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