[Eeglablist] Identifying induced responses

[Guillaume Rousselet]

Dear Keith,

Not sure which technique is the best, but you should try testing  
existing techniques on a good subject first to see if you get anything  
at all.

I think the most used technique to isolate the induce response  
consists in [1] computing the power spectrum of the ERP averaged  
across trials, [2] computing the average power spectrum of the single- 
trial ERP, [3] subtract 1 from 2.

Another technique consists in directly subtracting the mean ERP from  
each single-trial, and computing the power spectrum on the centred  
data, see ref:

Rousselet, G. A., Husk, J. S., Bennett, P. J., & Sekuler, A. B.  
(2007). Single-trial EEG dynamics of object and face visual  
processing. Neuroimage, 36(3), 843-862.

The function to compute time-frequency decompositions is newtimef.

The different steps of the 2 techniques above can easily be  
implemented using this function. For instance, you can compute centred  
data at one electrode E to feed into newtimef by doing:
data = squeeze(EEG.data(E,:,:)) - repmat(mean(squeeze(EEG.data(E,:,:)), 
2),1,size(EEG.data,3));

Best,

GAR




On 23 Sep 2009, at 16:11, Keith Yoder wrote:

> Hi all,
> I know that this problem has been discussed to some extent already  
> (e.g. http://sccn.ucsd.edu/pipermail/eeglablist/2008/002262.html),  
> but I've been unable to find a define solution.  What I'm attempting  
> to do is the portion of the response that is induced, rather than  
> evoked.
> Our paradigm involves a computer screen with five sections, each  
> flickering at a different frequency.  At any given time, one of the  
> sections is cued (the section is surrounded by a a bright red  
> line).  We are attempting to identify induced changes time-locked to  
> cue shifts.  I know that other authors have had some success using  
> the following general procedure:
>    1. Compute the time-domain average (ERP)
>    2. Perform a Fourier transform on the time-domain average
>    3. Subtract the Fourier transform coefficients from the Fourier  
> transform coefficients of each single trial
>    4. Calculate the amplitude spectrum of the subtracted Fourier  
> coefficents at each trial
> In theory, the resulting amplitude spectrum from step 4 should  
> isolate the induced response.  Before I implement the above steps, I  
> wanted to ask this list for opinions.
> Have others on this list found the procedure to be the most  
> effective method for isolating induced responses?  If not, what  
> methods have proved to be more effective?  Finally, are there EEGLAB  
> functions that can be manipulated (either by the appropriate  
> parameters or by small changes to the code itself) to perform some  
> or all of the above steps?
> Thanks in advance,
> Keith
> --
> Research Aide
> Belmonte Autism Lab
> Cornell University
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************************************************************************************
Guillaume A. Rousselet, Ph.D.

Lecturer

Centre for Cognitive Neuroimaging (CCNi)
Department of Psychology
Faculty of Information & Mathematical Sciences (FIMS)
University of Glasgow
58 Hillhead Street
Glasgow, UK
G12 8QB

The University of Glasgow, charity number SC004401

http://web.me.com/rousseg/GARs_website/

Email: g.rousselet at psy.gla.ac.uk
Fax. +44 (0)141 330 4606
Tel. +44 (0)141 330 6652
Cell +44 (0)791 779 7833


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