Diversity of characteristic frequency rate-intensity functions in guinea pig auditory nerve fibres

Rate-intensity functions at characteristic frequency (CF) were recorded from single fibres in the auditory nerve of anaesthetised guinea pigs. Within the same animal, CF rate-intensity functions, although probably forming a continuum, could be conveniently divided into three groups; (1) Saturating; reach maximum discharge rate within 30 dB of threshold, (2) Sloping-saturation; initially rapid growth in discharge rate leading to a slower growth in discharge rate but not saturating and (3) Straight; approximately constant increase in firing rate per decibel increase in sound pressure up to the maximum sound pressures used. Thresholds for individual fibres were plotted relative to compound action potential thresholds at the appropriate frequency. Fibres with straight CF rate-intensity functions had the highest thresholds. Fibres of the saturating CF sloping-saturation CF rate-intensity type had thresholds intermediate between saturating and straight. There was a close relationship between the type of CF rate-intensity function exhibited by a fibre and its spontaneous discharge rate. Fibres with saturating CF rate-intensity functions generally had high spontaneous discharge rates (greater than 18/s), whereas those with straight CF rate-intensity functions generally had low spontaneous discharge rates (less than 0.5/s). The majority of fibres with sloping-saturation CF rate-intensity functions had spontaneous rates between 0.5/s and 18/s. There was a negative correlation (r = -0.59) between the logarithm of the spontaneous discharge rate and relative threshold at CF with the lowest spontaneous rate fibres having the highest thresholds and vice-versa. This diversity of CF rate-intensity functions has functional implications for both frequency and intensity coding at high sound pressures in the mammalian auditory system.