The influence of spatial summation on human tactile directional sensibility

Spatial summation is known to influence the magnitude of sensation for stationary cutaneous stimuli. Yet analysis of moving stimuli may also be pertinent, since most stimuli that attract our attention involve movements over the skin surface. The present investigation dealt with the importance of spatial summation for the appreciation of the direction of motion for moving stimuli. The ability to detect the direction of motion was tested on the radial surface of the forearm with the two-alternative forced-choice method. Stimulation was performed with a rolling wheel, in order to exclude friction-generated activation of stretch receptors. Each subject was tested with two wheels with the same radius but different widths, 1 mm and 15 mm. On average, the subjects performed better with the wide wheel than with the narrow one for stimulation distances > or = 16 mm. This value also probably exceeds the threshold distance for directional discrimination for the narrow wheel, which indicates that spatial summation improves suprathreshold performance.