Haptic aftereffect of curved surfaces.

A haptic aftereffect of curved surfaces is demonstrated. Two spherical surfaces were presented sequentially to human subjects. They rested one hand on the first (conditioning) surface. After a fixed conditioning period they transferred their hand to the second (test) surface and judged whether the test surface was convex or concave. In experiment 1 the curvature of the conditioning surface was varied; the subject's judgment of convexity or concavity of the test surface was strongly shifted in the direction opposite to the curvature of the conditioning surface (negative aftereffect). Therefore, subjects judged a flat surface to be concave after being exposed to a convex surface. After a conditioning period of 5 s the shift was about 20% of the curvature of the conditioning surface. In experiment 2 the duration of the conditioning period was varied; the magnitude of the aftereffect could be described by a first-order integrator with a time constant of 2 s. In experiment 3 the time interval between the conditioning period and the touching of the second surface was varied; the magnitude of the aftereffect could be described by an exponential decay with a time constant of 40 s. It is concluded that the haptic aftereffect of curved surfaces is an important effect that occurs almost instantaneously and lasts for an appreciable period.