Effects of traverse length on human perioral directional sensitivity.

The capacity of 8 neurologically healthy adults to distinguish direction of motion on the skin overlying the mental foramen was determined. The velocity, orientation, and the length and width of skin traversed by the moving tactile stimuli were precisely controlled. Directional sensitivity, d', was found to depend on both stimulus velocity and the length of skin traversed. Since the relationship between d' and velocity at each traverse length was well described by a generalized gamma function, it was possible to quantitatively characterize the effects of changes in traverse length on the relationship between d' and velocity. Specifically, peak (i.e., maximal) directional sensitivity increased as the length of skin traversed was increased, yet the velocity which resulted in peak directional sensitivity (i.e., the optimal or model velocity) remained invariant over the range of traverse lengths investigated (0.35-1.0 cm). The effect of stimulus velocity on directional sensitivity was least at the longest traverse lengths used. The generalized gamma function model fit the relationship between directional sensitivity and velocity equally well at all traverse lengths studied. The results lead us to anticipate that stimuli of the type used in this study should prove valuable for the detection and quantification of disturbances in orofacial tactile spatiotemporal integration in patients with peripheral nerve injury.