Processing of Dynamic Spectral Properties of Sounds

Abstract

This chapter reviews the ways in which neurons at different levels of the auditory pathway respond to frequency modulations (FMs). The chapter describes the responses to FM at successive levels of the auditory pathway, and identifies trends in the ways dynamic changes in frequency are represented. Specificity for parameters such as the direction of frequency change is apparent at all levels above the auditory nerve (AN). However, at more central locations there is evidence for great specificity of response, topographic mapping of FM parameters, and differences in response properties between cortical subdivisions. There is considerable variation between datasets, and it is striking that many neurons respond best to rates of frequency change that are unlikely to occur in natural sounds. The chapter explains that the responses of neurons in the auditory pathway to frequency sweeps and sinusoidal FM depend on several parameters. Several features in the responses to sweeps observed at the cortical level have already emerged at the lowest levels of the auditory pathway. The relationship between the effective instantaneous frequency of the FM sweep and the slope of the frequency response area is apparent in the responses of cochlear nerve fibers, and selectivity for sweep direction is present in the responses of some neurons in the CN. However, there are quantitative differences in the responses between brainstem and cortical areas, and synaptic activity in the cortex enhances directional selectivity.