Re: Cochlear nonlinearity & TTS

[Monita Chatterjee <mchatterjee@xxxxxxxxxxxx>]

Navid:

The two are just flip sides of the same coin. At a particular place, the
peak of the frequency response shifts to lower frequencies as you
increase intensity. This means, for instance, that the place that is
tuned to, say, 2000 Hz at low intensities will respond most to, say,
1500 Hz at a somewhat higher intensity. Thus, basal locations respond
more and more to lower frequency tones as the intensity increases. If
you think in terms of the excitation pattern on the basilar membrane or
the traveling wave, you'll see that this means that, for a fixed
frequency input (say, 1500 Hz), the peak of the excitation pattern
shifts basalward as intensity increases.

Hope this helps..

Monita


M Chatterjee, Ph.D.
Asst Professor, Hearing and Speech Sciences
0100 LeFrak Hall
University of Maryland, College Park
College Park, MD 20742
(301) 405 7716
>>> Navid Shahnaz <nshahnaz@xxxxxxxxxxxxxxxxxx> 01/15/07 11:46 AM >>>
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.
Best
Navid
--------------------------------------------------------------------
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
Fax.604-822-6569
E-mail: nshahnaz@xxxxxxxxxxxxxxxxxx
Website:http://www.audiospeech.ubc.ca/school/faculty/navid/