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The "half-octave shift in temporary threshold shift (TTS)"
On Oct 11, Alain de Cheveigne' asked:
If you have a pure-tone generator, you'll have it even easier. Pull up the
sound level from 60 dB to 90 dB. Do you, or don't you, hear the
half-octave shift of BM tuning?
Non sequitur. Perhaps you can elaborate?
Yes, of course. Sorry for the delay. The question was if there is a
perceptional correlate to the apparent half-octave shift of basilar membrane
(BM) tuning when the sound level is changed from 60 dB to 90 dB.
Actually, there is a well-known perceptional correlate. But this can only be
observed under specific experimental conditions, which are described below.
Under normal conditions, a level shift from 60 dB to 90 dB does not shift
the perceptional quality of a pure-tone except for the loudness shift.
Because half an octave corresponds to ca 2 mm of cochlear length, the
spectral information for the brain would clearly differ, if the BM really
was the primary effector of hair cell excitation. The absence of a shift in
spectral perception is one of the many indications that the BM is NOT the
primary effector of hair cell excitation.
Now to the perceptional correlate that CAN be observed under certain
conditions. It is the effect of the "half-octave shift in temporary
threshold shift (TTS)", which is extensively documented in the literature
and which was a widely discussed topic in earlier decades of hearing
research (for a review, McFadden, 1986). A typical example of the effect is
that a high-level exposure at 2 kHz can cause a TTS at 3 kHz, while leaving
the hearing threshold at 2 kHz fully intact.
These experiments have not only demonstrated that the BM mechanics protects
hair cells against damage due to high-level sound exposure. They have also
demonstrated that it dampens hair cell exposure at the characteristic
frequency for sound levels between 60 and 90 dB. Because the sound level for
speech normally lies in the range of 60-80 dB, we can assume that the
damping mechanism of the BM also plays a significant role in speech
Patients suffering from endolymphatic hydrops, such as patients affected by
Meniere's disease, usually have a compromised BM mechanics due to the
expansion of the endolymph in the scala media of the cochlea. These patients
also suffer from the perceptional correlates of loudness intolerance and
reduced speech comprehension (Braun, 1996).
McFadden D (1986). The curious half-octave shift: evidence for a basalward
migration of the traveling-wave envelope with increasing intensity. In:
Salvi RJ, Henderson D, Hamernik RP, Coletti V (Eds). Basic and Applied
Aspects of Noise-induced Hearing Loss. Plenum Publishing, New York, pp.
Braun, M (1996) Impediment of basilar membrane motion reduces overload
protection but not threshold sensitivity: evidence from clinical and
experimental hydrops. Hear. Res. 97, 1-10.
Neuroscience of Music
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