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

[Andrei Medvedev]

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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