[FieldTrip] Projection of simulated source activity to the surface EEG

[Nina Omejc]

Dear Jan-Mathijs,

thank you very much for a quick and very comprehensive answer.

Based on some research I noticed that none of the papers I looked at, 
which were simulating the sources and projecting to the EEG, actually 
used volumetric atlases. Rather, all used surface based atlases, as you 
suggested, and probably for the same reason. I decided to follow your 
suggestion and first tried with the Desikan-Killiany Atlas based on 
freesurfer's fsaverage surface space. And since I've already started 
this issue, I wanted to ask you (it is the last time) regarding some 
questions and issues I've encountered.

i) Afaik, none of the atlases provide normals calculated up front. I 
calculated the normals for each dipole (center of mass of an atlas 
region) as a normalized, averaged normal of all normals that appear in 
one atlas region. There were as many normals as were triangles in a 
region and one normal was calculated as a cross product between two 
edges of a triangle (of a surface) and oriented outwards. Is that 
approach ok or is there nevertheless some standard way of doing it? 
Papers usually don't go in such details.

ii) I am slightly confused which type of surface should I take for the 
calculation of normals. Most likely that should be the original folded 
surface, and not modified pial, white or inflated, as are also the 
options? When I was looking at the results, they clearly (and logically) 
differ.

iii) could I still use volumetric atlas, where orientations of the 
cortical regions would be defined as commonly in the surface based atlas 
(perpendicular to the surface), even though it would be volumes instead 
of surfaces? In a sense that I take surface areas of the cortical 
volumes to calculate the normal, and then use that normal as orientation 
of the dipole, that is in a volume, not necessarily on a surface of a 
volume. Then, the subcortical regions could either be excluded in the 
projection (and only had influence through time series couplings in the 
simulation), or the orientation could be defined based on structural 
connectivity (e.g. sampling from probabilistic distribution determined 
based on structural connectivity)?  Thank you for any comment on that topic.


I hope I'm not asking too much. Thank you again for your time!

Best,

Nina