Why wet feels wet? A neurophysiological model of human cutaneous wetness sensitivity

Abstract

Although the ability to sense skin wetness and humidity is critical for behavioral and autonomic adaptations, humans are not provided with specific skin receptors for sensing wetness. It has been proposed that we "learn" to perceive the wetness experienced when the skin is in contact with a wet surface or when sweat is produced through a multisensory integration of thermal and tactile inputs generated by the interaction between skin and moisture. However, the individual roles of thermal and tactile cues and how these are integrated peripherally and centrally by our nervous system is still poorly understood. Here we tested the hypothesis that the central integration of coldness and mechanosensation, as subserved by peripheral A-nerve afferents, might be the primary neural process underpinning human wetness sensitivity. During a quantitative sensory test, we found that individuals perceived warm-wet and neutral-wet stimuli as significantly less wet than cold-wet stimuli, although these were characterized by the same moisture content. Also, when cutaneous cold and tactile sensitivity was diminished by a selective reduction in the activity of A-nerve afferents, wetness perception was significantly reduced. Based on a concept of perceptual learning and Bayesian perceptual inference, we developed the first neurophysiological model of cutaneous wetness sensitivity centered on the multisensory integration of cold-sensitive and mechanosensitive skin afferents. Our results provide evidence for the existence of a specific information processing model that underpins the neural representation of a typical wet stimulus. These findings contribute to explaining how humans sense warm, neutral, and cold skin wetness.

Neurophysiology of Skin Thermal Sensations

“…lower back), but that also, these correspond to regions presenting greater cold sensitivity (85). Finally, within the context of regional variability on wetness perception, it is worth noting that, while limited psychophysical evidence is available on whether skin wetness is perceived differently between hairy and glabrous skin, a recent study from this author has shown that, owing to its potentially higher thermal sensitivity, hairy skin presents higher wetness sensitivity than the glabrous skin of the finger pad (98).…”

“…Evidence for the fact that the central integration of inputs from peripheral thermal and tactile A-type nerve afferents could represent a specific sensory strategy developed by humans to sense wetness, has been recently provided in one of this author's studies (98).…”

“…This simple observation has led to a number of investigations exploring how other somatosensory stimuli (e.g. mechanical and tactile) could therefore contribute to the perception of skin wetness in the absence of cold sensations (16,98,103). As a result of such studies, evidence has emerged in support of the fact that when thermal cues are insufficient, individuals seems to rely on mechanical and tactile inputs (e.g.…”

“…stickiness) to sense skin wetness. This is particularly true under conditions of sweat-induced skin wetness (16,98,103). For example, under conditions of sweat-induced skin wetness, and especially when wearing clothing, skin wetness perception has been shown to be driven primarily by tactile stimuli in the form of intermittent mechanical interaction between skin, sweat and clothing (100,109,120,191).…”

“…To date, only two human studies have tackled the problem of what peripheral and central neural mechanisms (in the form of activity in first order thermo-and mechano-sensitive neurons and of central perceptual integration) could be involved in sensing wetness and humidity on our skin (98,205). These studies have shown that it reasonable to hypothesize that the peripheral mechanisms of thermo-and mechano-sensory integration could provide the neurophysiological bases for the first level of integration required for being able to sense skin wetness.…”