Dear list,
I've found some strange output when using spm_eeg_img2maps for plotting
thresholded spm scalp-time EEG results, and wondered if anyone has a
suggestion for how to fix this?
I computed group-level spms on EEG data at the sensor level, and then saved
the thresholded spm as an image. I then used spm_eeg_img2maps to plot this
F image as a scalp topography across time.
However, using img2maps, the resulting topographies are distorted and don't
really match the raw data topography. If I use the "results" window to
overlay my spm results on the mask.img, it plots the largest F value is the
same location where the raw EEG difference is maximal. However, using
img2maps to plot the same F image, the F topography is different, even
though I use the same raw data file for the channel locations. The
topographies are most distorted around the edges on the scalp, where
essentially everything seems to be wiped out. I've attached a document with
examples for comparison.
I've gone through spm_eeg_img2maps, and I think the problem stems from the
way this function transforms the image pixel matrix into an interpolated
scalp map. It seems to downsample the matrix into a channel vector by
assigning the nearest pixel value to a channel (on line 93), and then uses
spm_eeg_plotScalpData to interpolate those channels to create a scalp map.
However, because I have thresholded the spm map to only include F values
above a certain size and these are only sampled by 70 channels (quite
typical channel number for EEG), the resulting interpolation doesn't work
very well and the topography is really patchy and weirdly shaped, and some
large clusters around the edges are almost absent because they were not
directly under an electrode location. In fact, it looks like the electrodes
around the edges are actually masked out of the plot.
Do you think it might be possible to improve plotting with img2maps
somehow, for example by increasing the number of pixels that are used for
interpolation, or by widening the mask to include electrodes on the edges?
Any input would be very much appreciated!
Thanks for your time,
Zara
|