Long Integration Time for Accelerating and Decelerating Visual, Tactile and Visuo-tactile Stimuli

The human visual system is good at discriminating speed but not acceleration. However, as speed is seldom constant, it is important to be able to extract speed in conditions of acceleration and deceleration. We measured visual, tactile and bimodal speed-matching over a wide range of accelerations and decelerations in a 2IFC procedure. Both visual and tactile stimuli were generated on physical wheels etched with a sinusoidal profile. During different experimental sessions the wheels could be seen, or touched, or both. Comparisons between different unimodal and bimodal matched speeds revealed similar integration times for the two modalities, in both cases around one second, suggesting that it occurs at a relatively high level of processing. Bimodal precision of speed discrimination was better than unimodal discrimination, as predicted by the maximum likelihood model of optimal integration.