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Cochlear nonlinearity & TTS

Dear List
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)
Assistant Professor
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
E-mail: nshahnaz@xxxxxxxxxxxxxxxxxx