[FieldTrip] Beamformer: different length of baseline and post baseline interval

[Johanna Zumer]

Dear Anna,

Ideally for the common filter, you want the same amount of data T(s) per
condition, where T = N x tw (and N is number of trials and tw is timewindow
length).  In your case, if the baselines for each conditions can be
combined into one general baseline, and if you happen to have 100 trials
per condition, then T_baseline = 3 x 100 x 0.5s = 150s.  If you then use
1.5s length post-baseline, then T_each_condition = 100 x 1.5s = 150s, so
you now have equal T for each condition for the common filter.

However, in order to have an equal effect of tapers and edge-effects on the
different conditions, you should use equal time window lengths in
freqanalysis.  Thus it would be better to split your post-baseline data
into 3 segments of 500ms each before calling ft_freqanalysis, which again
gives T = 100 x 0.5 x 3 = 150s.

Cheers,
Johanna

2012/6/29 Anna Wilsch <wilsch at cbs.mpg.de>

>
> Dear Fieldtrippers,
>
> I'm trying to beamform my MEG data by building a common filter including
> three conditions and a baseline for each condition. The baseline intervals
> have a duration of 500 ms. I was wondering if it is ok if the post-baseline
> data are longer than that (1000 - 2000 ms). Does it have any negative
> impact on the cross-spectral-density matrix and/or the common filter? Would
> that still be a valid operation to do or is it necessary that baseline and
> post baseline data have the same length?
> Thank you for your comments.
>
> Cheers,
> Anna
>
>
> Anna Wilsch, Dipl.-Psych.
> Auditory Cognition Research Group
> Max Planck Institute for Human Cognitive and Brain Sciences
> Stephanstr. 1a - Leipzig, Germany
> (p) +49 (0)341 9940 2641
> (e) wilsch at cbs.mpg.de
>
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