evoked field amplitude

[Nathan Weisz]

> following the stimulus.  However, I noticed that during the course of the response, the dipole of the source seemed to shift slightly,
>

it really depends what you mean by "shift slightly". small shifts shouldn't be a big deal (i hesitate to give a rule of thumb). your dipole doesn't just capture activity from that one spot. of course if the movement is huge then placing a single dipole somewhere to represent that activity does not make sense.

> 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.
>
the standard BESA approach e.g. would be to use a "regional source" i.e. a dipole with 3 orthogonal orientations. you can then calculate the vector magnitude (pythagoras). that should take care of a "rotating" dipole.

this can also be done within fieldtrip if you don't have BESA.
> 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?
>
pro:
- in case of "simple" activations (e.g. sensory evoked ERPs) you can reduce the information from >100 sensors to e.g. 2 sources. the dipole positions could also be standardized across subjects which may increase your power later when doing statistics (sensor positions are impossible to standardize without using offline tricks)
- of course neighbouring sources capture almost identical activity (which is good regarding you issue of "slightly shifting" source; see above), however the mixing of diverse activities is far worse on a sensor level!

cons:
- some arbitrariness of placing sources in "cognitive" experiments. unless you have very good prior information where to expect activity to come from, doing sensor analysis first followed by some distributed sources solution seems more advisable. you could then still look at time courses of ROIs defined by your statistical contrast.

in the end, there is no general cookbook-recipe and you should decide based on your experiment. the best situation is when your sensor and source data give converging results :-)

best,
nathan


>
>
>
>
> 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.
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> http://www.ru.nl/fcdonders/fieldtrip/
>


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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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