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

[Tzvetan Popov]

Dear Georgios,

the reason is your cfg.bpfiltdir = ‘onepass’. This should be zero phase or twopass filter. Yet, you will still get filter artefacts at the edges. I suggest you should re-segment your data, e.g. -1000 to 500ms, filter and then re-segment to -500 to 0 again.
Good luck,
tzvetan


> Am 18.06.2020 um 14:14 schrieb george michael <gmichael_ at hotmail.com>:
> 
> 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
> 
> 
> 
> <bandpass_alpha_singlesubj.png>_______________________________________________
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