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

[Dr. Holger Hill]

hi, I've played a bit around modelling sine waves. Subtracting two 
identical (frequency, phase, amplitude) waves will cancel out the two 
waves. Then it doesn't matter if you compute the data according to 
statement one or two. If the waves are opposite in phase (lag 180°), the 
amplitude will be doubled if you apply the first method and remain the 
same according to the second method. So method one will create a higher 
spectral activity virtually. To compare spectral activities method 2 
should be correct.

Holger


Stanley Klein schrieb:
> 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 <mailto: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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-- 
Dr. rer. nat. Holger Hill
Johann Wolfgang Goethe Universität Frankfurt
Abteilung Allgemeine Psychologie II
Mertonstr. 17
60054 Frankfurt am Main
Tel.: 069-798-28678, 0170-4741842 (p)
Fax.: 069-798-23457
email: hill at psych.uni-frankfurt.de
www.psychologie.uni-frankfurt.de/abteilungen_und_bereiche/allgemeine_psychologie_II/index.html