The effect of reverberation on the representation of complex sounds in the lower auditory pathway

We rarely, if ever, have to detect sounds in quiet and the ability to distinguish signals under adverse conditions e.g. background noise or reverberant environments has important survival consequences. We are steadily increasing our knowledge of the neural mechanisms that exist to cope with background noise but we know little, if anything, about the neural response to reverberation. When listening to a sound in an enclosed space only part of the sound arrives directly at the listeners ears; the rest of the sound reflects from surrounding surfaces. These reflections arrive at the ear with various delays and sound levels and together form the reverberation. Most of us will have struggled with the effects of reverberation at some time in our lives, for example, being placed on speaker phone or listening to conversations in noisy rooms (e.g. cafeterias). These struggles are usually exacerbated with age. An increased understanding of the neural responses to sounds in the presence of reverberation will help in our quest to reduce its problems for the hard of hearing and in developing intelligent prostheses. At the end of this project we will have a much clearer idea about how the auditory nervous system responds to reverberation and also how it may compensate for its deleterious effects.

The sound waveform reaching the ear is typically made up of a mixture of sounds originating from a variety of different sources. In reverberant conditions this mixture of sounds also consists of reflections from nearby surfaces. The challenge for the brain is to interpret the sound waveform reaching the ear in terms of the events that produced it; this is often referred to as a 'scene analysis' problem. When discussing this 'scene analysis' problem Bregman (1990) distinguishes between primitive grouping, which involves the (probably innate) use of general properties of sound to determine the source of acoustic elements, and schema-based grouping, which is generally learned and therefore dependent on the listener's specific experience. Both processes influence the grouping of complex stimuli like speech. We hypothesize that the primitive grouping is a low-level phenomenon, taking place in the early stages of the auditory pathway. There are probably both low level and high level processes involved in the segregation of sounds in a reverberant environment. For instance pre-teenage children and elderly listeners (with normal hearing) are more susceptible to the effects of reverberation. These age related effects, combined with individual subject differences in susceptibility to the effects of reverberation argue for a high level process. However, underlying this high level process is a set of low level processes which we hypothesize operate in the auditory periphery and which underlie many of the perceptual phenomena associated with reverberation including perceptual compensation. Chief amongst these peripheral processes we suggest is adaptation. Thus, paradoxically, while adaptation may underlie some of the problems associated with reverberation it may also be partly responsible for perceptual compensation from reverberation.