Evidence of stochastic resonance in an auditory discrimination task may reflect response bias

Stochastic resonance is a mechanism that improves the detection of weak periodic signals when an optimal level of background noise is present. This counterintuitive concept is thought to involve a cooperative effect that arises out of a physical coupling between deterministic and random dynamics in nonlinear systems. We report the results of three psychophysical experiments designed to probe the auditory system for evidence of stochastic resonance. In all three experiments, participants judged which of two 1000-Hz tones was greater in intensity, with one of the tones embedded in a background of Gaussian noise. Experiment 1 used a two-alternative forced choice task and provided evidence of enhanced discriminability at an optimal level of noise. The aim of Experiment 2 was to determine whether the apparent enhancement observed in Experiment 1 was due to sensory processes or to a response bias. Receiver operating characteristic analysis supported an explanation based on decision processes, rather than a genuine sensory enhancement. An additional experiment (Experiment 3) was used to discount experimental artifacts.