[FieldTrip] Common beamforming filters for different groups?

Johanna Zumer johanna.zumer at donders.ru.nl
Fri Jul 6 14:36:04 CEST 2012

Dear Ulrich,

Only if the leadfields are the same, can you use the common filter.

If you have EEG data with standard electrode positions based on the cap
layout (rather than measured electrode positions) and using the standard
MNI MRI rather than subject specific MRI, then your leadfield will be the
same for all subjects.  Then for the covariance computation you might need
to consider some normalization (z-transform) of the data (using some
control window) so that differences in sensor values due to e.g.
sensitivity of electrodes across subjects, won't bias the result.

If you have MEG data, then the different head positions in the helmet will
mean different lead fields.  The only ways around that might be: if you do
something similar to correcting for head movement within a single session
(something like ft_headmovement but as far as I know is not implemented for
this multi-subject /session case) or if you ensured similar head position
between subject-pairs using ft_realtime_headlocalizer at the time of
acquisition; and in combination with using the MNI MRI rather than
individual subject MRI.

Perhaps someone else on the list can comment as to which is better, if the
common filter is a possibility? A common filter using sub-optimal
reconstruction for each subject if based on non-subject-specific MRI etc,
versus optimal reconstruction per subject but then issue of not a common

Another thing to do is to use the subject-specific inverse filter and do a
contrast within that subject (active versus baseline) and take this
contrast to the comparison at the paired-subjects group level. However, not
all experiments have the possibility of within-subject


2012/7/6 Pomper, Ulrich <Ulrich.Pomper at charite.de>

> Dear list members,
> I want to compare source data between two different groups (patients and
> controls). Each patient has an age and gender-matched control.
> Is it correct to compute a common filter for each pair of subjects? The
> TF-topographies do look different within each pair, so I would assume that
> the sources are somewhat different. However, as I'm going to statistically
> compare the two groups it would be nice to have common filters, as
> otherwise any finding could be due to different filters as opposed to
> different sources.
> Any help with this issue is greatly aprreciated!
> Cheers, Ulrich
> _______________________________________________
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> fieldtrip at donders.ru.nl
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