In this chapter we illustrate the relevance of stochastic resonance to auditory neural coding. This relates to natural auditory coding and also to coding by cochlear implant devices. Cochlear implants restore partial hearing to profoundly deaf people by trying to mimic, using direct electrical stimulation of the auditory nerve, the effect of acoustic stimulation.

Introduction

It is not surprising that the study of auditory neural coding involves stochastic phenomena, due to one simple fact – signal transduction in the ear is naturally a very noisy process. The noise arises from a number of sources but principally it is the Brownian motion of the stereocilia (hairs) of the inner hair cells that has the largest effect (Hudspeth 1989). Although the Brownian motion of the stereocilia appears small – typically causing displacements at the tips of the stereocilia of 2–3 nm (Denk and Webb 1992) – this is not small compared with the deflection of the tips at the threshold of hearing. Evidence exists that suggests, at threshold, the tip displacement is of the order of 0.3 nm (Sellick et al. 1982, Crawford and Fettiplace 1986), thus yielding a signal-to-noise ratio (SNR) of about –20 dB at threshold. Of course, under normal operating conditions the SNR will be greater than this, but, at the neural level, is typically of the order of 0 dB (DeWeese and Bialek 1995).

Given the level of noise and the fact that neurons are highly nonlinear makes the auditory system a prime candidate for observing stochastic resonance (SR) type behaviour.