Noise improves modulation detection by cochlear implant listeners at moderate carrier levels

Envelope detection and processing are very important for cochlear implant (CI) listeners, who must rely on obtaining significant amounts of acoustic information from the time-varying envelopes of stimuli. In previous work, Chatterjee and Robert [JARO 2(2), 159-171 (2001)] reported on a stochastic-resonance-type effect in modulation detection by CI listeners: optimum levels of noise in the envelope enhanced modulation detection under certain conditions, particularly when the carrier level was low. The results of that study suggested that a low carrier level was sufficient to evoke the observed stochastic resonance effect, but did not clarify whether a low carrier level was necessary to evoke the effect. Modulation thresholds in CI listeners generally decrease with increasing carrier level. The experiments in this study were designed to investigate whether the observed noise-induced enhancement is related to the low carrier level per se, or to the poor modulation sensitivity that accompanies it. This was done by keeping the carrier amplitude fixed at a moderate level and increasing modulation frequency so that modulation sensitivity could be reduced without lowering carrier level. The results suggest that modulation sensitivity, not carrier level, is the primary factor determining the effect of the noise.