Re: granular synthesis and auditory segmentation

[meijer@xxxxxxxxxxxxxxxxxxxxxxxxxxx]

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!
http://ourworld.compuserve.com/homepages/Peter_Meijer/

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