Dear SPM-M/EEG experts,
I have a couple of questions concerning source waveform extraction. I looked
at the source waveforms in 2 different ways: (1) using the 'mip' button in
the source localization gui and (2) using the function:
spm_eeg_inv_extract().
When trying to extract waveforms from auditory areas (e.g.: [-40 -18 7]) I
noticed two things. First, field intensities that are displayed by the gui
are flipped (i.e. positive = negative) with respect to the images displayed
when I use spm_eeg_inv_extract(). Do you have any idea why that is?
Interestingly, if I do:
D.inv{val}.source.XYZ = [-41 -18 8; -41 -18 8];
D.inv{val}.source.rad = 10;
D.inv{val}.source.label = {'1_A1'; '2_A1'};
D.inv{val}.source.fname = ['Source_3'];
Ds = spm_eeg_inv_extract(D);
source waveforms of the 2 sources are also flipped (i.e. positive =
negative), even though they should be extracted from the same source. (This
happens only for these coordinates and radius)
Second, even though waveforms look similar when I use slightly different A1
MNI coordinates (i.e. [-40 -18 7] & [-41 -18 8]), field intensities change
dramatically (e.g. from 0.05 to 30). I am hence wondering what field
intensities really mean and why they change so dramatically if I move only
little on the mesh?
Lastly, I would be great if you could advise what would be the most
reasonable approach to extract source waveforms from the primary auditory
cortex? I would like to average across all vertices that lie within A1. Is
there a possibility to use an image mask to do so?
Looking forward to your suggestions.
Anette
