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Abstract

In visual perception, observers usually tend to underestimate the speed of moving objects. Given that in natural visual scenes static–or slowly moving–objects are more likely to occur, this finding has been interpreted in terms of a Bayesian prior for low velocity, supposedly encoding the statistics of natural visual scenes. Here we investigate whether an analogous tendency to underestimate velocity also occurs in audition. Given that in natural scenes objects are mostly static, if priors for velocity are modality-independent, observers should present the same bias towards slow motion also in audition. Alternatively, given that to produce sounds objects must move, if priors for velocity are modality-specific, participants should overestimate the velocity of moving sounds. In a psychophysical task, a set of loudspeakers was arranged along a circle to create a virtual auditory space in which sounds rotate around participants. On each trial, participants were presented with two consecutive moving sounds, and they had to judge which one was faster in a two-alternative forced choice task. Given that the influence of priors on perceptual estimates becomes stronger as the stimuli become noisier, we experimentally manipulated the signal-to-noise ratio of the moving auditory stimuli. Results showed that noisier stimuli appeared to move slower. This finding suggests the presence of an analogous prior for low velocity in both vision and audition. Notably, such a prior seems to operate in a modality-independent fashion by encoding the statistical properties of the world, rather than the properties of the stimuli reaching the individual senses.