[Eeglablist] ITC question

[Brian Roach]

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