[FieldTrip] Projection of simulated source activity to the surface EEG
Nina Omejc
nina.omejc at ijs.si
Thu May 18 23:33:03 CEST 2023
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
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