The use of motion cues in the haptic sense of circularity.

We tested the hypothesis that speed cues are used to haptically identify changes in the curvature of the hand's trajectory. Subjects grasped the handle of a robotically-controlled manipulandum that was moved in the horizontal plane along various elliptical arcs following one of three different speed profiles. In one profile, a circular arc was traced at a constant speed whereas in the other two speed was constant for ellipses whose aspect ratios differed from unity. A two-alternative forced choice procedure was used to identify the ellipse that was sensed to be circular in each of the three experimental conditions. In unconstrained movements, speed varies with the radius of curvature. If speed cues are used to identify curvature during passive movements, one would expect that subjects' responses should be biased towards the ellipse traced at a constant speed. The results did not support this hypothesis, indicating that speed cues are not a major contributor in the haptic sensing of shape.