Visual, tactile and visuo-tactile motion discrimination

Abstract

We investigated visual and tactile motion perception by measuring velocity discrimination thresholds over a wide range of base velocities and spatial frequencies (0, 3.3, 5 & 10 c/deg). The stimuli used were physical wheels etched with a sinewave profile that could be seen and felt. Two similar wheels were simultaneously driven at specific velocities (either congruent or in conflict, in the same or opposite directions) by two independent, computer controlled, motors. Subject was presented with two separate intervals and required to discriminate in 2AFC interval contained the faster movement, using only visual, only tactile or bimodal information (viewing distance was maintained constant at 57 cm). Both bimodal and unimodal visual and tactile thresholds showed a characteristic “dipper function”, with the minimum at a given “pedestal duration”. The “dip” (indicating facilitation) occurred over the same velocity range (0.05 - 0.2 cm/sec) at all spatial frequencies and conditions. At detection levels we found no direction-specific facilitation for visual-tactile motion, but an overall improvement (about root two) in the bimodal detection and discrimination thresholds, that were well predicted by the maximum likelihood estimation model. Our results suggest that visual and tactile systems analyze motion with similar sensitivities, and information between them is integrated in an optimal manner.