Audio-visual integration in the perception of tap dancing

Psychophysical studies show that, except under certain specific circumstances, auditory and visual motion signals do not summate in a direction-specific manner, implying summation between the two sensory sources is more probabilistic than physiological. With biological motion, however, there is some evidence that the summation is directionally specific and more substantial (Brookes et al, Neuropsychologia, 2007). We have measured thresholds for discriminating light-point tip-tap dancing scenes using visual, auditory and combined information. A dance sequence was filmed with three distinct visual markers positioned on each foot. Both the visual and audio sequences were then thresholded, and the visual markers and auditory taps substituted with circular disks and a stereotyped tap, to produce controlled light-point sequences. Subjects were required to discriminate in forced-choice which of two 3-sec noise-embedded intervals contained a light-point dance sequence (rather than a scrambled sequence). Sensitivity was defined as the noise level corresponding to 75% correct discrimination. After equating visual and auditory sensitivity with uni-modal measurements, sensitivity was measured bimodally with the auditory sequence in-phase and out-of-phase. The in-phase presentation improved thresholds substantially, by a factor of about 2, while the out-of-phase presentation only by ¼ this value, more consistent with probability summation. The results suggest that unlike simple translational motion, biological motion shows complete audio-visual integration.