Hi Matt, Hi Fred,<br><br>thanks for your answers! It helped a lot! The
artifact seems indeed to be due to the 60 Hz refresh rate of our beamer.
I filtered the data again and the result is pretty clear (see attached
picture - one subject).<br>
<br>Thanks again and a nice weekend for you two!<br><br>Best,<br><br>Andreas<br><br><div class="gmail_quote"><blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex"><div class="HOEnZb">
<div class="h5"><br><div class="gmail_quote">2012/11/17 Frederic Roux <span dir="ltr"><<a href="mailto:f.roux@bcbl.eu" target="_blank">f.roux@bcbl.eu</a>></span><br>
<blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex">Hi Matt, Hi Andreas,<br>
<br>
Indeed, I remember seeing strong 60 Hz line noise as<br>
a result of the beamer refresh rate (60Hz).<br>
<div><br>
Best,<br>
Fred<br>
<br>
<br>
----- Original Message -----<br>
</div><div>From: "Matt Craddock" <<a href="mailto:matt.craddock@uni-leipzig.de" target="_blank">matt.craddock@uni-leipzig.de</a>><br>
To: "FieldTrip discussion list" <<a href="mailto:fieldtrip@science.ru.nl" target="_blank">fieldtrip@science.ru.nl</a>><br>
</div><div><div>Sent: Friday, November 16, 2012 4:50:06 PM<br>
Subject: Re: [FieldTrip] Problem with TFR calculation using multitaper<br>
<br>
Dear Andreas and Frederic,<br>
<br>
Another possibility would be a 60 Hz signal (a steady-state evoked<br>
potential to monitor refresh rate, for example) running throughout the<br>
trial, including in the baseline period. Given that you're using 11 Hz<br>
frequency smoothing, this would effectively be smeared across the 49-71<br>
Hz range.<br>
<br>
You'd have a much higher baseline signal in the 49-71 Hz range (because<br>
of the 60 Hz signal appearing in all those frequencies), but the<br>
strength of the actual signal in those ranges wouldn't differ. So, the<br>
signal-to-baseline ratio would be much lower (because your<br>
signal-to-noise ratio is worse), and that'd created the appearance of<br>
boundaries at the edges of the frequency smoothing (because at 71 Hz you<br>
are still getting the 60Hz noise, but at 72Hz, you are not, so your<br>
signal-to-noise ratio dramatically changes).<br>
<br>
I've attached a pic showing something similar caused by 50 Hz line noise<br>
(in EEG, with ~12Hz freq smoothing).<br>
<br>
So I'd suggest checking the FFT of your data and possibly putting in a<br>
notch at 60 Hz if there's a big peak there.<br>
<br>
Cheers,<br>
Matt<br>
<br>
<br>
On 16/11/2012 14:51, Frederic Roux wrote:<br>
><br>
> Dear Andreas,<br>
><br>
> if you used a 50 Hz notch filter in your<br>
> preprocessing, that is probably the reason<br>
> why you are seeing the 'artificial' cut<br>
> between 30 and 70 Hz.<br>
><br>
> My guess is that you ran your notch filter<br>
> on segmented epochs of a few seconds (resulting<br>
> in a low spectral resolution for your notch filter).<br>
> This will wipe out a ~10Hz box (in your case I'm just guessing)<br>
> around the line noise frequency.<br>
><br>
> Running the multitaper on top of that may explain<br>
> the pattern you are observing.<br>
><br>
><br>
> Best,<br>
><br>
> Fred<br>
> ----- Original Message -----<br>
> From: "Andreas Sauer" <<a href="mailto:sauer.mpih@googlemail.com" target="_blank">sauer.mpih@googlemail.com</a>><br>
> To: "FieldTrip discussion list" <<a href="mailto:fieldtrip@science.ru.nl" target="_blank">fieldtrip@science.ru.nl</a>><br>
> Sent: Friday, November 16, 2012 2:36:01 PM<br>
> Subject: [FieldTrip] Problem with TFR calculation using multitaper<br>
><br>
><br>
> Dear all,<br>
><br>
> I have encountered a problem when trying to calculate the power spectrum of my data and I was wondering if anyone could help me to determine the origin of this problem.<br>
><br>
> For the analysis I used multitapers (dpss) with a 11 Hz freq-smoothing and a time window with a length of 500 ms ending up with 10 tapers for the TFR calculation (the freq range is 20 - 200 Hz):<br>
><br>
><br>
> cfg = [];<br>
> cfg.output = 'pow';<br>
> cfg.channel = 'MEG';<br>
> cfg.method = 'mtmconvol';<br>
> cfg.taper = 'dpss';<br>
> cfg.keeptrials = 'no';<br>
> cfg.foi = [20:1:200];<br>
> cfg.toi = [-0.5:0.05:1.0]; % the preprocessed data length is 2 sec, from -0.8 to 1.2 s<br>
> cfg.tapsmofrq = ones(length(cfg.foi),1).*11; % spectral smoothing = +/- 11 Hz<br>
> cfg.t_ftimwin = ones(length(cfg.foi),1).*0.5; % length of time window = 500 ms<br>
> cfg.pad = 'maxperlen';<br>
><br>
> TFR_Mult_DPSS_10tapers_fmin20Hz_fmax200Hz = ft_freqanalysis(cfg,DataOut);<br>
><br>
> After calculation I did a baseline normalization:<br>
><br>
><br>
> cfg = [];<br>
> cfg.baseline = [-0.4 -0.1];<br>
> cfg.baselinetype = 'relchange';<br>
><br>
> TFR_HighGamma{subject,condition} = ft_freqbaseline(cfg,TFR_file.TFR_Mult_DPSS_10tapers_fmin20Hz_fmax200Hz);<br>
><br>
> averaged:<br>
><br>
><br>
> cfg = [];<br>
> cfg.channel = 'ParOcc'; % parietal and occipital channels<br>
> GA_TFR_HighGamma{condition} = ft_freqgrandaverage(cfg,TFR_HighGamma{:,condition});<br>
><br>
> and finally plotted the spectrum:<br>
><br>
><br>
> time(1) = 3; % -400 ms<br>
> time(2) = 27; % 800 ms<br>
> indFreq = [20 200];<br>
> freq = find(ismember(GA_TFR_HighGamma{i}.freq, indFreq) == 1);<br>
><br>
> power = squeeze(mean(GA_TFR_HighGamma{i}.powspctrm,1));<br>
> power = mean(GA_TFR_HighGamma{i}.powspctrm(:,:,time(1):time(2)),1);<br>
> power = mean(power,1);<br>
><br>
> fig = figure;<br>
> set(fig,'PaperUnits','centimeters');<br>
> set(fig,'Units','centimeters');<br>
> set(fig,'PaperSize',[15 15]);<br>
> set(fig,'PaperPosition',[0 0 14 14]);<br>
> set(gcf,'Color',[1 1 1]);<br>
><br>
> ha = gca;<br>
> pcolor(GA_TFR_HighGamma{i}.time(time(1):time(2)),GA_TFR_HighGamma{i}.freq,squeeze(power));<br>
> set(ha,'FontName','Arial','FontSize',10,'LineWidth',1.5);<br>
> shading interp;<br>
> xlim([-0.4 0.8]);<br>
> ylim([40 120]);<br>
> caxis([-.2 .2]);<br>
> hold on;<br>
> ha = gca;<br>
> plot([0 0],[40 120],'k--','LineWidth',1);<br>
> set(ha,'FontName','Arial','FontSize',10,'LineWidth',1.5);<br>
> set(ha,'YTick',[40 60 80 100 120]);<br>
> colorbar('SouthOutside');<br>
> xlabel('Time [sec]');<br>
> ylabel('Frequency [Hz]');<br>
><br>
> The resulting plot looks strange to me since there is this artifical "cut" at 70 Hz and at 50 Hz. I checked the FAQs but my plot looks different from the "strange" ones described there.<br>
> I also attached a screenshot of the MultiplotTFR and there you can see the strange "effect" clearly in channels MLO22 or MRO22 for example.<br>
><br>
> Is something wrong with my TFR calculation? Or does the problem arise from an earlier step in the preprocessing?<br>
><br>
> Any help or advice would be greatly appreciated!<br>
><br>
> Best,<br>
><br>
> Andreas<br>
><br>
<br>
<br>
--<br>
Dr. Matt Craddock<br>
<br>
Post-doctoral researcher,<br>
Institute of Psychology,<br>
University of Leipzig,<br>
Seeburgstr. 14-20,<br>
04103 Leipzig, Germany<br>
Phone: <a href="tel:%2B49%20341%20973%2095%2044" value="+493419739544" target="_blank">+49 341 973 95 44</a><br>
<br>
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</div></div></blockquote></div><br><br clear="all"><br></div></div><span class="HOEnZb"><font color="#888888">-- <br><div></div><div>Dipl.-Psych. Andreas Sauer</div></font></span><div class="HOEnZb"><div class="h5"><div>
Max Planck Institute for Brain Research</div><div>Deutschordenstraße 46</div><div>60528 Frankfurt am Main</div>
<div>Germany</div><div><br></div><div>T: <a href="tel:%2B49%2069%2096769%20278" value="+496996769278" target="_blank">+49 69 96769 278</a></div><div>F: <a href="tel:%2B49%2069%2096769%20327" value="+496996769327" target="_blank">+49 69 96769 327</a></div>
<div>Email: <a href="mailto:sauer.mpih@gmail.com" target="_blank">sauer.mpih@gmail.com</a></div><div><a href="http://www.brain.mpg.de" target="_blank">www.brain.mpg.de</a></div>
<br>
</div></div></blockquote></div><br><br clear="all"><br>-- <br><div></div><div>Dipl.-Psych. Andreas Sauer</div><div>Max Planck Institute for Brain Research</div><div>Deutschordenstraße 46</div><div>60528 Frankfurt am Main</div>
<div>Germany</div><div><br></div><div>T: +49 69 96769 278</div><div>F: +49 69 96769 327</div><div>Email: <a href="mailto:sauer.mpih@gmail.com" target="_blank">sauer.mpih@gmail.com</a></div><div><a href="http://www.brain.mpg.de" target="_blank">www.brain.mpg.de</a></div>
<br>