Is the visual system functionally adapted to the normal relationship between self-motion and its perceptual consequences? The perceived speed of visual motion that is qualitatively consistent with self-motion is reduced during walking (see Gigone & Durgin, this VSS). According to a Barlow’s (1991) inter-sensory inhibition model, the function of such speed reduction would be to improve visual discrimination of expected speeds during walking. Such speed subtraction should improve sensory discrimination for faster visual speeds at the cost of impairing the discrimination of lower speeds. Using 2AFC comparisons of visual speeds presented in a head-mounted display, we measured both (1) relative perceived visual speed while walking and while standing and (2) the perceptual discrimination of visual speeds viewed while walking or standing. The apparent reductions in perceived visual speed that resulted from walking corresponded to about 50% of walking speed. As predicted, discrimination of low visual speeds was impaired during walking, whereas discrimination of faster visual speeds was reliably improved. Although raised thresholds at low visual speeds, could be attributed to signal noise produced by headset jiggle, reduced JNDs for faster visual speeds provide strong evidence of improved absolute visual speed discrimination. In essence, it would appear that the coding space for visual speeds during walking is constricted to a smaller set of values within which greater precision of discrimination can be achieved at the cost of losing discrimination for low (and therefore unlikely) visual speeds. Clifford and Wenderoth (1999) have demonstrated a similar improvement in motion discrimination following simple motion adaptation. The present results are consistent with treating perceived speed reduction during walking as a contingent motion aftereffect with the important functional property of improving sensitivity to expected visual motion while walking.

Swarthmore College Faculty Research Grant, HHMI