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Re: physiological or ecological basis of auditory sharpness
Yes, undoubtedly, high CFs are required for localization. I would like to
mention a nice tutorial by Duda:
He includes an explanation of IED (interaural envelope delay) and Franssen
effect, and he summarizes:
'With some risk of oversimplification, we can generalize and say that in
reverberant environments it is the high-frequency energy, not the
low-frequency energy, that is important for localization.'
The question was: 'What might be the basis of (the extremely unpleasant)
auditory sharpness evoked by spectral components...'
As we understand from the tutorial, the basal turn of cochlea is designed
for localization of single events like clicks or bat calls rather than for
recognition of any continuous tone. Maybe, Duda intentionally wrote
high-frequency 'energy' - not power - as to indicate that high power, i.e.
the product of energy and time, is not relevant for natural sounds. By the
way, I reiterate my opinion that so called acoustic power should not be
confused with the power acting on the hair cells because the OHCs are
largely powered from stria vascularis. One can imagine that high power puts
hair cells, etc. at risk. However, can this risk be directly signalized to
the brain? Perhaps not. Perhaps, very high frequency causes discomfort
partially for similar reasons as does very low frequency. The auditory
system increases gain because it cannot easily attribute a matching pitch.
Also, localized efferent feedback would not make sense for natural signals
being typically much too brief for that, while it is impossible for
frequencies below the lowest CF. Furthermore, cochlea has to cope with
unwelcome misleading synchrony. Spontaneous rate and threshold of auditory
nerve fibers are crucial for that, as well as for loudness perception. So
the physiological basis seems to be diverse. We don't have to resort in the
Gibbs effect, and 15 kHz does definitely not play any role in human