Multi-sensory integration of spatio-temporal segmentation cues: one plus one does not always equal two

How are multiple, multi-sensory stimuli combined for use in segmenting spatio-temporal events? For an answer, we measured the effect of various auditory or visual stimuli, in isolation or in combination, on a bistable percept of visual motion ("bouncing" vs. "streaming"). To minimize individual differences, the physical properties of stimuli were adjusted to reflect individual subjects' sensitivity to each cue in isolation. When put into combination, perceptual influences that had been equipotent in isolation were substantially altered. Specifically, auditory cues that had been strong when presented alone were greatly reduced in combination. Evaluation of alternative models of sensory integration showed that the state of the visual bistable percept could not be accounted for by probability summation among cues, as might occur at the level of decision processes. Instead, the state of the bistable percept was well predicted from a weighted sum of cues, with visual cues strongly dominating auditory cues. Finally, when cue weights were compared for individual subjects, it was found that subjects differ somewhat in the strategy they use for integrating multi-sensory information.