THE AUDITORY PERIPHERY AS A SIGNAL PROCESSOR

Auditory signal processing begins long before the signal reaches the brainstem. The waveform is first split by the head into two waveforms. The pinnae introduce further spectral transformations that assist with estimating the elevation of the source. The ear canal and the middle ear action of the malleus incus and stapes attenuate potentially interfering very low and high frequency signals. Mechanical filtering of the signal in the cochlea can help to identify the spectral profile of the sound but may also have far reaching implications for segregating simultaneously present sounds sources through subsequent channel selection. The nonlinear response of the cochlea compresses the signal into a more manageable narrow dynamic range. It also cunningly fails to compress attenuated signals remote from best frequency with the effect of smearing information usefully across the array of mechanical filters. Electrical transduction at the inner hair cell transforms the signal in a number of ways. The process of adaptation makes it easier to identify the onset of a sound (or its modulation). Half-wave rectification and low pass filtering isolate the envelope of the signal for high frequency carriers while leaving the fine structure in place for low frequency signals. All this sophisticated signal processing occurs before the nervous system can become involved. As a consequence auditory modelling needs to replicate all of these processes if psychophysical phenomena are to be explained and predicted with quantitative accuracy. However, each one of these stages could easily occupy a lifetime of research. An important problem in modelling, therefore, is knowing how much detail is required and how much can be omitted. The answer lies in an understanding of what each stage contributes to the listening experience.