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Re: Is there considerable phase locking up to 6 kHz?

On Thursday, March 18, 2004 3:40 AM, Richard F. Lyon wrote:

> Martin, thanks for your comments.  I will be more than happy to adopt
> your suggestion and give up the old idea of "place pitch" as
> unsupportable by any evidence.
> However, I'm not sure I like your division of timing pathways.
> The first one (a), with "phase locking of resolved harmonics" seems
> to be rather narrowly construed.  Sound does not in general
> consistent of discrete sinusoids, since it is not in general
> periodic, so the mechanisms must be much more general.  Even for
> periodic sounds, partials are in general only partly "resolved".  The
> phase locking in the auditory nerve will follow the time structure of
> arbitrary waveforms at the different points along the cochlea.
> Conceptualizing in terms of sinusoids does more harm then good, in my
> opinion.

Yes, Dick, what the cochlea and then various nuclei in the auditory pathways
do is much more than delivering clean phase coupling to clean sinusoid
components. We see various techniques of band filtering and the final
results are probabilistic. But, as FFR studies showed (frequency following
responses recorded on the human scalp originating in the upper brain stem),
the neural population responses reflect a remarkably clean periodicity
coding upon complex periodic stimuli.

> The second pathway (b) seems funny, too.  Why would information be
> encoded "for the purpose of sound localization" and then used for a
> different purpose?

Because many, if not most, neurons in the cochlear nuclei and in the nuclei
of inferior colliculus are multiplexing units. Monaural delay coding in the
cochlear nuclei and binaural delay coding in nuclei of the superior olivary
complex does not occur in units that are exclusively engaged in sound
localization. Therefore, not only the results of delay analysis are
delivered upstream, but also the raw delay periods. The period detectors in
the midbrain then (periodicity tuned neurons) respond to these delays,
simply because they present period information.

The environments that our hearing has adapted to did not present stimuli
where proper and secondary pitch pathways can come up with conflicting
results. So there was no evolutionary advantage of fully separating the
mechanisms for sound localization and pitch extraction.


Martin Braun
Neuroscience of Music
S-671 95 Klässbol
web site: http://w1.570.telia.com/~u57011259/index.htm