[FieldTrip] Systematic electrode displacement

Paul Czienskowski paul_c at gmx.de
Fri May 13 11:27:01 CEST 2011

Dear fieldtrippers,

I've got an issue with electrode placement and I hope that someone has 
any idea how to tackle those issues.

For research on my diploma thesis I am fitting simulated data with the 
realistic brain shape I used to simulate the data as well as a icbm 
model (BEM models). I used the electrode system for the icbm152 brain 
provided by Robert Oostenveld and did some manual correction to the 
realistic brains (which were normalised to icbm brain) with the 
ft_electroderealign function.
I simulated the data in a position near the auditory cortex, since this 
is the location where we assume our real ERP.
In my fitted data I encountered strange systematic errors (especially 
some kind of bias on the z-axis), as seen on the 
boxplot_coord_20110407.png file (note that it's both age groups: 1 for 
old and 2 for young, scale is meters). While investigating on the origin 
of this bias, I made a box plot of the distance of the electrodes in the 
icbm model and the electrodes in each of the realistic models 
(dist_electrodes.png). Obviously there already is an bias in the 
electrodes placement. First I assumed this was caused my the manual 
correction of the electrodes I performed. To check this out, I fitted 
the very data with the manually corrected electrodes on the icbm brain. 
Still I found that there was a bias, mainly on the z-axis and - what 
astonished me even more, there was still a bias in the electrodes. This 
is observable in the boxplot_20110512_electrode_locations_matched.png 
(sorry for the German captions), x-Axis is coordinate and y-axis is the 
deviation of the electrode's positions.
Furthermore I simulated and fitted a position near the vertex (though 
still in the brain ;) ). There was not so much of a bias there, 
therefore I assume it's mainly the lateral and maybe posterior electrodes
causing the bias, not that much the superior.

Due to those results I assumed that it's got something to do with the 
automatic projection of the electrodes on the head surface. I had no 
time yet to compare those electrode positions manually (or better 
visually), but this is definitely on my agenda. I wondered, if anyone 
had an idea, how to improve this, for electrode deviations up to ~10 cm 
is way to much, asking me. I already checked if there maybe was any 
confusion in the order of the electrodes, but I just checked the labels 
and this does not support it.

Thanks in advance and best regards,
Paul Czienskowski

Paul Czienskowski
Björnsonstr. 25
12163 Berlin

Tel.: (+49)(0)30/221609359
Handy: (+49)(0)1788378772
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