Haptic feature extraction.

This study examined the process by which the shape of a haptically explored object is synthesized from the geometric characteristics of simpler constituent elements, such as arcs and ellipses. Subjects traced the outlines of virtual objects by means of whole arm movements. Each object consisted of the union of a large central ellipse and two smaller circles, extending upward and outward from the top left and right sides of the base. The sizes of the two circles and the eccentricity of the elliptical base were varied. After exploring the object's contour in the absence of vision, subjects reproduced the sensed shape by means of freehand drawing. Speed and force were modulated during the exploratory phase in a manner that suggested that subjects reacted to rather than predicted changes in curvature. Also, subjects typically devoted more time to exploring the part of the contour encompassing the two smaller circles. During drawing, individual features of the explored shape were reproduced with varying degrees of fidelity. Aspects related to the size and location of the smaller circles were reproduced better than was the eccentricity of the ellipse forming the base. Since subjects spent proportionally less time exploring the base, these results suggest that subjects selectively focused attention to regions of high spatial contrast and that the exploratory strategy introduced distortions in the haptically sensed shapes.

Effect of compliance on haptic perception of curvature.

The haptic sense of geometric properties such as the curvature of a contour is derived from somatosensory cues about the motions and forces experienced during exploratory actions. This study addressed the question of whether compliance, the relationship between force and displacement, influences haptic perception of curvature. Subjects traced a curved 30 cm long compliant contour by grasping the handle of a manipulandum and reported whether the contour curved towards or away from them. The contour at which there was equal probability of responding either way was taken to represent one that was sensed as being straight. The compliance of the contour was varied, being constant, greatest in the middle or greatest at the ends. Subjects exhibited a bias in what they sensed to be a straight edge. However, the actual handpath that was judged to be straight did not vary across the three compliance profiles. Our results rule out a hypothetical strategy in which an intended motion is planned and the actual trajectory is then inferred by sensing force feedback. Another strategy in which the force against the contour is controlled and the handpath is inferred from proprioceptive feedback is more consistent with the observations.