A frequency-dependent saturation evident in rate-intensity functions of the chinchilla auditory nerve

The shape of rate-intensity functions recorded from individual neurons of the auditory nerve using stimulus frequencies at and below the characteristic frequency have been both well-characterized and modeled by other researchers. However, previous studies of rate-intensity functions using stimulus frequencies above the characteristic frequency have primarily focused on the slopes of the rising phases of the functions. Hence, they did not determine whether rate-intensity functions recorded using stimulus frequencies above the characteristic frequency saturate, and, if so, at what firing rates the saturation occurs. In this study, rate-intensity functions have been obtained from neurons of the eighth nerve of the chinchilla in response to gated, sinusoidal stimuli in order to investigate saturation firing rates for frequencies above the characteristic frequency. For each neuron, rate-intensity functions were obtained for stimulus intensities up to 90 dB SPL at the characteristic frequency and at as many frequencies above the characteristic frequency as time would allow. These data clearly reveal that, for frequencies above the characteristic frequency, saturation occurs at a rate that decreases monotonically as the frequency of stimulation is increased. In addition, an empirical equation is given which summarizes the dependence of saturation on stimulus frequency for the data of this study.