pitch and unresolved harmonics (Chris Darwin )

Subject: pitch and unresolved harmonics
From:    Chris Darwin  <cjd(at)biols.susx.ac.uk>
Date:    Fri, 11 Oct 2002 10:36:41 +0100

Martin Braun wrote:

>I suggest we should never forget that this "host of very diverse stimulus
>configurations" is a lab artefact. No ears or brains are in any way adapted
>to "iterated ripple noise, or high-pass filtered click trains". They are not
>even adapted to "pure tones". These things do not occur outside the lab.
>
>Outside the lab all sound is broad-band noise with varying amounts of
>periodic components. If these are strong enough, we hear pitch, otherwise we
>don't.
>
>The advantage of detecting a pitch is that it helps the animal to identify
>and follow a sound source. In order to do this, the animal needs ONE pitch
>detector, not several. More than one would only lead to a more diffuse
>pitch.
>
>It is obvious that complex sound with only unresolved harmonics produces a
>pitch perception. But such sound is non-existent in a natural environment,
>and this pitch percept is weaker and slower than that caused by natural
>sound with resolved harmonics. Experiments with unnatural periodic sound are
>useful to test the limits of certain systems, but they never justify the
>conclusion that we have two or more pitch detectors. It would be more
>plausible to say that, upon artificial periodic sound, non-pitch systems can
>deliver information to the pitch system as a by-product.
>

While I sympathise with Martin Braun's view that purely from the point of view
of perceiving the pitch of  naturally occuring complex the resolved harmonics
are particularly important, we should also remember that harmonicity serves as a
way of binding together sounds across the whole spectrum.  The mechanisms that
estimate the periodicity of unresolved harmonic clusters are used to group those
clusters with the appropriate resolved harmonics for the purpose of identifying
the timbre of the sound.  If there is a mismatch between the periodicity of the
unresolved harmonics and of the resolved for a particular sound source, then our
ability to separate two simultaneous speech sounds or sentences suffers.  So an
unresolved-harmonic 'pitch' mechanism is certainly ecologically justified, but
not necessarily for the pitch percept.

Culling, J. F. and Darwin, C. J. (1993). "Perceptual separation of simultaneous
vowels: within and across-formant grouping by Fo," J. Acoust. Soc. Am. 93,
3454-3467.

Bird, J. and Darwin, C. J. (1998). "Effects of a difference in fundamental
frequency in separating two sentences," in Psychophysical and physiological
advances in hearing edited by A. R. Palmer, A. Rees, A. Q. Summerfield and R.
Meddis (Whurr, London), pp. 263-269.

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Chris Darwin, Experimental Psychology,
University of Sussex, Brighton BN1 9QG, UK
Phone: +44-1273-678409;  FAX: +44-1273-678058
Email: cjd(at)biols.susx.ac.uk
Home Page: http://www.biols.susx.ac.uk/Home/Chris_Darwin/
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