evoked field amplitude

Belluscio, Beth (NIH/NINDS) [E] BelluscB at NINDS.NIH.GOV
Thu Jun 3 18:00:29 CEST 2010

I want to calculate the strength of the evoked fields in response to a series of stimuli.  Initially, I tried doing this with the averaged data at sensors demonstrating an evoked field following the stimulus.  However, I noticed that during the course of the response, the dipole of the source seemed to shift slightly, and that this dipole was not consistently oriented following successive stimuli.  So I was concerned that by measuring only the response at the sensor, I would obtain a false sense of change in the underlying response.
  I decided to try looking at the response with the data converted into the corresponding planar gradient (I use a CTF MEG).  However, I was unable to find a way to obtain values of the field within an ROI with this data set.  Does anyone know how to do this?
  Lastly, it seems it might be best to compute the source and then evaluate the strength of the signal from an ROI in source-space.  What are the pros/cons of using sensor space vs. source space to measure the amplitude of a response?  Is there a convention within the field for the methodology for doing this?

Beth Belluscio, MD-PhD
Clinical Fellow
Human Motor Control Section
National Institute of  Neurological Disorders and Stroke

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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