Dependence of subjective traverse length on velocity of moving tactile stimuli.

Two series of experiments were performed to assess the effects of stimulus velocity on human subjects' perception of the distance traversed by a moving tactile stimulus. In all experiments, constant-velocity stimuli were applied to the dorsal surface of the left forearm; velocities ranging between 1.0 and 256 cm/sec were used. In some experiments the stimuli moved from distal to proximal over the skin, and in others they moved from proximal to distal. The length of skin contacted by the moving stimulus was defined by a plate having an aperture of 4.0 X 0.5 cm. In the first series of experiments, subjects were required to compare the distance traversed by a test stimulus delivered 2 sec after a standard stimulus, and also to report the on-locus and the off-locus of the brushing stimulus. In the second series of experiments, the subjects rated the perceived distance on the skin using a free-magnitude-estimation procedure. The data from both series of experiments defined the same relationship between stimulus velocity and perceived stimulus distance. More specifically, although the length of skin contacted by the stimulus was the same at all velocities, subjects' estimates of stimulus distance decreased with increasing stimulus velocity. In addition, the function relating estimates of stimulus distance to velocity was flat for velocities between 5 and 20 cm/sec, but possessed an appreciable negative slope at lower and higher velocities. It is interesting that the plateau of the relationship between perceived stimulus distance and velocity occurred within the range of velocities that human subjects employ to scan textured surfaces; it also corresponded precisely with the range of stimulus velocities at which the directional sensitivity of somatosensory cortical neurons and human subjects is optimal.