EFFORTFUL TOUCH WITH MINIMAL MOVEMENT

Perception of the extents of occluded rods was examined under conditions in which a rod was held as steadily as possible. A given rod was held horizontally, on one side of its center of mass, with one upward (U) and one downward (D) force. Extent was perceived when D and U were distributed over the surfaces of 1 hand, 2 hands, and a hand and a knee and if only D or only U was provided anatomically, the other being provided by an environmental support. Increasing the distance between D and U decreased perceived extent when both contact points were anatomical and when only 1 was anatomical. The first moment of the mass distribution, a constant, affected perceived extent more than gravitational torque, a variable. Rods of different lengths but of the same first moment were not distinguished. Results are discussed in terms of functionally equivalent, haptic perceptual instruments and the detection of invariants.