The ability to tell the direction of a motion across the skin deserve attention for being an easily observed function which provides a sensitive test for disturbances of the peripheral and central nervous systems. The mode of operation, on the other hand, of this tactile directional sensibility is still uncertain. 2. The dependence of directional sensibility on the contact load and distance of movement of a blunt metal tip, has now been determined for the skin of the forearm of normal subjects with the two-alternative forced-choice method. The testing was done under two conditions: elbow bent or straight. Straightening of the arm always reduced the accuracy of the directional sensibility. It also caused measurable changes of cutaneous mechanical properties, which presumably decreased the reliability of afferent information about lateral distension. 3. The average accuracy of the directional sensibility was found to be correlated linearly to the logarithm of the contact load, and straightening of the arm decreased the accuracy for each load by corresponding amounts. Similar relationships were found between the accuracy and the distance of movement. 4. Straightening of the arm did not cause any significant average reduction of the contact threshold for point stimulation of the same receptive field. A consistently lowered contact sensitivity, however, was observed for some of the subjects, which may have contributed to the reduction of the directional sensibility in these cases. 5. Correct directional estimations of the movement of the metal tip were obtained for a distance which was a fifth of the shortest distance for a corresponding estimation of the movement of a frictionless stimulus. The findings thus indicated that the friction between a moving object and the underlying skin, which can be mediated via stretch-sensitive cutaneous receptors, is critical for the determination of its direction of motion. 6. The present observations and previous observations by various authors are suggested to indicate that typical tactile directional sensibility depends on parallel processing of direction-selective data, and spatial data expressed as a function of time.
