OBSERVATIONS ON HUMAN TACTILE DIRECTIONAL SENSIBILITY

1. The ability to tell the direction of a motion across the skin deser ves attention for being an easily observed function which provides a s ensitive test for disturbances of the peripheral and central nervous s ystems. The mode of operation, on the other hand, of this tactile dire ctional sensibility is still uncertain. 2. The dependence of direction al 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 nor mal subjects with the two-alternative forced-choice method. The testin g 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 a bout lateral distension. 3. The average accuracy of the directional se nsibility 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 b etween 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 consis tently lowered contact sensitivity, however, was observed for some of the subjects, which may have contributed to the reduction of the direc tional sensibility in these cases. 5. Correct directional estimations of the movement of the metal tip were obtained for a distance which wa s a fifth of the shortest distance for a corresponding estimation of t he movement of a frictionless stimulus. The findings thus indicated th at the friction between a moving object and the underlying skin, which can be mediated via stretch-sensitive cutaneous receptors, is critica l for the determination of its direction of motion. 6. The present obs ervations and previous observations by various authors are suggested t o indicate that typical tactile directional sensibility depends on par allel processing of direction-selective data, and spatial data express ed as a function of time.