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<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D;mso-fareast-language:EN-US">Dear Arti,<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D;mso-fareast-language:EN-US"><o:p> </o:p></span></p>
<p class="MsoNormal"><span lang="EN-US" style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D;mso-fareast-language:EN-US">My guess is that is has to do with plotting relative change instead of e.g. absolute change. In the individual conditions,
you plot relative change to their own baselines, which is in the order of 0.2 or so.
<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US" style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D;mso-fareast-language:EN-US">Now when you subtract the 2 conditions, the difference in both baseline and activation epoch is probably small. So when
you plot the tfr of this, normalized by the baseline wpoch values of this difference signal, it might blow up the relative change values. You might want to plot just the absolute values of the difference TFR, since it is the difference in the activation window
that is most relevant to you (assuming that baseline values are not different between the conditions. Or you could contrast just the activation epochs (e.g., using the tfr of cond1 as ‘baseline’ for cond2), and plot relative change of that.<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US" style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D;mso-fareast-language:EN-US"><o:p> </o:p></span></p>
<p class="MsoNormal"><span lang="EN-US" style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D;mso-fareast-language:EN-US">I’ll illustrate it with some example values to show what I think goes ‘wrong’:<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US" style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D;mso-fareast-language:EN-US"><o:p> </o:p></span></p>
<p class="MsoNormal"><span lang="EN-US" style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D;mso-fareast-language:EN-US">Cond. 1: baseline: 4; activation 6; -> rel.change 0.5 (50% increase)<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US" style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D;mso-fareast-language:EN-US">Cond. 2: baseline 3.9; acitivation 6.3; -> rel change 0.6 (60% increase)<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US" style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D;mso-fareast-language:EN-US">Difference: baseline 0.1; activation 0.3 -> rel change 2.0 (200% increase)<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US" style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D;mso-fareast-language:EN-US"><o:p> </o:p></span></p>
<p class="MsoNormal"><span lang="EN-US" style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D;mso-fareast-language:EN-US">Best,<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US" style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D;mso-fareast-language:EN-US">Stan<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US" style="mso-fareast-language:EN-US"><o:p> </o:p></span></p>
<p class="MsoNormal"><span lang="EN-US" style="mso-fareast-language:EN-US">--<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US" style="mso-fareast-language:EN-US">Stan van Pelt, PhD<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US" style="mso-fareast-language:EN-US">Donders Institute for Brain, Cognition and Behaviour<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US" style="mso-fareast-language:EN-US">Radboud University<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US" style="mso-fareast-language:EN-US">Montessorilaan 3, B.01.34<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US" style="mso-fareast-language:EN-US">6525 HR Nijmegen, the Netherlands<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US" style="mso-fareast-language:EN-US">tel: +31 24 3616288<o:p></o:p></span></p>
<p class="MsoNormal"><b><span lang="EN-US" style="font-size:11.0pt;font-family:"Calibri",sans-serif"><o:p> </o:p></span></b></p>
<p class="MsoNormal"><b><span lang="EN-US" style="font-size:11.0pt;font-family:"Calibri",sans-serif">From:</span></b><span lang="EN-US" style="font-size:11.0pt;font-family:"Calibri",sans-serif"> fieldtrip-bounces@science.ru.nl [mailto:fieldtrip-bounces@science.ru.nl]
<b>On Behalf Of </b>Arti Abhishek<br>
<b>Sent:</b> woensdag 16 november 2016 5:09<br>
<b>To:</b> FieldTrip discussion list <fieldtrip@science.ru.nl><br>
<b>Subject:</b> [FieldTrip] Questions about time frequency analysis of EEG data<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US"><o:p> </o:p></span></p>
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<p class="MsoNormal" style="margin-bottom:12.0pt">Dear fieldtrip community,<o:p></o:p></p>
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<p class="MsoNormal" style="margin-bottom:12.0pt">I am running time frequency analysis on my EEG data and when I subtract the response between conditions, the difference condition has much higher amplitude. Could someone suggest what is wrong here?
<br>
<br>
Please find the image here:<br>
<a href="https://www.dropbox.com/s/kh5uzz8000fmh1b/tfr.pdf?dl=0">https://www.dropbox.com/s/kh5uzz8000fmh1b/tfr.pdf?dl=0</a><o:p></o:p></p>
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<p class="MsoNormal">and here is my script<br>
<br>
cfg = [];<br>
cfg.trials = find(s01_ica_clean_artrel_reref.trialinfo==102);<br>
cfg.channel = 'EEG'; <br>
cfg.method = 'wavelet'; <br>
cfg.width = 7; <br>
cfg.output = 'pow'; <br>
cfg.foi = 2:2:40; <br>
cfg.toi = -0.4:0.02:0.5; <br>
s01_cond1 = ft_freqanalysis(cfg, s01_cond1_preprocess);<o:p></o:p></p>
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<p class="MsoNormal">% difference<o:p></o:p></p>
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<p class="MsoNormal">cfg=[];<br>
cfg.parameter='powspctrm';<br>
cfg.operation ='x1-x2';<br>
s01_difference =ft_math(cfg, s01_cond1, s01_cond2);<o:p></o:p></p>
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<p class="MsoNormal">% plot<o:p></o:p></p>
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<p class="MsoNormal" style="margin-bottom:12.0pt">cfg = [];<br>
cfg.baseline=[-0.4 -0.1];<br>
cfg.zlim=[-0.5 0.5];<br>
cfg.xlim=[-0.4 0.5];<br>
cfg.baselinetype = 'relchange';<br>
cfg.layout = lay;<br>
cfg.interactive = 'yes';<br>
cfg.channel={'FZ'};<br>
subplot (1,3,1)<br>
ft_singleplotTFR(cfg, s01_cond1);<br>
title('deviant');<br>
subplot (1,3,2);<br>
ft_singleplotTFR(cfg, s01_cond2);<br>
title('standard');<br>
subplot (1,3,3)<br>
ft_singleplotTFR(cfg, s01_difference);<br>
title('difference');<o:p></o:p></p>
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<p class="MsoNormal">Thanks,<o:p></o:p></p>
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<p class="MsoNormal">Arti<o:p></o:p></p>
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