The "half-octave shift in temporary threshold shift (TTS)" (Martin Braun )


Subject: The "half-octave shift in temporary threshold shift (TTS)"
From:    Martin Braun  <nombraun@xxxxxxxx>
Date:    Sun, 21 Oct 2007 14:52:03 +0200
List-Archive:<http://lists.mcgill.ca/scripts/wa.exe?LIST=AUDITORY>

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 perception. 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). References: 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. 295-312. 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. Martin --------------------------------------------------------------------- Martin Braun Neuroscience of Music S-671 95 Klässbol Sweden web site: http://w1.570.telia.com/~u57011259/index.htm


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