[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
-------------- next part --------------
A non-text attachment was scrubbed...
Name: boxplot_coord_20110407.jpg
Type: image/jpeg
Size: 17510 bytes
Desc: not available
URL: <http://mailman.science.ru.nl/pipermail/fieldtrip/attachments/20110513/320398d2/attachment.jpg>
-------------- next part --------------
A non-text attachment was scrubbed...
Name: dist_electrodes.png
Type: image/png
Size: 8323 bytes
Desc: not available
URL: <http://mailman.science.ru.nl/pipermail/fieldtrip/attachments/20110513/320398d2/attachment.png>
-------------- next part --------------
A non-text attachment was scrubbed...
Name: boxplot_20110512_electrode_locations_matched.png
Type: image/png
Size: 6945 bytes
Desc: not available
URL: <http://mailman.science.ru.nl/pipermail/fieldtrip/attachments/20110513/320398d2/attachment-0001.png>
-------------- next part --------------
A non-text attachment was scrubbed...
Name: boxplot_20110512_coordinates.png
Type: image/png
Size: 8841 bytes
Desc: not available
URL: <http://mailman.science.ru.nl/pipermail/fieldtrip/attachments/20110513/320398d2/attachment-0002.png>
More information about the fieldtrip
mailing list