Re: What do we hear in High frequency hearing

[Søren Buus <buus@xxxxxxx>]

Based on psychophysical evidence, we also reached the conclusion that the high-frequency 
limit of the audiogram usually is due to the characteristic frequency at the very basal end of the cochlea.
We called it the "end of cochlea" hypothesis.

Buus, S., Florentine, M., and Mason, C. R. (1986). "Tuning curves at high frequencies and their relation to the absolute threshold curve," in Auditory Frequency Selectivity, edited by B. C. J. Moore and R. D. Patterson (Plenum, New York), pp. 341-350.

As for the mechanism for ultrasound perception, it seems possible that the
steep high-frequency slope of the "last" tuning curve does not continue
indefinitely. It could be that a limited attenuation in the rejection band on 
the high-frequency side of the auditory filter allows high-level ultrasound
to excite the inner hair cells at the basal end of the cochlea. The ultrasound
thresholds are usually quite high. For example, Corso (1963, Vol 35, 1738-1743)
report thresholds of 110 to 130 dB re 1.0 microbar for frequencies between 20
and 90 kHz. This compares to thresholds around 50 or 60 dB re 1.0 microbar
for frequencies between 6 and 14 kHz. Of course, it is difficult to 
translate the bone-conduction thresholds into SPLs
of air-conducted sound that would provide equivalent amplitude at, say,
the oval window, but the ultrasound thresholds seem high enough that they
could reflect finite attenuation in the rejection band of the auditory filter. 

Søren Buus

On Thursday, November 7, 2002, at 04:41 PM, David Mountain wrote:

> Those of us who work in cochlear mechanics generally believe that
> the high-frequency limit of the audiogram is due to the characteristic
> frequency at the very basal end of the cochlea (Ruggero and Temchin,
> 2002).  In the case of hearing ultrasound via bone conduction, it would
> seem that the acoustic signal would have to be bypassing the usual
> cochlear filtering process and stimulating the hair cells via a
> nonconventional path.  Otherwise, the very steep high-frequency slope
> associated with cochlear tuning would have severely attenuated the
> stimulus.