<div dir="ltr"><div><div><div><div><div><div>Hi Ulrich,<br><br></div>one possible explanation for this could be the subtraction of the mean when calling ft_freqanalysis. If you don't specify cfg.polyremoval explicitly, the mean is removed. Of course, changing the time-dimension will change the mean and therefore result in a different value used in the ft_preproc_polyremoval - step.<br>
<br></div>I tried this with a dataset, setting cfg.polyremoval to -1, thereby avoiding the demean results in virtually identical time-freq results for a short (800ms) or long (1000ms) interval. However, setting cfg.polyremoval to 0 (the default) will change the output.<br>
<br></div>Also, setting the padding to 5, as you did in your code, will increase the frequency resolution which again might influence the way the low frequencies contribute to your output.<br><br></div>If someone has a more in-depth explanation or if I got this completely wrong, please correct me!<br>
<br></div>Best, <br><br></div>Julian<br><br><div><span class="" style><span class="" style>
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</span></span></div><br></div><div class="gmail_extra"><br><br><div class="gmail_quote">On Tue, May 27, 2014 at 3:18 PM, Pomper, Ulrich <span dir="ltr"><<a href="mailto:Ulrich.Pomper@charite.de" target="_blank">Ulrich.Pomper@charite.de</a>></span> wrote:<br>
<blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex">Dear community,<br>
I have a question regarding time-frequency transformation.<br>
The data I want to analyse last from -800 to -200 ms before stimulus onset, I'm using a 400 ms taper.<br>
Importantly, want to make sure that no stimulus-evoked activity leaks into my prestim analysis.<br>
<br>
I first tried to cut the segment down to -1000 to 0 ms and used (zero) padding (see figure attached, top row). This would mean that the taper, at the last samplepoint of interest (-200ms), would include data up to 0ms<br>
Suprisingly, I get quite different looking results when using a longer data segment (-1200 to 0 ms) (fig. bottom row) without padding.<br>
<br>
Can someone explain to me what is going on here, and which approach would be more correct?<br>
Either, the padding in the first variant introduces artificial low frequency activity, or the taper in the second variant uses more than 400ms, at least for the low frequencies.<br>
Note that the difference between conditions (left coloumn) look almost identical, so whatever happens, it happens to both conditions very similarly.<br>
<br>
I hope someone can educate me on that matter.<br>
Cheers, Ulrich<br>
<br>
<br>
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%<br>
%% TF transform %%<br>
<br>
cfg = [];<br>
cfg.continuous = 'no';<br>
cfg.channel = 'all';<br>
cfg.output = 'pow';<br>
cfg.method = 'mtmconvol';<br>
cfg.foi = 2:0.5:35;<br>
cfg.toi = -1:0.01:0;<br>
cfg.taper = 'hanning';<br>
cfg.tapsmofrq = 2;<br>
cfg.t_ftimwin = 400<br>
% cfg.pad = 5;<br>
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