Re: physiological or ecological basis of auditory sharpness

["Daniel J. Tollin" <tollin@xxxxxxxxxxxxxxxxxxx>]

At 04:58 PM 9/16/2002 +0800, you wrote:
>Dear List,
>
>I am looking for literature about the physiological or ecological basis of
auditory sharpness evoked by spectral components at very high frequencies
(> 15 kHz). Such tones sound extremely unpleasant because (2) they approach
the upper limit of the human auditory range, or (2) high frequencies have
special meanings in mammal communication. Any comments will be very much
appreciated.
>
>Chen-Gia Tsai
>gia@snafu.de


Hi,
        I don't know much about 'sharpness', but I do know that the late great
Bruce Masterton hypothesized that the selective pressure for the evolution
of high-frequency hearing in mammals was not for communication purposes per
se, but rather for accurate sound source localization (Masterton et al.,
1969).  What follows is a summary of his arguments.  In a comparative
study, Masterton and his colleagues found that small mammals with small
head diameters tended to have higher frequency hearing ranges than larger
animals with larger heads (Masterton and Diamond, 1969).  The diameter of
the head, which roughly equals the distance that the two ears are
physically separated, is a primary factors contributing to the interaural
time difference (ITD) cue for sound localization.  A small head means a
small range of ITDs that are available for sound localization.  For
example, in some mammals with very small heads, such as some species of
bats and rodents, the maximum ITD experienced can be less than 50
microseconds.  These same mammals also have extraordinarily high frequency
hearing.  Humans experience a maximum ITD that is an order of magnitude
larger, and we have a much more limited range of hearing in terms of
frequency.
        What does high-frequency hearing do for localization?  The other binaural
cue for location, interaural level differences (ILDs), are created in part
due to an acoustic 'shadowing' effect whereby the amplitude of the stimulus
arriving at the ear opposite the source is reduced because some of the
acoustic energy in the stimulus is reflected from the leading side of the
head.  But this only occurs for sounds with wavelengths on the order of or
smaller than the diameter of the head.  What this means is that, roughly,
the smaller the head, the higher the frequency at which ILDs of any
appreciable magnitude are established.  The important thing Masterton
pointed out was that, acoustically, even though the ITDs in small mammals
could be extremely small and probably of limited usefulness for accurate
localization, the magnitude of the ILD could be quite substantial and
therefore very useful for localization PROVIDED that the frequency of the
sounds were high enough.  Yet in order for these high-frequency ILD cues to
be useful at all, the organisms must be able to hear them.  Hence, their
sensitivity to high frequencies.

Masterton B, Heffner H, and Ravizza R (1969). The evolution of
high-frequency hearing, JASA 45: 966-985.

Masterton B and Diamond IT (1969). Hearing: Central neural mechanisms. In:
Carterette EC and Friedman MP (eds) Handbook of Perception: Biology of
Perceptual Systems, Vol 3. New York, Academic, pp 408-448.


Cheers,

Daniel J. Tollin, Ph.D.
Assistant Scientist

University of Wisconsin                  FAX: (608)-265-5512
Department of Physiology                             Phone:(608)-265-5143
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