quiestions about planar transformation

Mark Drakesmith mark.drakesmith at POSTGRAD.MANCHESTER.AC.UK
Thu Mar 4 23:02:58 CET 2010

Hi all

I am currently a little confused as to how to properly use megplanar and
combineplanar for my 4D 148 axial gadiometer data. I would ideally like
to use planar gradients as it will make subsequent ERF and frequency
analysis easier to interpret. My question is when should I combine the v
and h components?

Looking  at some previous posts it is suggested that you should combine
the gradients on an individual basis, but when I do this i get an almost
flat amplitude across all sensors which doesn't look right (see attached
image, 'bad_planar'). When i use combineplanar after averaging, the data
the data looks ok (good_planar). I get the same result doing both
megplanar and combineplanar on the averaged data.

I just want to double check that the 'bad' planar fields i am getting
are correct and not due to a bug in field trip. Intuitively, I would
expect combining the components on the individual level world be more
accurate as it would prevent fields from opposeingly orientated dipoles
cancelling out when averaged.

If it is the case that you should use combineplanar after trial
averaging, how do you go about combining after statistical tests? i.e.
when you have a p-value or a test statistic for the v and h components
for each sensor, how would you combine these together?

Any help or advice would be appreciated.

Many thanks



Mark Drakesmith
PhD Student

Neuroscience and Aphasia Research Unit (NARU)
University of Manchester

The aim of this list is to facilitate the discussion between users of the FieldTrip  toolbox, to share experiences and to discuss new ideas for MEG and EEG analysis. See also http://listserv.surfnet.nl/archives/fieldtrip.html and http://www.ru.nl/neuroimaging/fieldtrip.
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