Why Wet Feels Wet? An Investigation Into the Neurophysiology of Human Skin Wetness Perception

Page ii ABSTRACT The ability to sense humidity and wetness is an important sensory attribute for many species across the animal kingdom, including humans. Although this sensory ability plays an important role in many human physiological and behavioural functions, as humans’ largest sensory organ i.e. the skin seems not to be provided with specific receptors for the sensation of wetness (i.e. hygroreceptors), the neurophysiological mechanisms underlying this complex sensory experience are still poorly understood. The aim of this Thesis was to investigate the neurophysiological mechanisms underpinning humans’ remarkable ability to sense skin wetness despite the lack of specific skin hygroreceptors. It was hypothesised that humans could “learn” to perceive the wetness experienced when the skin is in contact with a wet surface or when sweat is produced through a complex multisensory integration of thermal (i.e. heat transfer) and tactile (i.e. mechanical pressure and friction) inputs generated by the interaction between skin, moisture and (if donned) clothing. Hence, as both thermal and tactile skin afferents could contribute significantly to drive the perception of skin wetness, their role in the peripheral and central sensory integration of skin wetness perception was investigated, both under conditions of skin’s contact with an external (dry or wet) stimulus as well as during the active production of sweat. A series of experimental studies were performed, aiming to isolate the contribution of each sensory cue (i.e. thermal and tactile) to the perception of skin wetness during rest and exercise, as well as under different environmental conditions. It was found that it is not the contact of the skin with moisture per se, but rather the integration of particular sensory inputs which drives the perception of skin wetness during both the contact with an external (dry or wet) surface, as well as during the active production of sweat. The role of thermal (cold) afferents appears to be of a primary importance in driving the perception of skin wetness during the contact with an external stimulus. However, when thermal cues (e.g. evaporative cooling) are limited, individuals seem to rely more on tactile cues (i.e. stickiness and skin friction) to characterise their perception of skin wetness. The central integration of conscious coldness and mechanosensation, as sub-served by peripheral cutaneous A-nerve fibers, seems therefore the primary neural process underpinning humans’ ability to sense wetness. Interestingly, these mechanisms (i.e. integration of thermal and tactile sensory cues) appear to be remarkably consistent regardless of the modality for which skin wetness is experienced, i.e. whether due to passive contact with a wet stimulus or due to active production of sweat. The novelty of the findings included in this Thesis is that, for the first time, mechanistic evidence has been provided for the neurophysiological processes which underpin humans’ ability to sense wetness on their skin. Based on these findings, the first neurophysiological sensory model for human skin wetness perception has been developed. This model helps explain humans’ remarkable ability to sense warm, neutral and cold skin wetness.