evoked field amplitude

Stanley Klein sklein at BERKELEY.EDU
Thu Jun 3 19:02:45 CEST 2010

Beth, you ask about going to source space.
It's my understanding that people don't yet know how to reliably go from
sensor space to source space. The problem is that neighboring cortical areas
are often wired together and will fire together and since these neighboring
areas will likely have different folding patterns and different time
functions the inverse problem of going to sources has problems with both EEG
and MEG because of how close these neighbors are to each other. There are
two cases where there seem to be solutions, one is occipital cortex where
V1, V2, V3, V3a have retinotopic organization that can be identified with
fMRI/MRI. The idea here is that with tiny patches in a dartboard layout one
can overwhelm the above ambiguity by making use of the topographic layout of
cortex. Another area is in somatosensory and motor areas where the
homunculus mapping also allows neighboring areas to be separated. Luckily
fMRI can also identifying other regions that could be used in helping one go
from sensors to sources. However, the above manipulations aren't easily
carried out. In addition, there are many researchers who believe that ICA
can help do the job.

In summary it is healthy to be skeptical of some of the claims of success in
going from sensors to sources.

On Thu, Jun 3, 2010 at 9:00 AM, Belluscio, Beth (NIH/NINDS) [E] <
BelluscB at ninds.nih.gov> wrote:

>  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
> 301-402-3495
> ----------------------------------
> 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.
> http://listserv.surfnet.nl/archives/fieldtrip.html
> http://www.ru.nl/fcdonders/fieldtrip/

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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