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Re: AW: Cochlear nonlinearity & TTS

Navid, Richard and the listees,

I have heard a lot of speculation about the cochlear amplifier for many years. One of the questions that I have wondered about
as a signal processing engineer for many years, is with all the sophisticated nonlinearities, delays, amplifiers, filters
etc that are present in the auditory periphery, how does it "represent" an acoustic signal in the neural spike patterns
that emanate from the auditory periphery? (I guess everyone wonders about it.)
Is it possible to reconstruct the acoustic signal if you were able to measure/monitor the
spike patterns that are put out by all the auditory nerve fibers? What is the reconstruction 'algorithm"?
(I know about Egbert deBoer's reconstruction method for a single nerve fiber.) Is'n't the information about the signal
distributed across many, many nerve fibers? Should'nt the reconstruction take information from
all nerve fibers and fuse them to reconstruct the signal? Just wondering aloud. RK

Richard F. Lyon wrote:

At 9:17 AM -0800 1/16/07, Navid Shahnaz wrote:

Thank you Reinhart for your clarification. Does the cochlear amplifier works on both sides of the excitation pattern peak on the BM? or the amplifier operates wore efficiently at a place that is just above or toward the apex from the point of disturbance created by travelling wave? Operationally this point may be an ideal point as it is less likely saturates the amplifier due to sharp slope of the travelling wave on the apical side.


Both Monita and Reinhart have given good explanations, but let me add a bit.

The way I think of it, the active amplification is active everywhere, but it competes with the passive loss mechanisms, and is only significant at low enough levels. The active loss mechanism (damping) increases rapidly apically when a sine wave travels past a characteristic place. Because of the active gain, the response to a sine wave can travel further before it damps out; from the "passive peak" that Reinhart mentions, the peak response location can be further apical, up to about a half octave worth of place further, when the active amplification is significant, to the "active peak". The "net" amplification is positive (in dB per mm or whatever) before the response peak, and negative after the response peak, pretty much by definition of peak. That net includes the active gain, which saturates, and the passive loss, which doesn't, so it's level dependent.

In addition to the saturation that reduces the active gain at high level, there is also efferent control that turns down the gain in response to afferent response level and possibly other central control signals. This effect of efferent control of mechanical gain has been directly demonstrated, but I don't recall exactly who/when/where to cite right now.


fn:Ramdas Kumaresan