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Abstract:
(Invited Talks)
The main structure of the inner ear is located in nail-shaped
fluid-filled cavity of the temporal bone, called cochlea. This is
partitioned by a diametrally clamped strip of graded elasticity,
called basilar membrane (BM), on which a very sophisticated
structure, called organ of Corti, is installed. The organ of Corti
can be roughly modeled as a dense array of strongly nonlinear
harmonic oscillators interacting with each other by long-range
hydrodynamic forces, the proper frequencies of which decrease
exponentially along the BM so as to cover tonotopically a range of
some tens kHz. The cochlea is not only the seat of acoustic sensors
but also an active system capable of performing smart nonlinear
filtering of acoustic information. Throughout it, the acoustic
signals transmitted to the cochlear fluid by a piston driven by the
middle-ear ossicles, is parallelized, processed and interfaced with
the central nervous system in a manner that appears optimized for
the detection and discrimination of sound-source characteristics:
timbres, formants of voiced-sounds, sound transients etc. Among the
most relevant properties of this structure, one seems even
paradoxical: the cochlea works like a wide-band filter and a
frequency-selective filter at the same time. In this way, the
responses elicited tonotopically along the cochlea can follow
quickly any sort of sudden input-signal changes - like attacks,
interruptions, frequency changes - while preserving excellent
frequency discrimination capability. This happens because frequency
selectivity is not achieved by sharp tuning, as occurs in filter
banks, but depends on an effect, called tone-to-tone suppression,
which is caused by the combined action of certain nonlinear and
nonlocal characteristics of cochlear dynamics. Tone-to-tone
suppression makes the responses to frequency components of
sufficient intensity cancel out those to tones of proximal
frequency but lower intensity. This allows, in particular, a
spectacular improvement of the signal-to-noise ratio and explains
why hearing losses in humans caused by the weakening of cochlear
functionality cannot be compensated for by simple amplification of
the acoustic input.
Renato Nobili was born in Adria (RO, Italy) on 1 November
1937. Professor of General Physics at the Physics Department of
Padova University from 1970 to 1984 and Assistant Professor in
Institutions of Theoretical Physics since 1972. His research
activity passed from elementary particle physics to biophysics
and theoretical biology in 1984. Since 1991 his research was
mainly devoted to the development of a reliable theory of the
non-linear dynamics of the mammalian cochlea.
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