[FieldTrip] single-trial phase at stimulus onset using Hilbert transform

[george michael]

Dear Fieldtrip Community,

I am trying to extract the alpha phase at stimulus onset from single trials.
My approach is to :
1) Band-pass filter the trial segments from -500ms to 0 (±1Hz around individual alpha frequency).
I use the filter 'firws' and filter direction 'onepass' to avoid smearing poststimulus activity into the prestimulus interval.
2) Compute Hilbert transform to extract the instantaneous phase of the bandpass signal.
3) Select the phase at the last sample (t=0) as the one representing phase at stimulus onset.

 cfg=[];
cfg.channel = 'O1';
cfg.bpfilter='yes';
cfg.bpfreq=[foi-1 foi+1];
cfg.bpfilttype= 'firws';
cfg.bpfiltdir= 'onepass';
cfg.hilbert='angle';
phase = ft_preprocessing(cfg , trials_prestim);

 for tr=1:length(phase .trial)
       phase_onset(tr,:)=phase .trial{tr}(:,end); % select phase at last sample
 end

The problem is that the band-bass signal looks quite weird at the right edge, towards t=0s (see attached figure).

Do you have any idea what would be the reason for this problem?
And which would be the optimal way to extract phase at stimulus onset?

Many thanks,
Georgios
-----------------------
Georgios Michail, MSc.
Department of Psychiatry and Psychotherapy
Charite-Universitätsmedizin Berlin



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