[Mne_analysis] appropriate way to combine intrasubject dSPMs?

Dykstra, Andrew Andrew.Dykstra at med.uni-heidelberg.de
Thu Feb 7 06:12:37 EST 2013
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Thanks Martin.

I've tried A and something very similar to B, the only difference being 
I used the gain matrix from a single run (the first) instead of 
averaging the gain matrices across runs.  The solutions I obtained were 
different in two related ways: (1) the overall values were larger in B 
and (2) the activity meeting a certain dSPM threshold was more 
widespread.  I'll try averaging the gain matrices to see how much of a 
difference that makes in the computation of a single dSPM for the grand 
average.  I guess this will depend on the magnitude of the difference in 
head position.

Cheers,
Andy

-- 
Andrew R. Dykstra, PhD
Auditory Cognition Lab
Neurologie und Poliklinik
Universitätsklinikum Heidelberg
Im Neuenheimer Feld 400
69120 Heidelberg

"How small the cosmos.  How paltry and puny compared to human consciousness . . . to a single individual recollection." - Vladimir Nabokov

On 02/06/2013 05:45 PM, Martin Luessi wrote:
> 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
>>
>
>




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