[FieldTrip] Time Frequency Analysis - optimal trial segment length and artifact issues

[Raphaël Thézé]

Hello,

If your main problem are the blink artefacts, then doing an ICA during the
preprocessing should enable you to remove those quite easily, they usually
are the easiest artefacts, along with any rhytmic biological artefact such
as heart beat, to be picked up by the ICA. You can find a tutorial here:
http://www.fieldtriptoolbox.org/example/use_independent_component_analysis_ica_to_remove_eog_artifacts/

Best,
Raphaël Thézé


Le mer. 15 mai 2019 à 06:41, Poppy Watson <popwatson at hotmail.com> a écrit :

> Dear Fieldtrippers,
>
>
>
> I have some conceptual questions about the ideal trial segment length for
> eventual TFR (I’m running TFR in the 2-30Hz range).
>
> Of course for TFR, the longer the segments are the better. Although I have
> long ITIs and can theoretically take 3000-4000ms segments into the
> TFanalysis processing pathway – I currently don’t because of concerns about
> artifacts (particularly blinking) in these longer segments. I currently
> reject trials where artifacts are found within the specific 1 second range
> (plus an extra 250ms padding at the front) that I will eventually use for
> statistical analysis. I then baseline correct using the extra 250 ms (I
> know baseline correction is another discussion point but I have reason to
> do it here).I then trim the segments down to 1250ms length and use those in
> the TF_freqanalysis. But I know these are too short and there are issues in
> representing the lower frequencies on plots etc.
>
>
>
> So my query is – what if there are eye artifacts in the longer 3 second
> segments?? Does this not do strange things to the power representation of
> certain frequencies? Of course I could widen the time interval of my
> artifact rejection procedure but fear that too many trials will be rejected
> because people do need to blink at some stage and they are told to do this
> in the ITI so as not to do it during the crucial 1 second window (another
> concern is that within 200 ms of this critical analysis window participants
> start moving their eyes meaning that eye movements artifacts would
> definitely appear in any after-padding). Or – does it not matter if there
> are occasional artifacts in this ‘padded’ area that will not end up in
> statistical analysis? What do others do in these situations?
>
>
>
> Much appreciated
>
> P.Watson
>
>
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> https://doi.org/10.1371/journal.pcbi.1002202
>
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