SPATIAL CUES SERVING THE TACTILE DIRECTIONAL SENSIBILITY OF THE HUMANFOREARM

1. Tactile directional sensibility is considered to rely on the parall el processing of direction-contingent sensory data that depend on skin stretching caused by friction, and spatial cues that vary with time. A temperature-controlled airstream stimulus that prevented the activat ion of stretch receptors was used to investigate directional sensibili ty for the skin of the forearm. 2. The dependence on contact load and distance of movement was determined for normal subjects with a two-alt ernative forced-choice method. Testing was performed under two conditi ons, elbow bent or straight, Bracing the skin by straightening the arm did not alter the accuracy of the directional sensibility, in contras t to previous findings with stimuli that caused friction. 3. The accur acy of directional sensibility was correlated linearly to the logarith m of the distance of movement of the air jet. No correlation was found between accuracy and contact load, unlike findings with stimuli that cause friction. 4. Measurements were made with different subjects to d etermine the threshold distance at constant load. On average, subjects were able to distinguish direction with movements of less than or equ al to 8 mm. This acuity is sharper than has been reported with static stimuli. There was no correlation between subjects' threshold distance s for judging direction and spatial acuity measured with absolute poin t localization. 5. The ability to distinguish direction was poor for t he airstream stimulus compared with stimuli causing frictional contact with hairy skin. Nevertheless, the present findings are consistent wi th the suggestion that cutaneous spatial acuity is better for dynamic than for static stimuli.