[Eeglablist] ITC question

[Scott Makeig]

Brian -

The ITC result is likely correct, and comes from phase-locked activity 
of a non-sinusoidal response (it should be more or less triangular). The 
40-Hz sinusoid does not have harmonic spectral content, so no harmonic ITC.
Best luck - see my website bib for my early papers and thesis chapters 
on auditory 40-Hz driving, if you'd like.

Scott Makeig


Brian Roach wrote:

> EEGlab users,
>
> We have data on 26 healthy controls in a steady state driving paradigm 
> where the subjects hear 500msec click trains with a 1 ms click every 
> 25 ms (20 clicks total).  This produces nice power changes and phase 
> locking in the 40 Hz range.  Here: 
> http://peppy.med.yale.edu/EEGlabData/question.html you can find a 
> grand average ITC plot for these subjects and this stimulus in the 
> lower figure.  What you may also notice is that there appears to be 
> harmonic phase locking at 80Hz, 120Hz, 160Hz, 200Hz, and 240Hz.  I am 
> wondering if these higher frequency values are actually a result of 
> higher frequency phase-locking, or could they be a result of the 
> analysis - does the 40 Hz signal produce lower coherence values for 
> 80Hz, 120Hz, etc.  I might guess that the latter is the case because 
> the corresponding ERSP plot for the 26 controls only shows positive dB 
> values at 40Hz in the ERSP graphic.  To try and figure this out, I 
> produced a fake data set with ~40 Hz pure sine waves from 0-500ms and 
> ran timef on it.  The result is in the upper plot at 
> http://peppy.med.yale.edu/EEGlabData/question.html.  I'm not sure why 
> this pure sine wav produces high coherence values at all frequencies 
> and not just around 40Hz, but the answer to that question is less 
> important to me than the earlier question.  Thanks in advance for any 
> info, and please let me know if there are any parts of my question 
> that I can clarify.
>
> Brian
>
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