Dear Navid and List,
Two-tone suppression is explained, e.g., in Figure 1.21 of "An Introduction to the Psychology of Hearing" by B.C.J. Moore (Academic Press, 5th edition, 2003). Abscissa: frequency (of sine-tones). Ordinate: sound pressure levels.
Open circles: tuning curve (threshold versus frequency) of a single neuron with characteristic frequency of 8 kilohertz. Lowest point of tuning curve: 8 kilohertz, 10 decibel.
An open triangle indicates the first tone: sine-tone of 8 kilohertz, 27 decibel.
Black circles: sound pressure level of a second tone which, at each chosen frequency, leads to a reduction of the response of the above-mentioned neuron by exactly 20 percent.
The lowest black circle is at 26 decibel, 10 kilohertz (i.e., 0.32 octave higher than 8 kilohertz).
This second tone leads to a strong basilar-membrane vibration at the 10-kilohertz characteristic place, about 1.5 mm basal of the 8-kilohertz characteristic place. At the 10-kilohertz place, the 8-kilohertz travelling wave starts to be amplified by the cochlear amplifier (i.e., by the outer hair cells). That amplification is hindered by the 10-kilohertz vibration.
The effects of the sound pressure levels can be inferred, e.g., from Fig. 14 of the review "Mechanics of the Mammalian Cochlea", by Robles and Ruggero [Physiological Review 81 (2001) 1305-1351].
The curve of basilar-membrane displacement versus position has an "active" peak, comparatively high at low sound pressure levels, and, a few millimetres basal, a "passive" peak, comparatively high at high sound pressure levels. In humans, the distance between the two peaks is about 3 millimetres.
The active peak is comparatively weak at high levels because the outer hair cells are "saturated", i.e., are unable to deliver the amplification factor which they deliver at lower levels.
Dr. phil. nat.,
r. PSI and ETH Zurich,
Phone: 0041 56 441 77 72.
Mobile: 0041 79 754 30 32.
E-mail: reinifrosch@xxxxxxxxxx .
Datum: 15.01.2007 17:46
Betreff: Cochlear nonlinearity & TTS
I've been trying to find a good explanation on why TTS occur about half an octave to an octave higher than than the exposure frequency. When you look at BM displacement patterns (Johnstone, 1986; Ruggero et al., 1997) as a function of frequency for a given center frequency at multiple levels you will notice that first the cochlea will lose its nonlinearity at the best frequency at high levels and the best frequency shifts more toward the lower frequencies (apicalward); however, when you look at travelling wave on the BM for a given center frequency at multiple levels the best frequency shifts toward higher frequencies (basalward) with increasing levels (Ren, 2002). This level dependent shift has been proposed as an explanation for a shift in TTS. My question is why displacement of BM for the CF is more toward the apical side at high levels while the travelling wave is basalward. The latter proposes that the amplifier should be more apical to the CF; therefore, damaging this area will result in a shift in threshold toward more basal side. Your clarification is highly appreciated.
Navid Shahnaz, Ph.D., Aud. (C)
School of Audiology & Speech Sciences
Faculty of Medicine
University of British Columbia
5804 Fairview Ave., J. Mather Building
Vancouver, BC Canada V6T 1Z3
Tel. 604- 822-5953