Re: physiological or ecological basis of auditory sharpness ("Daniel J. Tollin" )

Subject: Re: physiological or ecological basis of auditory sharpness
From:    "Daniel J. Tollin"  <tollin(at)PHYSIOLOGY.WISC.EDU>
Date:    Mon, 16 Sep 2002 16:23:52 -0500

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(at) 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 290 Medical Sciences Building tollin(at) 1300 University Avenue Madison, WI 53706

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