[Eeglablist] Time-frequency analysis (subtraction first oranalysis first)

[Carolinec Brown]

Hi All
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 INDUCED (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.
 
Hope that helps
Caroline Brown
 
 
*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-
Dr Caroline C Brown
Lecturer in Biological Psychology
School of Psychology, University of West of England, Bristol
 
carolinec.brown at uwe.ac.uk <mailto:carolinec.brown at uwe.ac.uk>  
Tel: +44 (0) 117 328 3616

	-----Original Message-----
	From: eeglablist-bounces at sccn.ucsd.edu
[mailto:eeglablist-bounces at sccn.ucsd.edu] On Behalf Of Stanley Klein
	Sent: 15 April 2008 21:41
	To: eeglablist at sccn.ucsd.edu
	Subject: Re: [Eeglablist] Time-frequency analysis (subtraction
first oranalysis first)
	
	
	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.]
	
	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.
	
	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. 
	Stan
	
	
	On Tue, Apr 15, 2008 at 9:38 AM, Andrei Medvedev
<am236 at georgetown.edu> wrote:
	

		Hi All,
		
		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.
		
		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.
		
		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).
		
		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?
		
		Regards,
		Andrei.
		
		Georgetown University
		


		----- Original Message -----
		From: Arnaud Delorme <arno at cerco.ups-tlse.fr>
		Date: Sunday, April 13, 2008 1:53 pm
		Subject: Re: [Eeglablist] Time-frequency analysis
(subtraction first or analysis first)
		
		> Dear Hsu,
		>
		> only your first statement is correct. The second one
could be
		> correct if
		> you could pair the trials, but it would be very rare
that you would
		> want
		> to do this (since trials are recorded at different
times and are
		> usually
		> not paired between conditions). Look up the help of
the newtimef
		> function which allows computing differences between
power between
		> different conditions and newcrossf which allows
computing
		> difference
		> between phase coherence images.
		>
		> Best,
		>
		> Arno
		>
		> Hsu, Shen-Mou wrote:
		> > Dear list-memebers,
		> >
		> > Suppose that I am interested in comparing two
conditions A and B
		> in terms of their power and phase coherence. I was
wondering which
		> one of the following steps is more theoretically
correct. 1. After
		> segmentation, calculate the EEG differences between
the condition A
		> and B and then perform time-frequency analysis on the
differences.
		> 2. After segmentation, perform time-frequency analysis
on the EEG
		> data of the condition A and B respectively and then
compute the
		> power or phase coherence differences between two
conditions. Any
		> comments would be much appreciated.
		> >
		> > Many thanks,
		> >
		> > Shen-Mou Hsu
		> >
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