Dear FS team,

I have structural data from a training study involving different tDCS groups. I would like to define ROIs (masks) in MNI space corresponding to the approximate cortical area underlying the tDCS electrodes, which were placed on F3&F4 for one group and on P3&P4 in another group (10-20 system positions). I would therefore need to find the approximate centre coordinates, in MNI space, for the F3, F4, P3, P4 electrode positions; however, this is proving challenging.

Other papers that have done this (1, 2) have obtained coordinates for their electrodes (other positions than mine, unfortunately) using various types of converters, such as Brainstorm, Comet and the Muenster T2T Converter. However, it's proving very time-costly to get any of these to actually produce MNI coordinates at the cortical (not scalp) level for the electrode positions I mentioned. I suppose a scalp-to-cortex projection would have to be done, but I'm still working out how to do it. Clearly many approximations are involved so these won't be very accurate masks, but I want to at least try to define them first.

Also, assuming I find the centre coordinates, how would a surface-based mask, of a certain cortical depth and centred around those coordinates, be created in Freesurfer?

Many thanks!