Nonlinear effects of noise on phase-locked cochlear-nerve responses to sinusoidal stimuli

It is well known that, in a cochlear afferent axon with background spike activity, a sinusoidal stimulus (tone) of sufficiently low frequency will produce periodic modulation of the instantaneous spike rate, the alternating half cycles of which comprise excursions above and below the mean background spike rate. It also is known that if the amplitude of the stimulus is sufficiently small, the instantaneous spike rate follows very nearly a sinusoidal trajectory through these positive and negative excursions. For such cases, we define the AC responsiveness of a primary auditory afferent axon to be the amplitude of sinusoidal modulation of the instantaneous spike rate divided by the amplitude of the tone producing that modulation. In the experiments described in this paper, changes in AC responsiveness were followed during and after sudden changes in the background noise level. When the amplitude of the tone was sufficiently small relative to that of the noise, we found that the AC responsiveness can be strongly dependent on the time elapsed since the last change in noise level, while being nearly independent of the amplitude of the tone itself. Under those circumstances, after transitions between noise levels 20 dB apart, we observed changes in AC responsiveness that consistently followed time courses similar to those of the short-term mean (background) spike rate (approximating the adapting response to the noise alone), unfolding over several milliseconds or tens of milliseconds. At the time of the transition between noise levels, there was another change in AC responsiveness, which appeared to be instantaneous; as the noise level increased, the AC responsiveness immediately increased with it. This seemingly paradoxical effect and the similarity of the time courses of AC responsiveness and short-term mean spike rate both are consistent with a simple, descriptive model of spike generation involving the shifting of threshold along a bell curve.