Multisensory integration in speed estimation during self-motion.

This study assessed the relative contributions of visual and proprioceptive/motor information during self-motion in a virtual environment using a speed discrimination task. Subjects wore a head-mounted display and rode a stationary bicycle along a straight path in an empty, seemingly infinite hallway with random surface texture. For each trial, subjects were required to pedal the bicycle along two paths at two different speeds (a standard speed and a comparison speed) and subsequently report whether the second speed travelled was faster than the first. The standard speed remained the same while the comparison speed was varied between trials according to the method of constant stimuli. When visual and proprioceptive/motor cues were provided separately or in combination, the speed discrimination thresholds were comparable, suggesting that either cue alone is sufficient. When the relation between visual and proprioceptive information was made inconsistent by varying optic flow gain, the resulting psychometric functions shifted along the horizontal axis (pedalling speed). The degree of separation between these functions indicated that both optic flow and proprioceptive cues contributed to speed estimation, with proprioceptive cues being dominant. These results suggest an important role for proprioceptive information in speed estimation during self-motion.