Human, tactile, directional sensibility and its peripheral origins

Tactile directional sensibility is probably functionally important and deserves attention as it is known to be sensitive to many different disturbances of the somatosensory system. Therefore, the ability of healthy adults to determine the direction of motion of a light tactile stimulus travelling proximally or distally along a straight line on depilated, hairy skin of the forearm was examined with two-alternative, forced-choice technique. The aim was to investigate the relative importance of different types of afferent information which may be used for this purpose. A test was started with the moving stimulus covering a distance of no less than 2.5 mm, which was subsequently increased until the subject could report the direction of motion reliably. Afterwards, the distance was decreased until the subject could no longer do so. Three different stimulation conditions were used and for a point stimulator touching the skin it was found that the necessary distance decreased to 2.5 mm after a moderate increase of the vertical contact load. No such decrease was found when a frictionless air-stream point stimulator was used instead. The distances which had to be covered by the point stimulator touching the skin increased to values which were comparable to those obtained with the air-stream stimulator after the lateral extensibility of the skin had been diminished. This was achieved by attaching a surgical sticky plaster around the stimulated skin area. The present findings consequently indicated that optimal, tactile, directional sensitivity depends on peripheral afferent messages which signal the direction of lateral stretching of the skin.