[Mne_analysis] appropriate way to combine intrasubject dSPMs?

[Martin Luessi]

Hi Andy,

It seems to me that from a theoretic point of view you are right; 
averaging the dSPMs is incorrect due to noise normalization. However, 
that being said, I recently did a test with data that has 15 runs and 
~180 epochs/run:

A) Compute forward solutions, noise cov, inverse operator, dSPM for each 
run, average the dSPMs (what you are asking).

B) Create a single evoked response by averaging all epochs from all 
runs, noise cov using all epochs, average the forward solutions across 
runs, compute a single inverse operator, compute dSPM.

At least qualitatively the solutions obtained using A and B are almost 
identical for the data I have. Maybe in situations where you have 
significantly different head positions between runs it would be better 
to use A, but as you said, technically it is incorrect (unless the same 
noise cov and fwd operator are used for each run).

I hope this helps,

Martin

On 02/06/13 05:41, Dykstra, Andrew wrote:
> Hi all,
>
> What is the most appropriate way to combine intrasubject dSPMs, e.g.
> multiple runs of the same task within the same subject in which I cannot
> assume that the gain and noise covariance matrices are equivalent across
> runs.  Is it as simple as averaging the resulting dSPMs from each run.
> This would seem to make sense for the raw current estimates, but I'm
> unclear on the noise-normalized estimates.
>
> Thanks in advance,
> Andy
>


-- 
Martin Luessi, Ph.D.

Research Fellow

Department of Radiology
Athinoula A. Martinos Center for Biomedical Imaging
Massachusetts General Hospital
Harvard Medical School
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Charlestown, MA 02129

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