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

[Eelke Spaak]

Thanks a lot JM, Lasha, Antoine, Tineke, & Mats for all the fast responses!

Just to be clear: I agree that it's extremely unlikely that the peak
reflects an endogenous rhythm.

@Antoine: the projector is supposed to be very high-end (vPixx
Propixx) and I've used it in the past at 1440 Hz, which worked well.
So I would assume it's not a matter of mismatch between the GPU
refresh rate and the projector's. But this is worth checking with a
photodiode. Also I guess it's possible that I forgot to check the
computer's refresh rate on some of the recording sessions and that it
was actually 60 Hz instead of 120.

For now I think I will deal with this with a notch filter.

@Mats: the data were recorded between April and June of this year (2018).

Cheers,
Eelke


On Wed, 7 Nov 2018 at 17:23, Es, M.W.J. van (Mats)
<M.vanEs at donders.ru.nl> wrote:
>
> Hi Eelke,
>
> I actually had the exact same issue with one of my pilot datasets, recorded in February this year. It was recorded from the same MEG system in Nijmegen. I only found out a while later, so it wasn't possible to check if anything in the setup had changed. I assumed someone changed the refresh rate to 60 Hz and forgot to set it to the default 120 Hz afterwards, but I can't be certain. I do remember thinking whether it could be caused by the power shortages in Eastern Europe that were going on around that time, having heard that this affected internal circuitry of electrical devices. Not sure if it could cause this though. When were your data recorded?
> In the end I worked around this by putting a dft filter on 60 Hz, which worked reasonably well.
>
> Hope this mystery gets solved at some point!
> Cheers,
> Mats
>
> -----Original Message-----
> From: Eelke Spaak <e.spaak at donders.ru.nl>
> Sent: maandag 5 november 2018 17:21
> To: FieldTrip discussion list <fieldtrip at science.ru.nl>
> Subject: [FieldTrip] Sharp 60 Hz peak (in Europe!) in wPLI and Granger spectra
>
> 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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