Haptic and Visual Perception of an Affordance for Upright Posture

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Abstract
Perception of whether a slanted surface supports upright stance was evaluated visually and haptically. Observers stood approximately 1 m from a 1 m x 1 m plywood board whose angle of inclination varied randomly among seven settings (12-degrees, 17-degrees, 22-degrees, 27-degrees, 33-degrees, 39-degrees, and 45-degrees). For visual trials, observers looked at the surface. For haptic trials, the observers were blindfolded and explored the surface with a hand-held wooden dowel. The perceived behavioral category boundary (i.e., the slope that received 50% yes responses) did not differ for visual and haptic judgments. Although the two perceptual systems did not differ in the mean, participants took longer to make haptic judgments and were less confident in doing so. For both perceptual systems, response time increased and confidence decreased at the category boundary. A second experiment used ascending and descending methods of limits and found both enhanced contrast and hysteresis effects for both perceptual systems. Differences between perceptual systems were discussed in the context of efficiency of perception and attunement. Similarities between perceptual systems were considered in the context of amodal invariants and dynamical systems.