[FieldTrip] Electrode position effects on leadfield values
Schoffelen, J.M. (Jan Mathijs)
jan.schoffelen at donders.ru.nl
Sun May 1 18:16:45 CEST 2016
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.
Indeed this is well known.
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
Looks quite good to me!
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).
Fieldtrip pulls (or pushes) the electrodes to the head surface, to ensure that the computations make sense. If you want to know the details, grep for project_elec (which is the function that does this).
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).
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