________________________________________________

    %Compute Transfer Matrices__________________________________________________________________

    BrainStorm.iscalp = IDs(end); % Index of the outermost surface (scalp, supposedly)

    eeg_answ = '';
    meg_answ = '';

    % Check whether some transfer-matrix files already exist for current study

    fn_eeg = sprintf('%s_eegxfer_%s_%s.mat',OPTIONS.rooot, OPTIONS.BEM.Basis,OPTIONS.BEM.Test);
    fn_meg = sprintf('%s_megxfer_%s_%s.mat',OPTIONS.rooot, OPTIONS.BEM.Basis,OPTIONS.BEM.Test);
    if MEG
        test = exist(fn_meg,'file');
    elseif EEG
        test = exist(fn_eeg,'file');
    else MEG& EEG
        test = exist(fn_eeg,'file') & exist(fn_meg,'file');
    end

    if OPTIONS.BEM.ForceXferComputation | ~test

        % Force (re)computation of transfer matrices, even if files exist in current study folder

        %    if OPTIONS.Verbose, bst_message_window('Computing the BEM Transfer Matrix (this may take a while)....'), end

        global nfv
        nfv = bem_xfer(R_eeg,R_meg1,O_meg1,Vertices,Faces,Param(1).Conductivity,Param(1).mode, ...
            Param(1).basis_opt,Param(1).test_opt,Param(1).ISA,fn_eeg,fn_meg,Param.Ntess_max,OPTIONS.Verbose);

        if ~isempty(find(flaggrad))
            if OPTIONS.Verbose, bst_message_window({'Gradiometers detected',...
                        'Computing corresponding Gain Matrix. . .'}), end

            %fn_meg_2 = fullfile(pwd,[OPTIONS.rooot,'_megxfer_2.mat']);
            fn_meg_2 = sprintf('%s_megxfer2_%s_%s.mat',OPTIONS.rooot, OPTIONS.BEM.Basis,OPTIONS.BEM.Test);
            bem_xfer(R_eeg,R_meg2,O_meg2,Vertices,Faces,Param(1).Conductivity,Param(1).mode, ...
                Param(1).basis_opt,Param(1).test_opt,Param(1).ISA,fn_eeg,fn_meg_2,Param.Ntess_max,0); % Verbose = 0
            if OPTIONS.Verbose, bst_message_window('Gradiometer Channel Gain Matrix is Completed.'), end
        end

        if ~isempty(irefsens) % Do the same for reference channels
            %fn_meg_REF = fullfile(pwd,[OPTIONS.rooot,'_megxfer_REF.mat']);
            fn_meg_REF = sprintf('%s_megREFxfer_%s_%s.mat',OPTIONS.rooot, OPTIONS.BEM.Basis,OPTIONS.BEM.Test);
            bem_xfer(R_eeg,R_meg_REF,O_meg_REF,Vertices,Faces,Param(1).Conductivity,Param(1).mode, ...
                Param(1).basis_opt,Param(1).test_opt,Param(1).ISA,fn_eeg,fn_meg_REF,Param.Ntess_max,0);% Verbose = 0

            if ~isempty(find(flaggrad_REF))

                if OPTIONS.Verbose, bst_message_window({'Reference Channels detected',...
                            'Computing corresponding Gain Matrix. . .'}), end

                %fn_meg_REF2 = fullfile(pwd,[OPTIONS.rooot,'_megxfer_REF2.mat']);
                fn_meg_REF2 = sprintf('%s_megREFxfer2_%s_%s.mat',OPTIONS.rooot, OPTIONS.BEM.Basis,OPTIONS.BEM.Test);

                bem_xfer(R_eeg,R_meg_REF2,O_meg_REF2,Vertices,Faces,Param(1).Conductivity,Param(1).mode, ...
                    Param(1).basis_opt,Param(1).test_opt,Param(1).ISA,fn_eeg,fn_meg_REF2,Param.Ntess_max,0);% Verbose = 0
                if OPTIONS.Verbose, bst_message_window('Reference Channel Gain Matrix is Completed.'), end
            end
        end
    end

    % Computation of TRANSFER MATRIX Completed ___________________________________________________

    %%%% THIS PART SPECIFIES PARAMETERS USED TO GENERATE THE 3-D GRID %%%%%%%%%%

    if OPTIONS.BEM.Interpolative%1%~exist(BEMGridFileName,'file')

        if OPTIONS.Verbose, bst_message_window('Computing BEM Interpolative Grid. . .'), end

        BEMGridFileName = [OPTIONS.rooot,'grid.mat'];

        % update tessellated envelope with surfaces possibly modified by
        % BEM_XFER (downsampling, alignment, embedding etc.)
        Vertices = {nfv(:).vertices};
        Faces = {nfv(:).faces};
        gridmaker(Vertices,Faces,BEMGridFileName,OPTIONS.Verbose);

        if OPTIONS.Verbose,
            bst_message_window('Computing BEM Interpolative Grid -> DONE'),

            % Visualization of surfaces + grid points
            for k = 1:length(Vertices)
                [hf,hs(k),hl] = view_surface('Head envelopes & a subset of BEM interpolative grid points',Faces{k},Vertices{k});
                view(90,0)
                delete(hl)
            end
            camlight
            rotate3d on

            set(hs(1),'FaceAlpha',.3,'edgealpha',.3,'edgecolor','none','facecolor','r')
            set(hs(2),'FaceAlpha',.2,'edgealpha',.2,'edgecolor','none','facecolor','g')
            set(hs(3),'FaceAlpha',.1,'edgealpha',.1,'edgecolor','none','facecolor','b')

            hold on

            load(BEMGridFileName)
            hgrid = scatter3(Rq_bemgrid(1:10:end,1),Rq_bemgrid(1:10:end,2),Rq_bemgrid(1:10:end,3),'.','filled');

        end

    else

        global GBEM_grid

        % Grid points are the locations of the distributed sources
        if isempty(OPTIONS.Cortex) % CBB (SB, 07-May-2004)| Should work also for volumic grid
            errordlg('Please select a cortical grid for computation of BEM vector fields')
            return
        end

        try
            load(OPTIONS.Cortex.FileName); % Load tessellation supporting the source locations and orientations
        catch
            Users = get_user_directory;
            load(fullfile(Users.SUBJECTS,OPTIONS.Cortex.FileName)); % Load tessellation supporting the source locations and orientations
        end

        BEMGridFileName.Loc = Vertices{OPTIONS.Cortex.iGrid}; clear Vertices

        if OPTIONS.ApplyGridOrient % Take cortex normals into account
            ptch = patch('Vertices',BEMGridFileName.Loc','Faces',Faces{OPTIONS.Cortex.iGrid},'Visible','off');
            set(get(ptch,'Parent'),'Visible','off')
            clear Faces
            BEMGridFileName.Orient = get(ptch,'VertexNormals')';
            delete(ptch);
            [nrm,BEMGridFileName.Orient] = colnorm(BEMGridFileName.Orient);
            % Now because some orientations may be ill-set to [0 0 0] with the get(*,'VertexNormals' command)
            % set these orientation to arbitrary [1 1 1]:
            izero = find(nrm == 0);clear nrm
            if ~isempty(izero)
                BEMGridFileName.Orient(:,izero) = repmat([1 1 1]'/norm([1 1 1]),1,length(izero));
            end
            clear izero
        else
            BEMGridFileName.Orient = [];
        end

        %if OPTIONS.Verbose, bst_message_window(sprintf('Loading BEM interpolative grid points from %s', BEMGridFileName)), end

    end

    % This part computes the gain matrices defined on precomputed grid------------------------------------------------------------

    % Assign file names where to store the gain matrices
    if MEG & ~EEG
        bem_xfer_mfname = {fn_meg};
        %BEMGaingridFname = [OPTIONS.rooot,'_meggain_grid.mat'];
        BEMGaingridFname = sprintf('%s_MEGGainGrid_%s_%s.mat',OPTIONS.rooot, OPTIONS.BEM.Basis,OPTIONS.BEM.Test);
        test = exist(BEMGaingridFname,'file');
    elseif EEG & ~ MEG
        bem_xfer_mfname = {fn_eeg};
        %BEMGaingridFname = [OPTIONS.rooot,'_eeggain_grid.mat'];
        BEMGaingridFname = sprintf('%s_EEGGainGrid_%s_%s.mat',OPTIONS.rooot, OPTIONS.BEM.Basis,OPTIONS.BEM.Test);
        test = exist(BEMGaingridFname,'file');
    elseif EEG & MEG
        bem_xfer_mfname = {fn_meg,fn_eeg};
        %     BEMGaingridFname.MEG = [OPTIONS.rooot,'_meggain_grid.mat'];
        %     BEMGaingridFname.EEG = [OPTIONS.rooot,'_eeggain_grid.mat'];
        BEMGaingridFname.MEG = sprintf('%s_MEGGainGrid_%s_%s.mat',OPTIONS.rooot, OPTIONS.BEM.Basis,OPTIONS.BEM.Test);
        BEMGaingridFname.EEG = sprintf('%s_MEGGainGrid_%s_%s.mat',OPTIONS.rooot, OPTIONS.BEM.Basis,OPTIONS.BEM.Test);
        test = exist(BEMGaingridFname.MEG,'file') & exist(BEMGaingridFname.EEG,'file');
    end

    if 1%OPTIONS.BEM.ForceXferComputation | ~test% Recompute gain matrices when transfer matrices have been recomputed just before

        if OPTIONS.Verbose
            if OPTIONS.BEM.Interpolative
                bst_message_window('Computing the BEM gain matrix for interpolative grid. . .'),
            else
                bst_message_window('Computing the BEM gain matrix for source grid. . .'),
            end
        end

        t0 = clock;

        if OPTIONS.Verbose,
            if MEG & EEG
                bst_message_window('for MEG and EEG channels. . .')
            elseif EEG
                bst_message_window('for EEG channels. . .')
            elseif MEG
                bst_message_window('for MEG channels. . .')
            end
        end

        if length(bem_xfer_mfname) == 1 % xor(MEG,EEG)
            bem_gain(BEMGridFileName,bem_xfer_mfname{1},Param(1).ISA,BEMGaingridFname, OPTIONS.Verbose);
        else
            % MEG gaingrid matrix
            bem_gain(BEMGridFileName,bem_xfer_mfname{1},Param(1).ISA,BEMGaingridFname.MEG, OPTIONS.Verbose);
            % EEG gaingrid matrix
            bem_gain(BEMGridFileName,bem_xfer_mfname{2},Param(1).ISA,BEMGaingridFname.EEG, OPTIONS.Verbose);
        end

        if OPTIONS.Verbose, bst_message_window('-> DONE'), end

        if ~isempty(find(flaggrad)) % Compute forward model on the second set of magnetometers from the gradiometers array

            bem_xfer_mfname = sprintf('%s_megxfer2_%s_%s.mat',OPTIONS.rooot, OPTIONS.BEM.Basis,OPTIONS.BEM.Test);
            bem_gaingrid_mfname_2 = sprintf('%s_MEGGainGrid2_%s_%s.mat',OPTIONS.rooot, OPTIONS.BEM.Basis,OPTIONS.BEM.Test);

            if OPTIONS.Verbose, bst_message_window('MEG - completing gradiometers. . .'), end

            bem_gain(BEMGridFileName,bem_xfer_mfname,Param(1).ISA,bem_gaingrid_mfname_2,OPTIONS.Verbose);

            if OPTIONS.Verbose, bst_message_window('-> DONE'), end

            if MEG & EEG
                G1 = load(BEMGaingridFname.MEG);
            else
                G1 = load(BEMGaingridFname);
            end
            G2 = load(bem_gaingrid_mfname_2);
            % Apply respective weights within the gradiodmeters
            meg_chans = [OPTIONS.Channel(find(flaggrad)).Weight];
            w1 = meg_chans(1:2:end);
            w2 = meg_chans(2:2:end);

            G1.GBEM_grid(find(flaggrad),:) = w1'*ones(1,size(G1.GBEM_grid,2)).*G1.GBEM_grid(find(flaggrad),:)...
                +  w2' * ones(1,size(G1.GBEM_grid,2)).*G2.GBEM_grid(find(flaggrad),:);
            clear G2

            % is there special reference channel considerations?
            if ~isempty(irefsens)

                % Forward model on all reference sensors
                bem_xfer_mfname_REF = sprintf('%s_megREFxfer_%s_%s.mat',OPTIONS.rooot, OPTIONS.BEM.Basis,OPTIONS.BEM.Test);
                bem_gaingrid_mfname_REF = sprintf('%s_MEGGainGrid_REF_%s_%s.mat',OPTIONS.rooot, OPTIONS.BEM.Basis,OPTIONS.BEM.Test);

                if OPTIONS.Verbose, bst_message_window('MEG reference channels. . .'), end
                bem_gain(BEMGridFileName,bem_xfer_mfname_REF,Param(1).ISA,bem_gaingrid_mfname_REF,OPTIONS.Verbose);
                if OPTIONS.Verbose, bst_message_window('-> DONE'), end

                if ~isempty(find(flaggrad_REF)) % Gradiometers are present in reference channels

                    bem_xfer_mfname_REF2 = sprintf('%s_megREFxfer2_%s_%s.mat',OPTIONS.rooot, OPTIONS.BEM.Basis,OPTIONS.BEM.Test);
                    bem_gaingrid_mfname_REF2 = sprintf('%s_MEGGainGrid2_REF_%s_%s.mat',OPTIONS.rooot, OPTIONS.BEM.Basis,OPTIONS.BEM.Test);

                    if OPTIONS.Verbose, bst_message_window('MEG reference channels / completing gradiometers. . .'), end

                    bem_gain(BEMGridFileName,bem_xfer_mfname_REF2,Param(1).ISA,bem_gaingrid_mfname_REF2,OPTIONS.Verbose);

                    if OPTIONS.Verbose, bst_message_window('-> DONE'), end

                    GR = load(bem_gaingrid_mfname_REF);
                    GR2 = load(bem_gaingrid_mfname_REF2);

                    meg_chans = [OPTIONS.Channel(find(flaggrad_REF)).Weight];
                    w1 = meg_chans(1:2:end);
                    w2 = meg_chans(2:2:end);

                    GR.GBEM_grid(find(flaggrad_REF),:) = w1'*ones(1,size(GR.GBEM_grid,2)).*GR.GBEM_grid(find(flaggrad_REF),:)...
                        +  w2' * ones(1,size(GR.GBEM_grid,2)).*GR2.GBEM_grid(find(flaggrad_REF),:);

                    %GR.GBEM_grid(find(flaggrad_REF),:) = GR.GBEM_grid(find(flaggrad_REF),:) - GR2.GBEM_grid(find(flaggrad_REF),:);
                    clear GR2;
                end

                % Apply nth-order gradient correction on good channels only
                %Weight by the current nth-order correction coefficients
                %G1.GBEM_grid = G1.GBEM_grid - Channel(MEGndx(1)).Gcoef(find(ChannelFlag(irefsens)>0),:)*GR.GBEM_grid;

                G1.GBEM_grid = G1.GBEM_grid - OPTIONS.Channel(MEGndx(1)).Comment*GR.GBEM_grid;

            end

            GBEM_grid = 1e-7*G1.GBEM_grid;

            if MEG & EEG
                save(BEMGaingridFname.MEG,'GBEM_grid','-append');
            else
                save(BEMGaingridFname,'GBEM_grid','-append');
            end

        end

        % Detect EEG reference - if none is specified in the .Comment or Type fields,
        % and if none was passed through OPTIONS.EEGRef
        % -> Apply average-referencing of the potentials by default.

        if EEG
            % EEG Reference Channel
            EEGREFndx = good_channel(OPTIONS.Channel,[],'EEG REF');

            if MEG & EEG
                load(BEMGaingridFname.EEG,'GBEM_grid')
            else
                load(BEMGaingridFname,'GBEM_grid')
            end

            if isempty(EEGREFndx)% AVERAGE REF
                GBEM_grid = GBEM_grid - repmat(mean(GBEM_grid),size(GBEM_grid,1),1);
            else
                GBEM_grid = GBEM_grid(setdiff(EEGndx,EEGREFndx)-EEGndx(1)+1,:) - repmat(GBEM_grid(EEGREFndx-EEGndx(1)+1,:),size(GBEM_grid(setdiff(EEGndx,EEGREFndx)-EEGndx(1)+1,:),1),1);
            end

            if MEG & EEG
                save(BEMGaingridFname.EEG,'GBEM_grid','-append')
                clear GBEM_grid
                %             % Now save the combined MEG/EEG gaingrid matrix in a single file
                %             MEGEEG_BEMGaingridFname = strrep(BEMGaingridFname.EEG,'eeg','meg_eeg');
                %             Gmeg = load(BEMGaingridFname.MEG,'GBEM_grid');
                %             GBEM_grid = NaN * zeros(length(OPTIONS.Channel),size(Gmeg.GBEM_grid,2));
                %             GBEM_grid(MEGndx,:) = Gmeg.GBEM_grid; clear Gmeg
                %             eeg = load(BEMGaingridFname.EEG);
                %
                %             save_fieldnames(eeg,MEGEEG_BEMGaingridFname);
                %
                %             GBEM_grid(setdiff(EEGndx,EEGREFndx),:) = eeg.GBEM_grid; clear eeg
                %
                %             save(MEGEEG_BEMGaingridFname,'GBEM_grid','-append')
                %
                %             BEMGaingridFname = MEGEEG_BEMGaingridFname;
            else
                save(BEMGaingridFname,'GBEM_grid','-append')
            end

        end

        telap_meg_interp = etime(clock,t0);

        if OPTIONS.Verbose, bst_message_window(sprintf('Completed in %3.1f seconds', telap_meg_interp),...
                'Computing the Gain Matrix for the Interpolative Grid Points -> DONE'), end
    else
        if MEG & EEG
            if OPTIONS.Verbose, bst_message_window(sprintf('Loading 3D grid gain matrix from %s', BEMGaingridFname.EEG)),end
        else
            if OPTIONS.Verbose, bst_message_window(sprintf('Loading 3D grid gain matrix from %s', BEMGaingridFname)),end
        end
    end

    function g = gterm_constant(r,rq)
    %gterm_constant
    % function g = gterm_constant(r,rq)

    %<autobegin> -------- 20-Nov-2002 14:06:02 ------------------------------
    % ---- Automatically Generated Comments Block using auto_comments -----------
    %
    % Alphabetical list of external functions (non-Matlab):
    %   toolbox\rownorm.m
    %<autoend> ---------- 20-Nov-2002 14:06:02 ------------------------------

    if size(rq,1)  == 1 % Just one dipole
        r_rq= [r(:,1)-rq(1),r(:,2)-rq(2),r(:,3)-rq(3)];
        n = rownorm(r_rq).^3;
        g = r_rq./[n,n,n];
    else
        g = zeros(size(r,1),3*size(rq,1));
        isrc = 1;
        for k = 1:size(rq,1)
           r_rq= [r(:,1)-rq(k,1),r(:,2)-rq(k,2),r(:,3)-rq(k,3)];
           n = rownorm(r_rq).^3;
           g(:,3*(isrc-1)+1: 3*isrc) = r_rq./[n,n,n];
           isrc = isrc + 1;
       end

    end

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