Re: granular synthesis and auditory segmentation (at)

Subject: Re: granular synthesis and auditory segmentation
Date:    Mon, 19 Oct 1998 10:49:56 +0200

I'd like to thank you all for the lively discussion that followed my somewhat provocative reply to Richard Fabbri: > I don't know about your neurons, but mine completely fail > to replenish their synapses above about 1 or 2 kHz even > after plenty of coffee. Of course there is a role for > non-Fourier type processing too, but no simple scheme > covers the entire audible [20 Hz, 20 kHz] range. After sending this note, I realized that reactions would probably follow about the 4-5 kHz correlations in nerve fibers, and I appreciated Peter Cariani's clear account of the "volley principle" that easily explains how, e.g., 1 kHz neurons can lead to higher frequency periodicities in (stochastic) nerve activity. Also, I'd like to stress that I do not favour either a place or temporal theory of hearing, but I do expect (any) temporal processing to occur mainly via the various auditory channels that first result after mechanical (place-type) filtering has taken, umm, place. Again in line with what Didier Depireux said: > The half-wave rectification occurs _after_ the frequency > decomposition performed on the basilar membrane, i.e. > after you have decomposed the signal into frequency > channels. Temporal information is still available in these channels, so both "theories" can therefore be complementary, and there are many indications that they *are* complementary, with significant temporal processing at lower frequencies, under 1 or 2 kHz. I'd like now to present my simplified view of the world, and would appreciate hearing from the auditory experts how wrong my view is (totally wrong, or just inaccurate): 1. Human auditory neurons fail to replenish their synapses above 1 or 2 kHz. (Just like I had already stated.) 2. The "volley principle" leads to higher frequency periodicities in neural activity of the auditory nerve, say up to 4 or 5 kHz. This is confirmed by neurophysiological measurements (mainly in cats). 3. [My hypothesis] The *higher* frequencies (say 3 - 5 kHz) that arise due to the "volley principle" do *not* lead to *psychophysically* observable effects, in spite of the fact that the higher frequencies are measured *neurophysiologically* (invasively) within the auditory nerve. Reasoning behind my hypothesis: - Psychophysical effects involve auditory processing through much more wetware than only the auditory nerve, and the initial phase locking *could* easily be lost "along the road". - Binaural masking level differences (MLD's) are significant only up to 1 or 2 kHz (e.g., 1500 Hz mentioned in Brian Moore's Psychology of Hearing). My own informal auditory display experiments have confirmed these values. - Significant evolutionary pressure would be expected to try and raise the above MLD transition frequency to improve sound localization accuracy and cocktail party type filtering, so if it still didn't get us beyond 2 kHz there, why would it occur in any other psychophysically observable auditory effect? - Binaural phase detection also fails above ~1500 Hz. Note that I am not an auditory perception expert, and I am only trying to increase my understanding of the subject through (my) conjectures and (your) falsification. I'd love to hear about *psychophysical* auditory perception experiments that unambiguously demonstrate temporal processing in humans in the 3 to 5 kHz range! My expectation is that such results have not been found... Best wishes, Peter Meijer Soundscapes from The vOICe - Seeing with your Ears! McGill is running a new version of LISTSERV (1.8d on Windows NT). Information is available on the WEB at

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Electrical Engineering Dept., Columbia University