<body bgcolor="" background="" class="mceContentBody " ><p>Hi Bobby,</p>
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<p>for fundamental reasons there is (of yet?) no valid approach to get at "pure" induced activity. To see this, imagine a process that produces induced responses, but in a particular participant and day you get by chance mostly responses with relatively similar phases - such that you get a nonnegligible average (some would call it an ERP). Is it still induced? Did it convert to evoked (without the underlying process having changed) ?</p>
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<p>As at least three different mechanisms (additive evoked activity, phase rest of ongoing oscillations, asymmetric amplitude modulation of ongoing oscillations) have been shown to produce non-vanishing averages / ERPs I would also strictly advise against regressing out "evoked" activity from each trial to get induced activity.</p>
<p>I also do not see a need for analysis of "pure" induced activity. You can calculate power and inter-trial phase-coherence for each time-frequency bin. For the various peaks in your time-frequency representation you quote these two values and most people should be fine with this.</p>
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<p>Michael</p>
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<b>Von:</b> Adrian Bartlett <adrian.m.bartlett@GMAIL.COM><br /><b>Gesendet:</b> Mar 25, 2010 6:51:51 PM<br /><b>An:</b> FIELDTRIP@NIC.SURFNET.NL<br /><b>Betreff:</b> Re: [FIELDTRIP] Induced activity<br /><br />Hi Bobby,
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<div>This sounds like you may have a case of line noise contamination, which is usually seen at 50/60Hz (depends on which continent you are on). While I am not familiar with it's implementation in FIELDTRIP, there are notch filtering solutions to eliminate this contamination. Using multitaper FFT will usually result in the 0 power (post-notch-filtering) at ~60 Hz being smoothed over by the narrowband frequency bias.</div>
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<div>- A<br /><br />
<div class="gmail_quote">On Thu, Mar 25, 2010 at 1:33 PM, Bobby Stojanoski <span><<a href="mailto:stojanoski@utsc.utoronto.ca">stojanoski@utsc.utoronto.ca</a>></span> wrote:<br />
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<p class="MsoNormal" style="margin-bottom: 0.0001pt; line-height: normal;">Dear Fieldtrippers,<span> <br /></span></p>
<p class="MsoNormal" style="margin-bottom: 0.0001pt; line-height: normal;"><span> </span></p>
<p class="MsoNormal" style="margin-bottom: 0.0001pt; line-height: normal;">I am a relatively new user of fieldtrip and am very impressed!</p>
<p class="MsoNormal" style="margin-bottom: 0.0001pt; line-height: normal;"> </p>
<p class="MsoNormal" style="margin-bottom: 0.0001pt; line-height: normal;">I am interested in comparing differences at certain frequencies – induced 40-100 Hz – between 2 experimental conditions.<span> </span>My understanding was that I can calculate induced activity in the gamma range by calculating the power for each trial (subject average -- freqanalysis) and then averaging across subjects (grand average -- freqdescriptives/freqgrandaverage).</p>
<p class="MsoNormal" style="margin-bottom: 0.0001pt; line-height: normal;"> </p>
<p class="MsoNormal" style="margin-bottom: 0.0001pt; line-height: normal;">To my dismay, when I plotted the results of my grandaverage I found a band of power at 55 - 65 Hz for the entire duration of my epoch. I should add this was not the case when I plotted power across trials for each participant. <span> </span></p>
<p class="MsoNormal" style="margin-bottom: 0.0001pt; line-height: normal;"> </p>
<p class="MsoNormal" style="margin-bottom: 0.0001pt; line-height: normal;">Earlier discussions mention computing induced+evoked (using freqanalysis and freqgrandaverage) and subtracting that from evoked (using timelockanalysis+freqanalysis) to get extract induced only activity. However, later posts suggest that this is not a valid approach.</p>
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<p class="MsoNormal" style="margin-bottom: 0.0001pt; line-height: normal;"> </p>
<p class="MsoNormal" style="margin-bottom: 0.0001pt; line-height: normal;">1. Where have I made my mistake?</p>
<p class="MsoNormal" style="margin-bottom: 0.0001pt; line-height: normal;"> </p>
<p class="MsoNormal" style="margin-bottom: 0.0001pt; line-height: normal;">2. If ((induced+evoked)-evoked)) is not valid, what is the correct approach to calculating induced activity at 40 - 100 Hz?</p>
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<p class="MsoNormal" style="margin-bottom: 0.0001pt; line-height: normal;">Any help would be greatly appreciated!</p>
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<p class="MsoNormal" style="margin-bottom: 0.0001pt; line-height: normal;"> </p>
<p class="MsoNormal" style="margin-bottom: 0.0001pt; line-height: normal;">Thank you</p>
<p class="MsoNormal" style="margin-bottom: 0.0001pt; line-height: normal;">Bobby Stojanoski</p>
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<p>The aim of this list is to facilitate the discussion between users of the FieldTrip toolbox, to share experiences and to discuss new ideas for MEG and EEG analysis.</p>
<p><a href="http://listserv.surfnet.nl/archives/fieldtrip.html" target="_blank">http://listserv.surfnet.nl/archives/fieldtrip.html</a></p>
<p><a href="http://www.ru.nl/fcdonders/fieldtrip/" target="_blank">http://www.ru.nl/fcdonders/fieldtrip/</a></p>
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</div>
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<p>----------------------------------</p>
<p>The aim of this list is to facilitate the discussion between users of the FieldTrip toolbox, to share experiences and to discuss new ideas for MEG and EEG analysis.</p>
<p>http://listserv.surfnet.nl/archives/fieldtrip.html</p>
<p>http://www.ru.nl/fcdonders/fieldtrip/</p>
</blockquote>
</span></span></span></span></span></body>
<p>----------------------------------</p>
<p>The aim of this list is to facilitate the discussion between users of the FieldTrip toolbox, to share experiences and to discuss new ideas for MEG and EEG analysis.</p>
<p> http://listserv.surfnet.nl/archives/fieldtrip.html</p>
<p> http://www.ru.nl/fcdonders/fieldtrip/</p>