Overview: Cochlear Neurobiology

Abstract

All environmental and biologically significant sounds change from frequency to frequency and from instant to instant. The hearing organ must be capable of analyzing time-varying frequency components and representing them in a spatiotemporal array of neural discharges in the fibers of the auditory nerve. When a signal is analyzed, its frequency spectrum and temporal characteristics are reciprocally related. The simplest signal in terms of its spectrum is an ongoing sine wave, corresponding to a pure tone in acoustics, and has only one spectral component, the frequency of the sound. This simple spectral line is achieved by having a signal that, at least in theory, lasts forever. In contrast, the simplest temporal signal, an impulse, or a click in acoustics, is infinitesimally short in duration but, in theory, contains all frequencies. Of course, natural sounds are in a continuum between these extremes; they have finite duration and finite spectral breadth. However, the reciprocal relation holds. The shorter the signal, the broader its frequency content and, conversely, the longer the signal, the narrower the band of frequencies that represents it. A device constructed to analyze signals is constrained in the same way. In order to ensure good frequency resolution of the signal, one must process in narrow-frequency bands and observe for long durations. In contrast, to resolve short temporal features in the signal, the analysis must be relatively broad band. These are conflicting requirements for which advanced ears must find a solution. Let us inquire about the severity of the requirements.