<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.0 Transitional//EN">
<HTML><HEAD>
<META http-equiv=Content-Type content="text/html; charset=iso-8859-1">
<META content="MSHTML 6.00.2800.1522" name=GENERATOR>
<STYLE></STYLE>
</HEAD>
<BODY bgColor=#ffffff>
<DIV><FONT face=Arial size=2>Hi all,</FONT></DIV>
<DIV><FONT face=Arial size=2></FONT> </DIV>
<DIV><FONT face=Arial size=2>I have a few questions: </FONT></DIV>
<DIV> </DIV>
<DIV><FONT face=Arial size=2>first, in order to look at induced power rather
than evoked power, one can subtract the power spectrum of the evoked potential
(average over trials) from the power spectrum (average of power in single
trials). Is this not the same as first subtracting the average over trials from
every trial and then doing a single freqanalysis? If not, would it then mean you
have to do a procedure like: do 1) freqanalysis on preprocessed data 2)
timelockanalysis on preprocessed data, then freqanalysis 3) take (result of
1)-(result of 2)?</FONT></DIV>
<DIV> </DIV>
<DIV><FONT face=Arial size=2>second, does a similar approach also exist for
coherence? For example, in case of presenting a flash to a subject, different
sensors will get common input that is time and phase locked, resulting in high
coherence values, however for this no coupling between the sensors needs to
exist. Is it possible to correct for this to get an "induced"
coherence?</FONT></DIV>
<DIV> </DIV>
<DIV><FONT face=Arial size=2>third, in principle the phase spectrum can give
information additional to the power spectrum: in the case of a power spectrum,
it can give the latency of the response if one fits a linear function to the
phase spectrum. How can this be done in FieldTrip? My first approach would
be:</FONT></DIV>
<DIV> </DIV>
<DIV><FONT face=Arial size=2>cfg = [];<BR>cfg.method = 'mtmfft';<BR>cfg.output =
'fourier'; % complex spectrum<BR>cfg.tapsmofrq =
4;<BR>[freq]=freqanalysis(cfg,data) % data is preprocessed data</FONT></DIV>
<DIV> </DIV>
<DIV><FONT face=Arial size=2>the power spectrum is
then:<BR>powspctrm=squeeze(mean(freq.fourierspctrm.*conj(freq.fourierspctrm)));</FONT></DIV>
<DIV> </DIV>
<DIV><FONT face=Arial size=2>the phase spectrum is then simply unwrapping the
angle of the complex spectrum and trying to fit a line on this?:<BR>phsspctrm =
squeeze(mean(unwrap(angle(freq.fourierspctrm))));</FONT></DIV>
<DIV> </DIV>
<DIV><FONT face=Arial size=2>Besides practical advice I'd much appreciate
references to any standard papers on this procedure.</FONT></DIV>
<DIV> </DIV>
<DIV><FONT face=Arial size=2>related to this, how could one use the phase of the
cross-spectrum to get an idea of the latency of covariance at some
frequency?</FONT></DIV>
<DIV> </DIV>
<DIV><FONT face=Arial size=2>Many thanks, best,
Jasper</FONT></DIV></BODY></HTML>
<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>