Dear SPM (and M/EEG) experts,
I came across a number of comprehension questions regarding the 3D
source reconstruction of EEG data using SPM5. I want to use SPM to
reconstruct induced EEG Power within specified time-frequency windows
for two different conditions. Therefore, I feed epoched single trial
data(say 80 Trials for Cond1, 80 Trials for Cond2) into source
reconstruction. After meshing coreg, forward model computation and model
inversion, I can define TF-windows, chose between induced/evoked
activity, and finally get niftis for further analysis - everything works
fine and looks good.
However, it is not fully clear to me what the resulting solutions
actually depict. For example, when I'm interested in the sources of
say, induced alpha 800-1200 ms after stim onset, I can define this
contrast of interest only after all model inversion, ReML estimation and
so on is already completed. Critically, shouldn't such contrasts of
interest be specified before the model is actually inverted? (Given that
the solution should specifically describe activity in this TF window).
Or, put differently, what data is actually used for the (generic?) model
inversion? Does it just somehow use all data, including all trials,
conditions, samples, prestimulus interval and so on? Or, how can a
single model inversion later account for evoked as well as for induced
activity in full TF space, seperately for different conditions? In the
MIP window, only the inverse solution's average (i.e. evoked) response
is depicted. How can this inverse solution then contain (or preserve)
induced activity, which I can define later on? I realize that when I
select an induced contrast (after inversion), the program appears to
work through each single trial (again?). What happens in this step? Is
the model re-estimated? Or does SPM just use the initial inversion
solution (which is somehow based on the raw signal - or on the average
signal only?) to account for the specified TF activity in the single
trials? Finally, does this procedure in fact image induced power only
(i.e. total power - evoked power), or does it actually image total
power, or anything completely different?
Sorry for so many questions but manual and mailing list remain
relatively silent about these points. All my questions may be subsumed
under:
Can I really get the specific sources of e.g., induced alpha power
800-1200 ms after stim onset, using the source reconstruction currently
implemented in SPM?
best regards
Bernhard Spitzer
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