Subject: Re: comodulation release of masking (CMR) From: Neil Todd <TODD(at)FS4.PSY.MAN.AC.UK> Date: Mon, 5 Oct 1998 21:57:29 GMTDear Steven > >> >I think there is no consensus about the processing of CMR >> >stimuli although we recently argued that the envelope cross correlation >> >(not to mistake with the envelope cross covariance or correlation >> >coefficient) may be helpful in accounting for CMR data >> >(van de Par and Kohlrausch, 1998a/b: JASA 103 pp 3605-3620; 1573-1579). >> >In the paper on page 3605 we even describe a model that accounts >> >for CMR data. >> > >> >Steven >> > >> > >> >> I think we are agreed that some form of cross-correlation mechanism is >> probably a good horse to bet on, I wonder though if you could clarify >> your distinction between different forms of cross-correlation. > >The difference between both forms of correlation can >be seen looking at their definitions: > >In other words: for the cross covariance the mean (or DC) >component of the envelopes is first removed (the -<x> and >-<y> terms in the equation). > >This removal of the DC component is what is problematic about >the cross covariance in my opinion. To see this, consider two tonal >carriers at different frequencies that are modulated >with the same sinusoidal modulator except that the >modulators differ in phase by either zero or pi radians. >Now subjects have to detect the change in modulator phase. > >Independent of the modulation depth the envelope cross covariance >will either be +1 or -1, indicating >that detectability of modulator phase difference >is independent of modulation depth. This is a problem >because eventually, when the modulation depth is very small >subjects will have great difficulties detecting modulator >phase differences. > >The envelope cross correlation doesn't suffer >from these problems because the envelope cross correlation >will depend on the modulation depth. > >> The >> cross-correlation mechanism I proposed in Todd (1996, Network: Computation >> in Neural Systems. 7, 349-356) was a product-moment on the cosine phase >> of the envelope modulation power spectrum. > >If I read this correctly you calculate correlations between envelope spectra >instead of correlations between temporal envelopes. >In that case the DC component of the envelope will be preserved >within the spectrum. As far as I can see there will be no >problems with regard to the type of stimuli that I described >above. > >Best wishes, > >Steven van de Par > Ok, I'm with you now. Yes, you are correct to say that the envelope power spectrum preserves the DC as you call it (although all psychophysical stimuli are of finite duration, hence the 'DC' is really a relatively slow AC). So, whether one computes the cross-covariance or cross-correlation of envelope power spectra, the resultant value will be sensitive to modulation depth in the signals you describe. Such a measure is effectively a second order correlation, since an indirect estimate of power spectrum may be obtained from the Fourier transform of the autocorrelation. In other words it's a cross-correlation between two indirect autocorrelations, and hence a hybrid accross-channel/within-channel mechanism. As I say, I think there is agreement on the existence of some kind of cross-correlation mechanism as mediating grouping, but how that mechanism is realized in the brain is another matter. Cheers Neil McGill is running a new version of LISTSERV (1.8d on Windows NT). Information is available on the WEB at http://www.mcgill.ca/cc/listserv