[FieldTrip] Sharp 60 Hz peak (in Europe!) in wPLI and Granger spectra

[Eelke Spaak]

Fellow FieldTrippers,

I have identified two points of interest in the brain, between which I
want to compute connectivity metrics. One is in early visual cortex,
the other is in right temporal lobe, somewhat medial (compatible with
hippocampus, but exact interpretation not relevant right now). I
reconstructed activity for these grid points using LCMV beamformer (on
MEG data) and computed source-level Fourier spectra (taper = 'dpss',
tapsmofrq = 3) after applying a band-stop filter around 50 Hz and
harmonics. Using ft_connectivityanalysis, I computed both debiased
wPLI and Granger causality between the two points of interest.

In both spectra, I see a clear peak at 60 Hz in the grand average
across 36 subjects, which is also there in the majority of individual
subjects (though very strong only in 2/36). Plots are attached. Now
this would be exciting news if indeed it turns out to be a true highly
band-limited gamma effect!

But of course I suspect that this peak could very well be artifactual.
I can't think of any artifact source I may have missed that would
cause this, though. The projector refresh rate during the experiment
was set to 120 Hz, and not 60. (Also note this is European data so AC
frequency is 50 Hz and not 60, hence the band stop I mentioned
earlier.)

Does anyone have any idea what might be causing this sharp peak? (A
genuine effect after all?)

Thanks,
Eelke
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