[FieldTrip] question on beamformers

[Bastiaansen, Marcel]

Dear all,

I have the following conceptual issue with using beamforming:

I have data from 2 conditions, A and B. Both contain a prestimulus 
interval and a poststimulus interval.

When plotting the TF representations of power, using a *relative* 
baseline (poststimulus interval / prestimulus interval), a strong 
difference emerges between the conditions in the beta frequency range. 
Especially if I plot the *difference between the conditions*, the beta 
effect is long-lasting, narrow-band and topographically focused. Ideal 
for beamforming, I'd say.

In setting up the beamformer analysis, I am faced with the following 
issue: the effect is most clearly present in the *relative* power 
changes, so I would want to compute spatial filters for both the pre and 
the post time intervals. My hunch would be to compute common spatial 
filters for both pre intervals (preA and preB), and common filters for 
both post intervals (postA and postB). Then, (after projecting the 
trials through the filters), compute the relative power again (pre/post 
for each condition). Does this approach make sense?

Next question is then, at which point in the analysis pipeline should 
one do the condition subtraction? (I'd want to do that because at the 
TFR level the effect comes out most clearly in the condition difference)


Best,
Marcel



-- 
dr. Marcel C.M. Bastiaansen.

Max Planck Institute for Psycholinguistics
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mail: marcel.bastiaansen at mpi.nl
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and

Donders Institute for Brain, Cognition and Behaviour
Centre for Cognitive Neuroimaging
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phone: + 31 24 3610 882
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mail: marcel.bastiaansen at fcdonders.ru.nl
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