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

[Julie Onton]

I agree that the time/frequency analysis should be performed first, and THEN subtraction.
Subtraction of the EEG time courses (ie, in the time domain) will give a very different result
compared to the (frequency domain) ERSP subtraction. EEGLAB's newtimef() called from the
commandline performs this very operation on two datasets corresponding to two different
conditions.

Best, Julie

-- 
Julie Onton, PhD
Swartz Center for Computational Neuroscience
Institute for Neural Computation
University of California, San Diego
(858) 458-1927 ext 17
http://sccn.ucsd.edu/~julie

> 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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