[FieldTrip] [Fieldtrip] Analysis Options for SSVEPs

[Nathan Han]

Hi Jan-Mathijs,

Thank you very much for your detailed message. I will keep what you said in
mind.

Thanks,
Nathan

On Fri, Feb 24, 2023 at 7:35 PM <fieldtrip-request at science.ru.nl> wrote:

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>    1. Re: [Fieldtrip] Analysis Options for SSVEPs
>       (Schoffelen, J.M. (Jan Mathijs))
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> ----------------------------------------------------------------------
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> Message: 1
> Date: Thu, 23 Feb 2023 11:12:43 +0000
> From: "Schoffelen, J.M. (Jan Mathijs)"
>         <janmathijs.schoffelen at donders.ru.nl>
> To: FieldTrip discussion list <fieldtrip at science.ru.nl>
> Subject: Re: [FieldTrip] [Fieldtrip] Analysis Options for SSVEPs
> Message-ID: <6D6ED6A7-9FC7-433E-AC7D-BD34E613C54F at donders.ru.nl>
> Content-Type: text/plain; charset="utf-8"
>
> Hi Nathan,
>
> This question is not at all dumb, it’s good one!
>
> A classical SSVEP analysis indeed estimates the power of the evoked
> response (i.e. first average the trials, then compute the power spectrum).
> This works well under the valid assumption that the timing of the
> individual stimuli is 1) very precise, and 2) not jittered across trials.
>
> Now, if you want to estimate time-varying changes in the evoked power -
> e.g. to use it as a ‘marker’ for the attentional focus - you need to keep
> in mind that the specificity of your spectral estimate (+your sensitivity
> to detect changes over time)  (i.e. the extent to which the estimate at -
> say - 15 Hz reflects only the energy in the signal at 15 Hz or that it also
> contains  energy of close by frequencies -> a phenomenon that relates to
> the notion of spectral leakage, and the time-frequency trade off) depends
> on your experimental stimuli and your analysis parameters. In your example,
> using frequencies of 13 and 15 Hz, they may not be sufficiently far apart
> in the spectral domain in order to be sufficiently reliably resolved given
> your most likely choice sliding time window that you are going to use for
> your analysis.
>
> Concretely, the kernel that you will use for your time-frequency analysis
> should have a bandwidth that is small enough to be able to separate the
> stimulus frequencies. This requires a sliding time window width of at least
> one second (because a time window of 1 second yields a spectral resolution
> of 1 Hz), which is probably too long for your experimental design (because
> it would require trials that are correspondingly much longer than that time
> window obviously). Therefore, I suspect that you may need to think a bit
> (and discuss with your co-workers) the optimal stimulation frequencies for
> both hemispheres.
>
> Another thing to keep in mind w.r.t. the optimal frequencies, is that
> ideally the lower order harmonic frequencies of the stimuli don’t overlap,
> because it may be interesting later on to look at the power of the
> harmonics as well, and in this case those harmonics will still distinguish
> between stimuli.
>
> Good luck with your experiment,
>
> Jan-Mathijs
>
>
> > On 23 Feb 2023, at 10:35, Nathan Han via fieldtrip <
> fieldtrip at science.ru.nl> wrote:
> >
> > Hi all,
> >
> > I'm still quite a novice with Fieldtrip and signal processing so please
> forgive me if this question is dumb :)
> >
> > I'm running an experiment where there is a visual stimulus on the left
> and right side of the screens, the purpose of which is to elicit SSVEPs.
> One circle flickers at 13Hz (e.g., the left side) and the other flickers at
> 15Hz (e.g., the right side). I would like to analyse the change in visual
> spatial attention over the course of the trial and one way I was thinking
> was that if attention switches from the left to the right side of the
> screen, that 13Hz power would reduce while 15Hz power would increase over
> the course of the trial.
> >
> > I'm not sure if that even makes sense or if it is possible. If it does,
> I would like to ask how should I approach this analysis?
> >
> > Kind regards,
> > Nathan
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