[Eeglablist] plotphase in time-frequency analysis

Dorothy Bishop Dorothy.Bishop at psy.ox.ac.uk
Sat Jul 10 09:48:37 PDT 2010


Thanks to those who have commented on the blog I did on ERSP.
I am writing a bit more, but have a query about phase of ITC.

I've created simple simulated waveforms, made by summing sine waves with some variation from trial to trial of phase,  and adding a bit of noise, and then setting phase to be constant just for part of the waveform, t1 to t2.
 I was surprised to see that where I would have expected a block of positive ITC (ie in the interval t1-t2), I had instead a sequence of blocks of +ve and -ve ITC.
Visually there is an abrupt transition from blocks of red to blocks of  blue.

I can get the output to look as I expect it by setting 'plotphase' to 'off', and I notice that in some sample scripts, this option is set.

Does this mean that the polarity of ITC is irrelevant? I had thought that +ve meant phase was positively correlated from trial to trial, and -ve mean it was negatively correlated, but i then started to wonder whether the circular nature of phase meant that when something is totally in phase it might be equally likely to come out as + or -? 
Yours a bit confusedly.
thanks






Dorothy Bishop
Professor of Developmental Neuropsychology
Department of Experimental Psychology
University of Oxford
OX1 3UD
Website: http://psyweb.psy.ox.ac.uk/oscci/
Blog: http://deevybee.blogspot.com/
tel: +44 (0)1865 271369
fax: +44 (0)1865 281255
 





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