[FieldTrip] psd for brief segments

[Stephen Whitmarsh]

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>:

> 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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> https://doi.org/10.1371/journal.pcbi.1002202
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