[FieldTrip] partial artifact rejection for time-frequency analysis

Schoffelen, J.M. (Jan Mathijs) jan.schoffelen at donders.ru.nl
Mon May 14 15:55:35 CEST 2018

Dear Matthias,

Don’t worry. The partial rejection method indeed cuts original trials in smaller portions, but it preserves the local time axes, relative to the original trials.
Since ft_freqanalysis returns per ‘segment’ TFRs with numeric data only for those portions of data for which the time windows are entirely filled with data (and nans otheriwse, with the average across segments is computed as a nanmean) there is no worry about ‘edge artifacts’ as such. The only thing to keep in the back of your head is that different time-frequency points may end up having slightly different degrees of freedom in the average.

Best wishes,

On 14 May 2018, at 14:39, Matthias Franken <Matthias.Franken at UGent.be<mailto:Matthias.Franken at UGent.be>> wrote:

Dear all,

I have an EEG dataset that consists of quite long trials (>20s). Therefore, For artifact rejection I would like to only reject the part of the trial that contains the artifact, rather than throwing away the entire trial, and thus I will use the cfg.artfctdef.method = ‘partial’; option in ft_rejectartifact.
However, this leads to some trials being cut in two shorter “trials” (i.e., the part before the artifact and the part after the artifact). I’m guessing this will lead to edge artifacts in my time-frequency analysis, not only at the edges of the original trials, but also at time points where artifacts were cut out. SO I’m not sure how to deal with this.

I guess one option would be to perform the time-frequency analysis first, and afterwards cut out the artifacts, although (1) that might lead to the artifact smearing into data points that were not cut out, depending on the time window width during frequency analysis, and (2) the artifacts identified visually using ft_databrowser are define din ample numbers, and the result from ft_freqanalysis downsamples the data.

Did anyone face similar issues or have ideas on how to get around this?

Matthias Franken
postdoctoral researcher
Experimental Psychology Department
Ghent University

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