Nonlinearities in auditory-nerve fiber responses to bandlimited noise

Discharge rate was measured as a function of spectral level for noise bursts of one bandwidth and center frequency. Such rate-level functions were measured for a number of bandwidths; either the low- or high-cutoff frequencies were set at fiber characteristic frequency (CF). Rate-level functions were also measured, simultaneously, for single tones at CF. We define dynamic range as the range in descibels over which rate increases from 10% to 80% of the maximum driven rate to CF tones. When pooling data across CF in single cats, dynamic range is an increasing function of fiber threshold for CF tones and noise stimuli. Narrow bands of noise produce rate-level functions that are similar to those for CF tones. For noise bands centered above CF, rate-level functions become less steep as bandwidth is increased, and are always monotonic. For wide bands of noise centered below CF, rate-level functions can be nonmonotonic or appear to plateau at rates less than the saturation rate to CF tones. Thus, wide bands of noise centered above or below CF can produce lower discharge rates than do narrow bands at the same spectral level. This rate reduction has properties similar to those for two-tone suppression. The suppressive effects observed for bandlimited noise are most pronounced on low spontaneous units and least pronounced on high spontaneous units.