[FieldTrip] Power from LCMV method ?

Schoffelen, J.M. (Jan Mathijs) jan.schoffelen at donders.ru.nl
Thu Nov 26 17:17:02 CET 2020


Dear Christodoulos,

The ‘pow’ field is obtained by sandwiching the sensor level covariance between the spatial filter, i.e.   w’ * C * w.
The ‘mom’ field is obtained by left multiplication of the spatial filter with  the sensor level average,  w’ * avg.
Computing the variance from the w’*avg will indeed yield something that is different from the ‘pow’.

This is because the sensor-level covariance is typically the average of the single trial covariances, and not the covariance of the trial average.

Best wishes,
Jan-Mathijs

PS: the ’noise’ is typically obtained by sandwiching a scaled identity matrix between the spatial filters, i.e. w’ * Identitymatrix * w * <a number>, where the <a number> is usually the smallest singular value of the matrix. I would take this with a grain of salt, since the noise is rarely diagonal, nor does the smallest singular value accurately reflect the noise level.



On 26 Nov 2020, at 16:15, Christodoylos Karittevlis <christodoylos_k at hotmail.com<mailto:christodoylos_k at hotmail.com>> wrote:

Dear Fieldtrip community,

I have one question. I am using the beamformer LCMV method for source localization of simulated data (1 dipole in a sphere).
I am trying to figure out how the source.avg.power is estimated from the function ft_sourceanalysis. Can anyone give me some explanation on that or maybe some directions where I can find more details on the estimation of power/noise using the function ft_sourceanalysis.
I tried to estimate it from the source.avg.mom but I could not really produce the power.

Thank you in advance.

Kind regards,
Christodoulos Karittevlis
PhD Candidate, ESR11- iCONN network

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