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<DIV><SPAN class=753362310-17042008><FONT face=Arial color=#0000ff size=2>Hi
All</FONT></SPAN></DIV>
<DIV><SPAN class=753362310-17042008><FONT face=Arial color=#0000ff size=2>I
think, if I'm following this debate correctly that there are two ways of
identifying high-frequency (gamma) activity; that which is time-locked or evoked
and can be either carried out on all trials then averaged, or averaged and then
T-F transformed; and that which "jitters" with each trial, and is defined as
<STRONG>INDUCED </STRONG>(Tallon-Baudry & Bertrand 1999), where the ERP
is subtracted from each trial and then each trial is T-F transformed, with
a grand average of spectral component of interest being created from the
output.</FONT></SPAN></DIV>
<DIV><SPAN class=753362310-17042008><FONT face=Arial color=#0000ff
size=2></FONT></SPAN> </DIV>
<DIV><SPAN class=753362310-17042008><FONT face=Arial color=#0000ff size=2>Hope
that helps</FONT></SPAN></DIV>
<DIV><SPAN class=753362310-17042008><FONT face=Arial color=#0000ff
size=2>Caroline Brown</FONT></SPAN></DIV>
<DIV> </DIV>
<DIV> </DIV>
<DIV align=left><FONT face=Arial size=2>
<DIV align=left><SPAN class=349154416-21122004><FONT face=Arial
size=2>*-*-*-*-<SPAN class=349154416-21122004><FONT face=Arial
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<DIV align=left><SPAN class=349154416-21122004><FONT face=Arial size=2><SPAN
class=349154416-21122004><FONT face=Arial size=2><SPAN
class=349154416-21122004><FONT face=Arial size=2><SPAN
class=349154416-21122004><FONT face=Arial size=2><SPAN
class=349154416-21122004>Dr Caroline C
Brown</SPAN></FONT></SPAN></FONT></SPAN></FONT></SPAN></FONT></SPAN></DIV>
<DIV align=left><FONT face=Arial size=2>Lecturer in Biological
Psychology</FONT></DIV>
<DIV align=left><FONT face=Arial size=2>School of Psychology, University of West
of England, Bristol</FONT></DIV>
<DIV align=left><FONT face=Arial size=2></FONT> </DIV>
<DIV align=left><A href="mailto:carolinec.brown@uwe.ac.uk"><FONT face=Arial
size=2>carolinec.brown@uwe.ac.uk</FONT></A><FONT face=Arial size=2>
</FONT></DIV>
<DIV align=left><FONT face=Arial size=2>Tel: +44 (0) 117 328
3616</FONT></DIV></FONT></DIV>
<BLOCKQUOTE dir=ltr style="MARGIN-RIGHT: 0px">
<DIV></DIV>
<DIV class=OutlookMessageHeader lang=en-us dir=ltr align=left><FONT
face=Tahoma size=2>-----Original Message-----<BR><B>From:</B>
eeglablist-bounces@sccn.ucsd.edu [mailto:eeglablist-bounces@sccn.ucsd.edu]
<B>On Behalf Of </B>Stanley Klein<BR><B>Sent:</B> 15 April 2008
21:41<BR><B>To:</B> eeglablist@sccn.ucsd.edu<BR><B>Subject:</B> Re:
[Eeglablist] Time-frequency analysis (subtraction first oranalysis
first)<BR><BR></FONT></DIV>It looks like there is some consensus on whether to
subtract first and then the TF or vice versa. That's nice. [On the other hand
subtracting first is a nice way to get rid of ERP, but there are better ways,
as described next.]<BR><BR>Andrei, I'm not sure I understood your last comment
or question, but I have a related question. Whenever one does time-frequency
power plots I would think that one should ALWAYS first get the time locked
average and subtract it off of all the individual trials. Then one could
do a TF plot of each. How many on this list do that? I suspect that
people mix together the standard evoked response and also the phase varying
response. Why do that since its so easy to show the the two TF plots
separately.<BR><BR>Also I've heard rumors that saccades and microsaccades are
responsible for most EEG gamma oscillations. So one should also put those
events into a separate category too. Too bad things are complicated. But it
makes life interesting. <BR>Stan<BR><BR>
<DIV class=gmail_quote>On Tue, Apr 15, 2008 at 9:38 AM, Andrei Medvedev <<A
href="mailto:am236@georgetown.edu">am236@georgetown.edu</A>> wrote:<BR>
<BLOCKQUOTE class=gmail_quote
style="PADDING-LEFT: 1ex; MARGIN: 0pt 0pt 0pt 0.8ex; BORDER-LEFT: rgb(204,204,204) 1px solid">Hi
All,<BR><BR>I think this was just a small mistake confusing options 1 and 2.
I believe so because it is option 1 (not 2) which would require pairing of
trials to do EEG subtraction first, which is indeed a rare
possibility.<BR><BR>To me, it also looks like option 2 is more correct
because TF analysis (in its most common 'spectral perturbation' or 'induced
activity' version) looks for changes in spectra regardless of phase. This is
why if you analyze only one condition, you do TF first and then average
trials. Similar thing should then be done when comparing two conditions,
that is, TF first.<BR><BR>With one condition, you can also do averaging
first and then TF, in this case you would have the so-called 'evoked'
responses in the frequency domain (instead of 'induced' responses mentioned
above). Evoked activity shows you the frequency components phase-locked to
the stimulus (a more strict form of time locking). If you try to do similar
thing with two conditions (trials should be paired somehow but there is no
'natural' way to pair them, only in some special circumstances), you will
have a problem of phase relations between conditions and may get different
answers (such as sum/subtraction of in-phase/out-of-phase sine waves, as
other people point out). This would be a very different response and I
believe nobody is doing this. But theoretically, this type of response can
be explored as well (if you have a 'natural' way of pairing
trials).<BR><BR>BTW, I haven't tried to use TF decomposition in EEGLAB
applied to the averaged ERP (i.e., averaging of trials first, then TF
resulting in an 'evoked' response for one condition). Has anyone tried
this?<BR><BR>Regards,<BR>Andrei.<BR><BR>Georgetown University<BR>
<DIV>
<DIV></DIV>
<DIV class=Wj3C7c><BR><BR>----- Original Message -----<BR>From: Arnaud
Delorme <<A
href="mailto:arno@cerco.ups-tlse.fr">arno@cerco.ups-tlse.fr</A>><BR>Date:
Sunday, April 13, 2008 1:53 pm<BR>Subject: Re: [Eeglablist] Time-frequency
analysis (subtraction first or analysis first)<BR><BR>> Dear
Hsu,<BR>><BR>> only your first statement is correct. The second one
could be<BR>> correct if<BR>> you could pair the trials, but it would
be very rare that you would<BR>> want<BR>> to do this (since trials
are recorded at different times and are<BR>> usually<BR>> not paired
between conditions). Look up the help of the newtimef<BR>> function which
allows computing differences between power between<BR>> different
conditions and newcrossf which allows computing<BR>> difference<BR>>
between phase coherence images.<BR>><BR>> Best,<BR>><BR>>
Arno<BR>><BR>> Hsu, Shen-Mou wrote:<BR>> > Dear
list-memebers,<BR>> ><BR>> > Suppose that I am interested in
comparing two conditions A and B<BR>> in terms of their power and phase
coherence. I was wondering which<BR>> one of the following steps is more
theoretically correct. 1. After<BR>> segmentation, calculate the EEG
differences between the condition A<BR>> and B and then perform
time-frequency analysis on the differences.<BR>> 2. After segmentation,
perform time-frequency analysis on the EEG<BR>> data of the condition A
and B respectively and then compute the<BR>> power or phase coherence
differences between two conditions. Any<BR>> comments would be much
appreciated.<BR>> ><BR>> > Many thanks,<BR>> ><BR>>
> Shen-Mou Hsu<BR>> ><BR>>
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