The Auditory System - Overview

Why the auditory system?

To study information processing in groups of neurons, it is helpful to use a system with well defined inputs, and well defined anatomy. There are several systems that meet these criteria, including various invertebrate preparations. However in vertebrates the requirement for well defined inputs is met best at the peripheral levels of sensory systems. 

The auditory system has two principal advantages over the other sensory systems for the study of information processing mechanisms:

• The representation of sound in the auditory nerve has been well studied over the past 40 years, and responses to simple sounds, such as tones, noises and clicks, as well as sounds as complex as portions of speech, have been sufficiently well characterized that sophisiticated computational models exist that capture many features of the nerve's responses. The auditory nerve represents these sounds in a fairly consistent way from one fiber to the next. Each auditory nerve fiber receives input from a single cochlear inner hair cell, which imparts sensitivity to a limited range of frequencies, similar to a bandpass filter. Individual fibers representing one frequency vary in their sound level threshold (the intensity of sound at which they change their firing rate), and this is (inversely) correlated with their spontaneous activity in silence. Thus, the input to the brain, although not completely homogeneous, is well characterized and can be reasonably represented computationally.

The first stage of auditory information processing occurs in the cochlear nucleus, where the relatively uniform discharge of auditory nerve fibers is transformed into a set of distinct ascending parallel pathways that emphasize different features of the sensory environment. These transformations can be understood in terms of the types of analysis that the auditory system must perform to accomplish its numerous perceptual tasks. This divergence of processing underlying these transformations is supported by defined subsets of cochlear nucleus neurons that have distinct patterns of afferent synaptic connection, dendritic architecture, neurotransmitter receptor expression, and ion channel expression. The ability to examine the mechanisms used by different cell types to process an essentially common input allows a comparative approach, and has helped us to understand the importance of different neuronal mechanisms in accomplishing different information processing tasks. The apparent direct correspondence between ion channel expression and the types of integration carried out by particular auditory neurons has particularly helped us understand the mechanisms of information processing in these cells.