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

[Tzvetan Popov]

Using filters is always a “problem”. A symmetric filter will at least not misplace your signal of interest. My reply referred to the “weirdness at the right edge”. 

Best
tzvetan


> Am 18.06.2020 um 14:44 schrieb george michael <gmichael_ at hotmail.com>:
> 
> Dear Tzvetan,
> 
> thanks you for the quick reply.
> Wouldn't be a problem to use "onepass-zerophase" or "twopass" given that I am interested in phase at stimulus onset?
> Don't both filter-types smear poststimulus activity into the prestimulus interval?
> 
> Best,
> Georgios
> From: fieldtrip <fieldtrip-bounces at science.ru.nl> on behalf of Tzvetan Popov <tzvetan.popov at uni-konstanz.de>
> Sent: Thursday, June 18, 2020 14:27
> To: FieldTrip discussion list <fieldtrip at science.ru.nl>
> Subject: Re: [FieldTrip] single-trial phase at stimulus onset using Hilbert transform
>  
> 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 <mailto: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
>> 
>> 
>> 
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