Effect of hydration on the tactile and thermal sensitivity of the lip.

The vermilion lip is a body site particularly susceptible to water loss. Therefore, the role of hydration in tactile perception at the lip was investigated. A series of measures of tactile performance and response were obtained from 22 female subjects, namely: (1) the subjective assessment of lip feel, (2) tactile sensitivity, (3) spatial acuity, (4) thermal sensitivity, and (5) the subjective assessment of thermal stimulation. These measures were obtained from lips in their natural (untreated) state, and lips that had been treated using a hydrating preparation. The preparation altered the subjective feel of the lips consistent with the treatment increasing lip hydration and compliance. Hydrated lips showed greater sensitivity to light touch, and there was a trend toward the lip's thermal sensitivity being altered consistent with the lip treatment having a physical cooling effect. Spatial acuity was unaltered by the state of lip hydration. The sensitivity changes on hydration were proposed to have mechanical basis.