[FieldTrip] psd for brief segments

[mubeen afzal]

Thanks very much Stephen for your input. My understanding for many of the clinical outcome studies when they talk about 'peak paroxysm frequency' they look at the slow wave frequency. However you are probably right, frequency analysis doesnt make sense if there is one cycle spike/wave only. I think some of the quantitative studies leave out brief fragment in the method sections (This is an excellent recent paper on quantitative analysis - Ivan C. Zibrandtsen, Jonas M. Nielsen, Troels W. Kjaer, Quantitative characteristics of spike-wave paroxysms in genetic generalized epilepsy, Clinical Neurophysiology)

I might try out what you suggested in terms of electrode comparison of power spectrum.

Could you also please direct me on how to plot multi-channel powerspectrum in field trip once we have the freqanalysis structure. Frequency vs Powspctrm for all channels in one figure? .e 27 channel lines plotted for power(y-axis) vs frequency (x-axis) and ignore the time domain? I have done it in brainstorm but I have been wanting to do it in fieldtrip but havent worked it out as yet.

[cid:52a16327-3d10-42f2-9631-0103f9876be6]

Regards,
Mubeen


________________________________
From: fieldtrip <fieldtrip-bounces at science.ru.nl> on behalf of Stephen Whitmarsh <stephen.whitmarsh at gmail.com>
Sent: Thursday, March 4, 2021 10:18 AM
To: FieldTrip discussion list <fieldtrip at science.ru.nl>
Subject: Re: [FieldTrip] psd for brief segments

Hi Mubeen,

I think it depends partly on what you consider "reliable". Any time series can be expressed in the frequency domain, and that would be an accurate representation.
Typically reliability would be a consideration based on how reproducible a phenomenon can be shown to be, and in this case you only have one event. However, it occurs on multiple electrodes, so that might give you a sense of reliability (And the possibility of averaging the spectrum)

Perhaps try something like this:
A) Do an FFT on every electrode in the green timewindow (cfg.method = 'mtmfft' in ft_freqanalysis, no need for tapers).
B) Do an FFT on every electrode in a window of equal size right before.
Plot the results in the same figure: B in black, A in red. Plot an average for both time windows as well. You can see whether you can spot a difference in the spectra that is consistent over electrodes.
You can then try out different window sizes to see how much that makes a difference.

In the end though, expressing something in terms of frequencies only makes sense to me if there is some periodicity in the signal, so I am not sure a frequency analysis is appropriate for describing (the low frequency) part of spike-waves. As I said earlier, a peak detection (perhaps after some high-pass filtering) might be more to the point, depending on what you're after.

Hope this helps,
Stephen




Op do 4 mrt. 2021 om 06:12 schreef mubeen afzal <mubafzal at hotmail.com<mailto:mubafzal at hotmail.com>>:
Hi fieldtrip team,

I wanted to ask the experts here if power can be calculated on such brief segment of generalised spike/wave. In the 30 minute recording this subject had only this single spike. Can a peak PSD be reliably calculated for such a brief segment and if so what window size and taper should ideally be used. I am using 4 second windows for other subjects who have 2 or more cycles of the spike/slow wave. I am not really interested in time-resolved frequency band. Just the freq-power relationship. This is quite confusing for me. All help will be highly appreciated.

Regards,
Mubeen

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