[FieldTrip] Normalization of beamformer leadfields

[Klink-3, N.E.C. van]

Dear all,

I want to do SAM beamformer source localization on single trial EEG data. I would like to normalize the leadfields to correct for depth, like mentioned in the lmcv beamformer tutorial: (http://www.fieldtriptoolbox.org/tutorial/beamformer_lcmv)

cfg                  = [];
cfg.elec             = hdr.elec;  % electrode distances
cfg.headmodel        = vol;   % volume conduction headmodel
cfg.grid             = grid;  % normalized grid positions
cfg.channel          = {'EEG'};
cfg.normalize        = 'yes'; % to remove depth bias (Q in eq. 27 of van Veen et al, 1997).
lf                   = ft_prepare_leadfield(cfg);

However when I look what happens with cfg.normalize='yes', the following script is used in ft_compute_leadfield, from line 570:

  case 'yes'
    for ii=1:Ndipoles
      tmplf = lf(:, (3*ii-2):(3*ii));
      if normalizeparam==0.5
        % normalize the leadfield by the Frobenius norm of the matrix
        % this is the same as below in case normalizeparam is 0.5
        nrm = norm(tmplf, 'fro');
      else
        % normalize the leadfield by sum of squares of the elements of the leadfield matrix to the power "normalizeparam"
        % this is the same as the Frobenius norm if normalizeparam is 0.5
        nrm = sum(tmplf(:).^2)^normalizeparam;
      end
      if nrm>0
        tmplf = tmplf ./ nrm;
      end
      lf(:, (3*ii-2):(3*ii)) = tmplf;
    end

This seems to me as independent of the dipole location, and does not use an estimate of the noise spectrum as in Eq 27 of van Veen et al 1997.
DICS beamformer has the option to estimate the noise spectrum with 'projectnoise', but SAM beamformer does not have that option. SAM does something with noise and a lambda, which is noise regularization I guess (beamformer_sam from line 102). I use Fieldtrip 20170212.

My main question: how do I correct the leadfields for depth bias?

Thanks in advance,
Nicole

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