Review of the roles of temporal and place coding of frequency in speech discrimination

Numerous studies have demonstrated that the frequency spectrum of sounds is represented in the neural code of single auditory nerve fibres both spatia lly and temporally, but few experiments have been designed to test which of these two representations of frequency is used in the discrimination of co mplex sounds such as speech and music. This paper reviews the roles of plac e and temporal coding of frequency in the nervous system as a basis for Fre quency discrimination of complex sounds such as those in speech. Animal stu dies based on frequency analysis in the cochlea have shown that the place c ode changes systematically as a function of sound intensity and therefore l acks the robustness required to explain pitch perception tin humans), which is nearly independent of sound intensity. Further indication that the plac e principle plays a minor role in discrimination of speech comes from obser vations that signs of impairment of the spectral analysis in the cochlea in some individuals are not associated with impairments in speech discriminat ion. The importance of temporal coding is supported by the observation that injuries to the auditory nerve, assumed to impair temporal coherence of th e discharges of auditory nerve fibres, are associated with grave impairment s in speech discrimination. These observations indicate that temporal codin g of sounds is more important for discrimination of speech than place codin g. The implications of these findings for the design of prostheses such as cochlear implants are discussed.