Tactile discrimination of thickness.

The ability of human subjects to discriminate plane metal plates of different thickness was measured using of forced-choice paradigm. The plates, made by electroplating a thin layer of copper onto flat brass shims, were gripped between the thumb and the index finger. Subjects were presented with either 2 standard plates (0.2 mm thick), or a standard plate and a test plate that was slightly thicker, and were required in state which alternative had occurred. When the edges of the plates could not be touched, a difference in thickness of about 0.075 mm could be discriminated. Surprisingly, when the edges were included in the grip, performance did not improve. All hypotheses of strategies used by the subjects required them to sense the angles of the finger joints with a precision of about 0.1 degrees.