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

[Stanley Klein]

Dear Arnaud,
Are you sure about your recommendation of the first statement being correct?
Suppose the two conditions happen to be quite similar in producing alpha or
gamma oscillations. Since the two conditions are the same, one would like
the desired outcome to be a *cancellation *of the two power plots. However,
since the occurrence of the oscillations likely happen at different time
points (and with different phases) early subtraction would still leave the
oscillations and the outcome would be equal or even stronger power rather
than cancellation. Or am I thinking about it improperly?
Stan

On Sun, Apr 13, 2008 at 10:53 AM, Arnaud Delorme <arno at cerco.ups-tlse.fr>
wrote:

> 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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