Tactile discrimination of curvature by humans using only cutaneous information from the fingerpads

Spherically curved surfaces were applied, with controlled force, to the fingerpads of human subjects; their fingers were immobilized. The curvature of the surfaces was characterised by the reciprocal of the radius of curvature. In scaling experiments, the subjects' perceived magnitude of curvature increased markedly with an increase in the curvature of the stimulus. An increase in contact force resulted in a slight decrease in perceived curvature. Four discrimination experiments were performed using a forced choice paradigm. Subjects could discriminate, at the 75% level, a flat surface (zero curvature) from a convex curvature of 4.9 m-1 (radius of curvature 204 mm) and from a concave curvature of 5.4 m-1 (radius 185 mm). When discriminating 2 convex spherical surfaces, subjects could discriminate a curvature of 144 m-1 from a curvature of 158 m-1 (radii 6.95 and 6.33 mm respectively), and could discriminate a curvature of 287 m-1 from one of 319 m-1 (radii 3.48 and 3.13 mm respectively); the Weber fraction is about 0.1. Contact areas between the curved surfaces and the fingerpad skin were estimated. There was approximate correspondence between contact areas and the scaling functions.