[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
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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