Single-fibre and whole-nerve responses to clicks as a function of sound intensity in the guinea pig

This paper describes a study of the intensity dependence of click-evoked responses of auditory-nerve fibres in relation to the simultaneously recorded compound action potential (CAP). Condensation and rarefaction clicks were presented to normal hearing guinea pigs over an intensity range of 60 dB. The recorded poststimulus time histograms (PSTHs) were characterized by the latency (t_p, amplitude (A_p) and synchronization (S_p) of their dominant peak, parameters that are particularly important for the understanding of the CAP. For all fibres t_p decreased monotonically with increasing intensity, in a continuous way for fibres with high characteristic frequency (CF 3 kHz), and in discrete steps of one CF-cycle for low-CF (CF≦ 3 kHz) fibres. An additional analysis of PSTH envelopes revealed that average latency shifts with intensity are similar for all CFs above 2 kHz. For the all fibres A_p increased monotonically with intensity; the increase was stronger and maximum values were larger for low-CF than for high-CF fibres. A schematic model PSTH was then formulated on the basis of the experimental data. A sum of these model PSTHs from a hypothesized fibre population was convolved with an elemental unit response (Versnel et al., 1992) in order to simulate the compound action potential. Synthesized CAPs agreed with experimental CAPs in their main aspects.