Cochlear amplifier

The cochlear amplifier is a positive feedback mechanism within the cochlea that provides acute sensitivity in the mammalian auditory system. The main component of the cochlear amplifier is the outer hair cell (OHC) which increases the amplitude and frequency selectivity of sound vibrations using electromechanical feedback.

The cochlear amplifier was first proposed in 1948 by T. Gold. This was around the time when Georg von Békésy was publishing articles observing the propagation of passive travelling waves in the dead cochlea.

Thirty years later the first recordings of emissions from the ear were captured by D.T. Kemp. This was confirmation that such an active mechanism was present in the ear. These emissions are now termed otoacoustic emissions and are produced by what we call the cochlear amplifier.

The first modeling effort to define the cochlear amplifier was a simple augmentation of Georg von Békésy's passive traveling wave with an active component. In such a model, a lop sided pressure about the Organ of Corti is hypothesized which actively adds to the passive traveling wave to form the active traveling wave. Perhaps the most popular example of this model was defined by Neely, S.T. and Kim, D.O. The definition of the active traveling waves require forward and backward traveling waves to be generated in the cochlea, as proposed by Shera, C.A. and Guinan, J.J.

Contention still surrounds the existence of the active traveling wave. Recent experiments conducted by T. Ren show that emissions from the ear occur with such a fast response that the slowly propagating active traveling waves can not exist. The only explanation for fast emission propagation is the dual of the active traveling wave, the active compression wave. Active compression waves were proposed as early as 1980 by P.J. Wilson due to older experimental data. However, they were widely disregarded by the research community until stronger experimental proof confirming these early experiments against the active traveling wave was produced.

Thirty years after Kemp's experimental proof of the existence of Gold's cochlear amplifier and sixty years after the proposal of Gold's cochlear amplifier, the active-compression-wave cochlear amplifier was defined by M.R. Flax and W.H. Holmes. In this model the active pressure is equal on both sides of the Organ of Corti and this produces very fast propagating pressure waves which generate extra activity within the cochlea and emissions through the middle/outer ear. This original OHC compression model was taken further to explain a 'mixed mode' cochlear amplifier in 2011, where the apex and base of the cochlea are modelled by the same system proposed by Flax and Holmes, however experimentally capture different modes of stimulation as proposed by Guinan.

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