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Dear Georgios,
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<div class="">Note that the default filter direction implemented for the ‘firws’ filter is onepass-zerophase. I recommend to use this (thus not to specify ‘onepass’), but in addition to widen up (as per Tzvetan’s suggestion) the epoch to avoid filter edge effects
in your time window of interest.</div>
<div class="">Note that you ask for a very narrow passband on very short data segments. This is in principle not really meaningful, since narrow filters require long filterorders (i.e. they need to integrate the original signal in a long window). As long as
they are integrating from the left only, the phase estimates won’t be affected by the stimulus induced transient.</div>
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<div class="">Best wishes,</div>
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<div class="">Jan-Mathijs</div>
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<div><br class="">
<blockquote type="cite" class="">
<div class="">On 18 Jun 2020, at 14:44, george michael <<a href="mailto:gmichael_@hotmail.com" class="">gmichael_@hotmail.com</a>> wrote:</div>
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Dear Tzvetan,</div>
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thanks you for the quick reply.</div>
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Wouldn't be a problem to use "onepass-zerophase" or "twopass" given that I am interested in phase at stimulus onset?</div>
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Don't both filter-types smear poststimulus activity into the prestimulus interval?<br class="">
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Best,</div>
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Georgios<br class="">
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<div id="divRplyFwdMsg" dir="ltr" class=""><font face="Calibri, sans-serif" style="font-size: 11pt;" class=""><b class="">From:</b><span class="Apple-converted-space"> </span>fieldtrip <<a href="mailto:fieldtrip-bounces@science.ru.nl" class="">fieldtrip-bounces@science.ru.nl</a>>
on behalf of Tzvetan Popov <<a href="mailto:tzvetan.popov@uni-konstanz.de" class="">tzvetan.popov@uni-konstanz.de</a>><br class="">
<b class="">Sent:</b><span class="Apple-converted-space"> </span>Thursday, June 18, 2020 14:27<br class="">
<b class="">To:</b><span class="Apple-converted-space"> </span>FieldTrip discussion list <<a href="mailto:fieldtrip@science.ru.nl" class="">fieldtrip@science.ru.nl</a>><br class="">
<b class="">Subject:</b><span class="Apple-converted-space"> </span>Re: [FieldTrip] single-trial phase at stimulus onset using Hilbert transform</font>
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<div class="" style="word-wrap: break-word; line-break: after-white-space;">Dear Georgios,
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<div class="">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.</div>
<div class="">Good luck,</div>
<div class="">tzvetan</div>
<div class=""><br class="">
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<blockquote type="cite" class="">
<div class="">Am 18.06.2020 um 14:14 schrieb george michael <<a href="mailto:gmichael_@hotmail.com" class="">gmichael_@hotmail.com</a>>:</div>
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Dear Fieldtrip Community,</div>
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I am trying to extract the alpha phase at stimulus onset from single trials.<span class="x_Apple-converted-space"> </span><br class="">
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My approach is to :<br class="">
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1) Band-pass filter the trial segments from -500ms to 0 (<span class="">±</span>1Hz around individual alpha frequency).</div>
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I use the filter 'firws' and filter direction 'onepass' to avoid smearing poststimulus activity into the prestimulus interval.<span class="" style="left: 612.92px; top: 156.986px; font-size: 16.4px; font-family: sans-serif;"></span><br class="">
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2) Compute Hilbert transform to extract the instantaneous phase of the bandpass signal.<span class="x_Apple-converted-space"> </span><br class="">
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3) Select the phase at the last sample (t=0) as the one representing phase at stimulus onset.</div>
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<span class=""> cfg=[];<br class="">
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<span class="">cfg.channel = 'O1';<br class="">
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<div class="">cfg.bpfilter='yes';<br class="">
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<div class="">cfg.bpfreq=[foi-1 foi+1];<br class="">
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<div class="">cfg.bpfilttype= 'firws';<br class="">
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<div class="">cfg.bpfiltdir= 'onepass';</div>
<div class="">cfg.hilbert='angle';</div>
<div class="">phase = ft_preprocessing(cfg , trials_prestim);</div>
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<span class=""> for tr=1:length(phase .trial)</span><br class="">
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<div class=""> phase_onset(tr,:)=<span class="">phase<span class="x_Apple-converted-space"> </span></span>.trial{tr}(:,end); % select phase at last sample<br class="">
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<span class=""> end</span></div>
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The problem is that the band-bass signal looks quite weird at the right edge, towards t=0s (see attached figure). </div>
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Do you have any idea what would be the reason for this problem?<span class="x_Apple-converted-space"> </span><br class="">
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And which would be the optimal way to extract phase at stimulus onset?</div>
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Many thanks,<br class="">
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Georgios</div>
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Georgios Michail, MSc.</div>
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Department of Psychiatry and Psychotherapy</div>
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Charite-Universitätsmedizin Berlin<br class="">
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<span id="x_cid:CD0CE12F-5A7D-457E-BDE1-8E2D4C11F836@fritz.box" class=""><bandpass_alpha_singlesubj.png></span><span class="" style="font-family: Helvetica; font-size: 12px; font-style: normal; font-variant-caps: normal; font-weight: normal; letter-spacing: normal; text-align: start; text-indent: 0px; text-transform: none; white-space: normal; word-spacing: 0px; text-decoration: none; float: none; display: inline !important;">_______________________________________________</span><br class="" style="font-family: Helvetica; font-size: 12px; font-style: normal; font-variant-caps: normal; font-weight: normal; letter-spacing: normal; text-align: start; text-indent: 0px; text-transform: none; white-space: normal; word-spacing: 0px; text-decoration: none;">
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