DICS, complex filter coeffcients and Virtual Electrodes

[Michael Wibral]

Dear Fieldtrippers,

I was wondering about the correct way to get SAM-like virtual electrodes using DICS. I naively tried to compute some virtual electrodes using the DICS filter coeffcients on the raw timecourses, but that obviously doesn't work because DICS filter coefficients are complex (at least in Fieldtrip 20081208 ?). Next thing I thought about was to FFT the single trial, filter it and then inverse FFT it, because the DICS filters are supposed to work in the frequency domain. But that shouldn't work either, because we have iFFT(filter(FFT(trial))) = iFFT(FFT(filter(trial))) = filter(trial), because all operation are linear.

Next thing I am puzzled about is that the Filters (A) times the leadfield matrix (L) should be the identity matrix: AL=1 (see the Gross, 2001, PNAS paper). If A is complex - as it is in Fieldtrip -, then we would also have a complex leadfield, which seems odd to me.

Any suggestion would be appreciated,
Michael


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