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.

Skin temperature detection thresholds have been used to measure human cold and warm sensitivity across the temperature continuum. They exhibit a sensory zone within which neither warm nor cold sensations prevail. This zone has been widely assumed to coincide with steady-state local skin temperatures between 32 and 34°C, but its underlying neurophysiology has been rarely investigated. In this study we employ two approaches to characterize the properties of sensory thermoneutrality, testing for each whether neutrality shifts along the temperature continuum depending on adaptation to a preceding thermal state. The focus is on local spots of skin on the palm. Ten participants (age: 30.3 ± 4.8 yr) underwent two experiments. Experiment 1 established the cold-to-warm inter-detection threshold range for the palm’s glabrous skin and its shift as a function of 3 starting skin temperatures (26, 31, or 36°C). For the same conditions, experiment 2determined a thermally neutral zone centered around a thermally neutral point in which thermoreceptors’ activity is balanced. The zone was found to be narrow (~0.98 to ~1.33°C), moving with the starting skin temperature over the temperature span 27.5–34.9°C (Pearson r = 0.94; P < 0.001). It falls within the cold-to-warm inter-threshold range (~2.25 to ~2.47°C) but is only half as wide. These findings provide the first quantitative analysis of the local sensory thermoneutral zone in humans, indicating that it does not occur only within a specific range of steady-state skin temperatures (i.e., it shifts across the temperature continuum) and that it differs from the inter-detection threshold range both quantitatively and qualitatively. These findings provide insight into thermoreception neurophysiology.

The ability of human hands and feet to convey skin thermal sensations is an important contributor to our experience of the surrounding world. Surprisingly, the detailed topographical distribution of warm and cold thermosensitivity across hands and feet has not been mapped, although such sensitivity maps exist for touch and pain. Using a recently developed quantitative sensory test, we mapped warm and cold thermosensitivity of 103 skin sites over glabrous and hairy skin of handsand feet in male (30.2±5.8y) and female (27.7±5.1y) adults matched for body surface area (M 1.77±0.2m2; F 1.64±0.1m2; p=0.155). Our findings indicated that warm and cold thermosensitivity varies by 5-fold across glabrous and hairy skin of hands and feet, and that hands are twice as sensitive as the feet. Opposite to what is known for touch and pain sensitivity, we observed a characteristic distal-to-proximal increase in thermosensitivity over both hairy and glabrous skin (i.e. from fingers/toes to body of hands and feet), and found that hairy skin is more sensitive than glabrous. Finally, we show that body-surface-area-matched males and females presented small differences in thermosensitivity, and that these differences are constrained to glabrous skin only. Our high-density thermosensory micromapping provides the most detailed thermosensitivity maps of hands and feet in healthy young adults available to date. These maps provide a window into the peripheral and central mechanisms of thermosensory integration in humans, and will help in guiding future developments in smart skin and sensory neuroprostheses, in wearable energy-efficient personal comfort systems, and in sport and protective clothing.

‘Personal comfort systems’ and thermally active clothing are able to warm and cool individual building occupants by transferring heat directly to and from their body surfaces. Such systems would ideally target local body surfaces with high temperature sensitivities. Such sensitivities have not been quantified in detail before. Here we report local thermal sensations and sensitivities for 318 local skin spots distributed over one side of the body, measured on a large number of subjects. Skin temperature changes were induced with a thermal probe 14 mm in diameter, and subjective thermal sensations were surveyed after 10 s. Our neutral base temperature was 31 °C and the spot stimulus was ±5 °C. Cool and warm sensitivities are seen to vary widely by body part. The foot, lower leg and upper chest are much less sensitive than average; in comparison, the cheek, neck back, and seat area are 2–3 times as sensitive to both cooling and warming stimuli. Every body part exhibits stronger sensitivity to cooling (1.3–1.6 times stronger) than to warming. Inter-personal differences and regional variance within body parts were observed to be 2–3 times greater than potential sex differences. These high-density thermal sensitivity maps with appended dataset provide the most comprehensive distributions of cold and warm sensitivity across the human body.