Gender-, side- and site-dependent variations in human perioral spatial resolution.

Twenty-eight right-handed, young adults participated in a sensory testing experiment to evaluate spatial resolution at 10 positionally matched sites on the right- and left-hand sides of the face. An adaptive psychophysical (i.e. tracking) procedure was used to estimate the threshold spatial separation for perceiving two points of contact at each site. Estimates of the threshold at one site on both sides of the face were also obtained with a method-of-limits procedure similar to that employed for clinical evaluation of patients. In addition, each individual was asked to rate (i) his(her) overall facial sensitivity to touch and (ii) the degree to which he(she) could discern subtle changes in lip, cheek and chin position during speech, chewing and facial expression. Analysis of the estimates of the threshold separation obtained with the tracking procedure revealed a significant effect of gender (p < 0.04) and of site (p < 0.001). Females were more spatially sensitive than males: average threshold separations were 1.55 mm less. Most notably, the threshold increased ninefold with distance posterolaterally from the oral opening. The vermilion of the upper lip was the most spatially sensitive site (population geometric mean = 2.4 mm) and the preauricular skin the least spatially sensitive site (20.9 mm). Significant effects of side and of interactions among gender, side and site were not observed. The estimates obtained with the method-of-limits procedure were very similar to those obtained with the tracking procedure: the latter were 0.67 mm less on the average. Individuals' ratings of overall facial sensitivity to touch were similar for males and females (p > 0.70). Females, however, reported greater ability to discern subtle changes in lip, cheek and chin position than males (p < 0.03). The ratings of this sensory function correlated negatively with the estimates of the threshold separation on the vermilion of the upper lip (p < 0.03).