[FieldTrip] mne vs lcmv methods

Belluscio, Beth (NIH/NINDS) [E] BelluscB at ninds.nih.gov
Mon Sep 26 21:17:04 CEST 2011


Hi fellow Fieldtripers-
  I am learning about the pros & cons of different methods for source localization.  For some of my data sets, I want to localize evoked activity (ERFs).  In one of them, I anticipated a single unilateral primary site of activation and MNE worked very nicely for this.  In the second, I am using auditory stimuli and therefore anticipated bilaterally synchronous activity.  As was to be expected, (I think) MNE gave a single site of activity near the midline.  I tried a parallel approach, but specifying lcmv as the method for ft_sourceanalysis as follows:


cfg = [];

cfg.method = 'lcmv';

cfg.grid = leadfield_02_stim1;

cfg.vol = vol;

cfg.lambda = 1e8;

cfg.checksize = inf;   %I've been inserting this and the next line because I've gotten error messages about the size.  This didn't happen before, but these lines avoid the problem

cfg.checkconfig ='loose';

source3CB24_02_stim1 = ft_sourceanalysis(cfg, tlkCB24_02_stim1)

The output has a time dimension 1x601, and a pos dimension of 8196x3 dimension, but the avg.pow is only 1x8196.  I interpret this to mean that the analysis averaged the power over some time interval instead of computing the power in each leadfield position for each time point.  When I use the mne method, the output is clearly power for each time point, and produces a beautiful movie, as indicated in the tutorial.

My questions about the lcmv technique are:

1.       Is there a fundamental difference between the lcmv approach and the mne approach whereby it cannot compute the power at each time point, but only gives a single average value for a time period?

2.       If so, how do I specify the time interval of interest (ie that containing the main peak of the ERF) in ft_sourceanalysis?

3.       I thought one advantage of either MNE or LCMV was that I could indicate my a priori hypothesis of the anticipated localization of the source activity.  Do I do this with cfg.grid?  Is there a way to indicate the number of anticipated dipoles?

Thanks in advance for your help-
Beth.

Beth Belluscio MD-PhD
Clinical Fellow
Human Motor Control Section
NINDS, NIH
301-402-3495

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