| Otoacoustic emissions are sounds produced by the cochlea of the inner ear. For example, if two tonal stimuli at frequencies f1 and f 2 are presented to the ear, a third spectral component can be measured in the ear canal at frequency 2f1-f2, named the cubic distortion product otoacoustic emission (DPOAE). For low levels of f 1 and f2, the dominant cause of distortion is believed to be the nonlinear operation of the cochlear amplifier, the biochemically-powered internal motor behind the ear's remarkable frequency selectivity and sensitivity. Since most cases of hearing loss result from an impairment of this amplification process, DPOAEs provide a fast, objective test of hearing status. However, because their levels correlate only weakly with audiometric thresholds, use of DPOAEs in the clinic is limited to a pass or fail screening for hearing defects.; In a recent paper, David Mills (J Acoust Soc Am 103:507–523) proposed a novel algorithm for estimating the gain of the cochlear amplifier, not from DPOAE level, but from the manner in which the level decreased when a third stimulus tone, the suppressor, was applied simultaneously with f1 and f2. In this thesis, the validity of Mills' algorithm is investigated using two approaches. First, a relationship between suppression and gain is derived on the basis of a small number of assumptions motivated by established empirical findings. Second, it is shown, to a very high degree of certainty, that there exists a relationship between DPOAE suppressibility and hearing threshold (an indirect measure of active gain) in humans. It is concluded that DPOAE suppression is a direct, noninvasive window on the gain of the cochlear amplifier. It has tangible diagnostic potential and warrants further study. |
