Anisotropy and spatial tactile acuity on human lips

ObjectiveTo investigate tactile anisotropy on human lips.MethodsSpatial tactile acuity was assessed with a three-alternative, forced-choice grating orientation task. Circular probes with horizontal (parallel to lip), vertical (perpendicular to lip), or oblique (45° right of vertical) grooves and ridges of equal width were applied (n = 60) to the midline of each lip. Participants (n = 13) were asked to state the grating orientation whilst blindfolded. The percentage of correct responses was plotted as a function of the log gap width. Data were fitted with a four-parameter sigmoid function. Response bias was assessed (n = 13) with application of a smooth polished Perspex probe. 65.5%, 71.5%, and 63.0% correct was adopted as the threshold estimate for the vertical, horizontal, and oblique orientations based on the measured response bias.ResultsAcross orientations, the threshold on the upper lip (1.5 ± 0.9 mm) was significantly greater than on the lower lip (1.0 ± 0.7 mm; P = 0.006). However, there was no significant main effect of orientation or orientation-by-lip interaction on threshold.ConclusionTactile anisotropy is absent on human lips.SignificanceThe lack of anisotropy is surprising given that anisotropy is present on fingers and that afferent input and sensory processing for human lips and fingers share similarities.

► Spatial tactile acuity is greater on the upper lip than on the lower lip in humans. ► A property of spatial tactile acuity known as anisotropy (or directional dependence) is absent on human lips. ► The lack of anisotropy on human lips is surprising given that anisotropy is present on the finger and that mechanoreceptor afferents that supply human lips exhibit similar characteristics to those of the hand.