[FieldTrip] Electrode position effects on leadfield values

[Maris Skujevskis]

Dear Fieldtrip users,

It is common knowledge that due to noise/measurement error, it is hard, if
not impossible to match Polhemus digitized electrode locations with the MR
head shape precisely. Even after manual and iterative alignment some
electrodes remain "hanging" in the air above the head surface, while some
others get buried below the head surface. Here is an illustration of what I
mean: https://goo.gl/aSm7YU

I am wondering what effect this type of electrode misplacement has on the
values in the leadfield matrix as calculated by ft_prepare_leadfield? How
does fieldtrip deal with electrodes placed in such a way? It seems that
there is some magic involved there, since in theory the lead field matrix
values for the electrodes not touching the surface of the head should be
zero (which is not the case in the lead field matrix produced by
ft_prepare_leadfield).

I am currently testing how moving a "floating" and a "buried" electrode to
its nearest point on the scalp affects the leadfield matrix values. I do
not have any definite answers yet, but I did want to check in with some
more experienced users out there before spending too much time on this (and
possibly reinventing the wheel, so to speak).


Thanks!
Maris
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