Body mapping of cutaneous wetness perception across the human torso during thermo-neutral and warm environmental exposures

High-density thermal sensitivity maps of the body of people with multiple sclerosis: Implications for inclusive personal comfort systems

Inclusive thermal comfort solutions should accommodate the need of clinical groups such as people with Multiple Sclerosis (pwMS), who experience abnormal thermal sensitivity. The aim of this study was to develop high-density body maps of temperature sensitivity in pwMS to inform the design of patient-centred personal comfort systems. Fourteen pwMS (6 M/8 F; 48.6 ± 10.0 y) and 13 healthy individuals (CTR; 5 M/8 F; 47.8 ± 10.4) underwent a quantitative sensory test in a thermoneutral environment, during which they rated their local thermal sensations arising from the application of warm (39°C) and cold (27°C) stimuli to 115 bilateral body sites across the face, torso, upper and lower limbs. We used a z-transformation to create maps of hypo- and hyper-sensitivity for each individual MS participant using normative CTR data. We found that 50% of pwMS (N = 7/14) presented a loss of cold sensitivity over the upper limb, and a loss of warm sensitivity over the feet. Furthermore, 36% of pwMS (N = 5) presented warm hyper-sensitivity over the upper limb. Finally, cold sensitivity loss and warm sensitivity gain were more evenly distributed and affected a greater proportion of skin sites in MS (i.e. cold hypo-sensitivity = 44% of tested sites; warm hyper-sensitivity = 14%) than warm sensitivity loss (i.e. 10%), which was more focused on sites such as the feet. Our findings highlight the need to consider "thermosensory corrective power" when designing personal comfort systems, to accommodate either thermosensory loss or gain in pwMS. Our approach to clinical body mapping may support this process and help meeting the unique thermal needs of vulnerable individuals.

Biophysical, thermo-physiological and perceptual determinants of cool-seeking behaviour during exercise in younger and older women

Women continue to be under-represented in thermoregulatory research despite their undergoing unique physiological changes across the lifespan. This study investigated the biophysical, thermo-physiological, and perceptual determinants of cool-seeking behaviour during exercise in younger and older women. Eleven younger (25 ± 5 years; 1.7 ± 0.1 m; 63.1 ± 5.2 kg) and 11 older women (53 ± 6 years; 1.7 ± 0.1 m; 65.4 ± 13.9 kg) performed a 40-min incremental cycling test in a thermoneutral environment (22 ± 1.7°C; 36 ± 4% relative humidity). Throughout the test, participants freely adjusted the temperature of a cooling probe applied to their wrists to offset their thermal discomfort. We continuously recorded the probe-wrist interface temperature to quantify participants' cool-seeking behaviour. We also measured changes in participants' rate of metabolic heat production, core and mean skin temperatures, and skin wetness. Finally, we body-mapped participants' skin heat, cold and wetness sensitivity. Our results indicated that: (1) older and younger women exhibited similar onset and magnitude of cool-seeking behaviour, despite older women presented reduced autonomic heat-dissipation responses (i.e., whole-body sweat losses); (2) older women's thermal behaviour was less determined by changes in core temperature (this being a key driver in younger women), and more by changes in multiple thermo-physiological and biophysical parameters (i.e., physical skin wetness, temperature and heat production); (3) older women did not present lower regional skin thermal and wetness sensitivity than younger women. We conclude that predictions of female cool-seeking behaviours based on thermo-physiological variables should consider the effects of ageing. These findings are relevant for the design of wearable cooling systems and sports garments that meet the thermal needs of women across the lifespan.

Thermal illusions for thermal displays: a review

Thermal illusions, a subset of haptic illusions, have historically faced technical challenges and limited exploration. They have been underutilized in prior studies related to thermal displays. This review paper primarily aims to comprehensively categorize thermal illusions, offering insights for diverse applications in thermal display design. Recent advancements in the field have spurred a fresh perspective on thermal and pain perception, specifically through the lens of thermal illusions.

The characterization of thermal perception in recreational surfers wearing wetsuits

The purpose of this study was to characterize the perception of heat loss, comfort, and wetness in recreational surfers wearing wetsuits, to compare these data with changes in skin temperature reported in prior studies, and to examine the impact of wetsuit thickness, zipper location, and accessory use on thermal sensation and comfort. Following their surf session, nine-hundred and three male (n = 735) and female (n = 168) recreational surfers responded to a series of questions regarding thermal comfort/sensation, wetsuit characteristics, and surfing history. Average whole body thermal sensation rating was 0.8 ± 3.6 on a scale of -10 to +10 and average whole body thermal comfort rating was 1.5 ± 1.2, midway between "just comfortable" and "comfortable." Overall, surfers felt coldest in their feet, hands, and head. Under their wetsuits, surfers felt the coldest, wettest, and least comfortable in their chest, lower legs, lower arms, and upper back. Wetsuit accessory use had the greatest impact on regions identified as coldest, least comfortable, and wettest. These data suggest that wetsuit design should focus on optimizing water access points and improving accessories for the feet, hands, and head.

Body mapping of skin friction coefficient and tactile perception during the dynamic skin-textile interaction

The clothing fabric and skin interact continuously across the many regions of users' bodies during wear, which can lead to both physical skin damage and discomfort. Therefore, this investigation aimed to explore the regional differences in skin friction, tactile perception, and sensitivity in both females and males during the skin-textile interaction. The static and dynamic friction coefficient and textile perception (texture, stickiness, pleasantness, and discomfort) were measured across the 36 selected testing body areas by using a friction measurement device. The results revealed there was a significant difference in skin friction, tactile perceptions, and sensitivity across the various body regions. The anterior neck had the highest skin friction in both females and males, and participants generally rated higher texture perception in their anterior aspects compared to posterior and lateral regions. There was no significant difference in skin friction, tactile perception ratings, and sensitivity between females and males. Practitioner summary: This study sought to examine regional variations in skin friction, tactile perception, and sensitivity during the skin-textile interaction. There was a significant difference in skin friction, tactile perceptions, and sensitivity across the various body regions and no significant sex effect on skin friction, tactile perception ratings, and sensitivity.

Cutaneous thermosensory mapping of the female breast and pelvis

Differences in skin thermal sensitivity have been extensively mapped across areas of the human body, including the torso, limbs, and extremities. Yet, there are parts of the female body, such as the breast and the pelvis for which we have limited thermal sensitivity data. The aim of this study was to map cutaneous warm and cold sensitivity across skin areas of the breast and pelvis that are commonly covered by female underwear. Twelve young females (21.9 ± 3.2 years) reported on a 200 mm visual analogue scale the perceived magnitude of local thermal sensations arising from short-duration (10 s) static application of a cold [5 °C below local skin temperature (T_sk)] or warm (5 °C above local T_sk) thermal probe (25 cm²) in seventeen locations over the breast and pelvis regions. The data revealed that thermal sensitivity to the warm probe, but not the cold probe, varied by up to 25% across the breast [mean difference between lowest and highest sensitivity location was 51 mm (95% CI:14, 89; p < 0.001)] and up to 23% across the pelvis [mean difference between lowest and highest sensitivity location: 46 mm (95% CI:9, 84; p = 0.001)]. The regional differences in baseline T_sk did not account for variance in warm thermal sensitivity. Inter-individual variability in thermal sensitivity ranged between 24 and 101% depending on skin location. We conclude that the skin across the female breast and pelvis presents a heterogenous distribution of warm, but not cold, thermal sensitivity. These findings may inform the design of more comfortable clothing that are mapped to the thermal needs of the female body.

Skin wetness sensitivity across body sites commonly affected by pain in people with migraine

Objective

The objective of this study was to evaluate skin wetness perception and thermal sensitivity in people with migraine and similar healthy controls.

Background

Environmental triggers, such as cold and humidity, are known triggers for pain in people with migraine. Sensory inputs might be implicated in such heightened responses to cold‐humid environments, such that a migraine‐induced hypersensitivity to cold wetness could be present in people with migraine. However, we lack empirical evidence on skin thermal and wetness sensitivity across skin sites commonly associated with reported pain in migraine, such as the forehead.

Methods

This prospective cross‐sectional observational study, conducted in a university hospital setting, evaluated skin wetness perceptions and thermal sensations to wet non‐noxious warm‐wet, neutral‐wet, and cold‐wet stimuli applied to the forehead, the posterior neck, and the index finger pad of 12 patients with migraine (mean and standard deviation for age 44.5 ± 13.2 years, 7/12 [58%] women) and 36 healthy controls (mean and standard deviation for age 39.4 ± 14.6 years, 18/36 [50%] women).

Results

On the forehead, people with migraine reported a significantly higher wetness perception than healthy controls across all thermal stimulus (15.1 mm, 95% confidence interval [CI]: 1.8 to 28.5, p = 0.027, corresponding to ~ 15% difference), whereas no significant differences were found on the posterior neck nor on the index finger pad. We found no differences among groups in overall thermal sensations (−8.3 mm, 95% CI: −24.0 to 7.3, p = 0.291; −7.8 mm, 95% CI: −25.3 to 9.7, p = 0.375; and 12.4 mm, 95% CI: −4.0 to 28.9, p = 0.133; forehead, posterior neck, and index finger, respectively).

Conclusion

These findings indicate that people with migraine have a heightened sensitivity to skin wetness on the forehead area only, which is where pain attacks occur. Future studies should further explore the underlying mechanisms (e.g., TRPM8‐mediated cold‐wet allodynia) that lead to greater perception of wetness in people with migraine to better understand the role of environmental triggers in migraine.

Ageing reduces skin wetness sensitivity across the body

NEW FINDINGS

What is the central question of this study? Ageing impairs the skin's thermal and tactile sensitivity: does ageing also induce loss of skin wetness sensitivity? What is the main finding and its importance? Older adults show an average 15% loss of skin wetness sensitivity, with this sensory deficit being mediated by a combination of reductions in skin's tactile sensing and hydration status. These findings increase knowledge of wetness sensing mechanisms across the lifespan.

ABSTRACT

Humans use sensory integration mechanisms to sense skin wetness based on thermal and mechanical cues. Ageing impairs the skin's thermal and tactile sensitivity, yet we lack evidence on whether wetness sensing also changes with ageing. We mapped local skin wetness and temperature sensitivity in response to cold-, neutral- and warm-wet stimuli applied to the forehead, neck, lower back, dorsal foot, index finger and thumb, in 10 Younger (22.4 ± 1.1 years) and 10 Older (58.2 ± 5.1 years) males. We measured local skin temperature and conductance (i.e., a marker of hydration status) at the tested sites, to establish the role of skin's thermal and mechanical parameters in ageing-induced changes in wetness sensing. Irrespective of body site, Older reported overall lower wetness perceptions than Younger across all wet-stimulus temperatures (mean difference: -14.6 mm; 95% CI: -4.3, -24.9; P = 0.008; ∼15% difference). When considering regional wetness sensitivity, the effect of ageing was more pronounced in response to the cold-wet stimulus over the lover back (mean difference Older vs. Younger: -36.8 mm; 95% CI: -68.4, -5.2; P = 0.014; ∼37% difference) and dorsal foot (mean difference: -37.1 mm; 95% CI: -68.7, -5.5; P = 0.013; ∼37% difference). We found no differences between age groups on overall thermal sensations (P = 0.744) nor local skin temperature (P = 0.372); however, we found that Older presented overall lower skin conductance than Younger (mean difference: -1.56 μS; 95% CI: -0.49, -2.62; P = 0.005), which corresponded to an ∼78% reduction in skin hydration. We conclude that skin wetness sensing decreases with ageing primarily due to age-induced changes in skin mechanics and tactile sensitivity.

An examination of five theoretical foundations associated with localized thermosensory testing

Purpose

To assess five theoretical foundations underlying thermosensory testing using local thermal stimuli.

Methods

Thermal sensation, discomfort and the confidence of thermal sensation scores were measured in 9 female and 8 male volunteers in response to 17 physical contact temperature stimuli, ranging between 18–42 °C. These were applied to their dorsal forearm and lateral torso, across two sessions.

Results

Thermal sensation to physical temperature relationships followed a positive linear and sigmoidal fit at both forearm (r² = 0.91/r² = 0.91, respectively) and lateral torso (r² = 0.90/ r² = 0.91, respectively). Thermal discomfort to physical temperature relationships followed second and third-order fits at both forearm (r² = 0.33/r² = 0.34, respectively) and lateral torso (r² = 0.38/r² = 0.39, respectively) test sites. There were no sex-related or regional site differences in thermal sensation and discomfort across a wide range of physical contact temperatures. The median confidence of an individual’s thermal sensation rating was measured at 86%.

Conclusion

The relation between thermal sensation and physical contact temperature was well described by both linear and sigmoidal models, i.e., the distance between the thermal sensation anchors is close to equal in terms of physical temperatures changes for the range studied. Participants rated similar thermal discomfort level in both cold and hot thermal stimuli for a given increase or decrease in physical contact temperature or thermal sensation. The confidence of thermal sensation rating did not depend on physical contact temperature.

Cooling During Exercise May Induce Benefits Linked to Improved Brain Perfusion

The aim of this study was to evaluate the impact of using a cooling vest during physical exercise (per-cooling) in humid and temperate conditions (≈22°C, ≈80% relative humidity) on perceptual and physiological responses (tissue oxygenation and heart rate). 20 physically active men performed twice a 30-min cycling exercise at 70% of their theoretical maximum heart rate while using an activated (experimental condition) and a deactivated (control condition) cooling system in a randomized crossover study. Heart rate and tissue (cerebral and muscular) oxygenation were continuously measured during exercise and recovery, and skin temperature was measured every 10 min. Perception of temperature, humidity and comfort were assessed at the end of the recovery period. Results showed a decrease in trunk skin temperature (p<0.05), a faster heart rate recovery and an increase in the concentration of total hemoglobin at the brain level (p<0.05) compared with control condition. Moreover, an improved subjective rating of thermal sensations, wetness and comfort compared to control values (p<0.05) was noted. In conclusion, wearing a cooling vest during submaximal exercise improves perceptual and physiological responses in humid temperate conditions, which may be due to a better blood perfusion at the brain level and a better parasympathetic reactivation.

Thermal sensitivity mapping - warmth and cold detection thresholds of the human torso

Skin as the largest organ of the human body accomplishes many important functions, including thermoregulation. In this context, investigating cold (CDT) and warmth detection thresholds (WDT) constitutes an important research branch, and investigating thermal thresholds has a significant impact on the clothing and fabric textile industry. In this regard, not only the extremities, but also torso regions are of high relevance. However, only few examinations have conducted detailed mapping studies of the human torso. Additionally, some of these studies show certain methodological limitations. Furthermore, the issue of whether cutaneous thermal sensitivity is gender-dependent is still controversial. Therefore, the present study investigated the cutaneous thermal sensitivity (CDT, WDT) of 42 male and female young and healthy subjects. Measurements were taken at 11 anatomical regions. We found that gender plays an important role when investigating thermal thresholds: Females tended to be more sensitive than males. We also found considerable differences between the tested regions, even within the anterior torso, for example. We identified locations which were constantly sensitive (lower back), while others were consistently insensitive (e.g. scapula). We also detected greater data variability for males compared to females, and for WDT compared to CDT. Furthermore, mainly for WDT, we found a proximal-to-distal increase of thermal torso and upper arm sensitivity. In line with previous investigations, our subjects were more sensitive to cold than to warmth. The findings of this study have important implications. First, our data may complement basic research, e.g. in terms of reference data of body regional maps. Second, our data provides important insights that could be leveraged in the textile industry, and also used to optimize current broadly applicable test methods and tools, like thermal manikins and thermophysiological models.

Evidence for the involvement of peripheral cold-sensitive TRPM8 channels in human cutaneous hygrosensation

In contrast to other species, humans are believed to lack hygroreceptors for sensing skin wetness. Yet, the molecular basis of human hygrosensation is currently unknown, and it remains unclear whether we possess a receptor-mediated sensing mechanism for skin wetness. The aim of this study was to assess the role of the cutaneous cold-sensitive transient receptor potential melastatin-8 (TRPM8) channel as a molecular mediator of human hygrosensation. To this end, we exploited both the thermal and chemical activation of TRPM8-expressing cutaneous Aδ cold thermoreceptors, and we assessed wetness sensing in healthy young men in response to 1) dry skin cooling in the TRPM8 range of thermosensitivity and 2) application of the TRPM8 agonist menthol. Our results indicate that 1) independently of contact with moisture, a cold-dry stimulus in the TRPM8 range of activation induced wetness perceptions across 12 different body regions and those wetness perceptions varied across the body following regional differences in cold sensitivity; and 2) independently of skin cooling, menthol-induced stimulation of TRPM8 triggered wetness perceptions that were greater than those induced by physical dry cooling and by contact with an aqueous cream containing actual moisture. For the first time, we show that the cutaneous cold-sensing TRPM8 channel plays the dual role of cold and wetness sensor in human skin and that this ion channel is a peripheral mediator of human skin wetness perception.

Illusion of Wetness by Dynamic Touch

Humans perceive wetness on contact with a dry-cold material; however, the magnitude of wetness that can be perceived using dynamic touch remains unclear. This study assessed how the type of touch, namely hand movement (either statically or dynamically) and pressing force (either low or high pressure), affect the perception of wetness. The participants judged the magnitude of perceived wetness after four types of touch of four stimuli comprising four fabrics of varying water content and surface temperatures. Overall, the perceived wetness was differed between static and dynamic touch independent of pressure and the participants scored the dry-cold stimulus as relatively dry for dynamic touch. Furthermore, cluster analysis revealed individual differences in the recognition of wetness in dynamic touch conditions. These results revealed the variability in the mechanisms used by humans to perceive wetness. Additionally, we discussed the optimal methods to reproduce the wetness perception using this illusion.

Wearing a Cooling Vest During Half-Time Improves Intermittent Exercise in the Heat

Endurance and intermittent exercise performance are impaired by high ambient temperatures. Various countermeasures are considered to prevent the decline in exercise performance in the heat, convenient, and practical cooling strategies attracts attention. The purpose of this study was to investigate the effect of wearing a new type of cooling vest which cooled torso and neck during half-time (HT) on intermittent exercise performance that imitated intermittent athletic games. All measurements on the experiments were carried out with the bicycle ergometer. Eight male soccer players performed a familiarization session and two experimental trials of a 2 × 30 min intermittent cycling exercise protocol, which consisted of a 5 s maximal power pedaling (body weight ×0.075 kp) every minutes separated by 25 s unloaded pedaling (80 rpm) and rest (30 s) in the heat (33.0°C; 50% relative humidity). The two trials included cooling-vest condition (VEST) and control condition (CON), and the difference is with or without wearing cooling vest imposed for 15 min at HT. Mean and peak power output, rectal (Tre) and skin temperature (neck, upper back, chest, right upper arm, and thigh), heart rate (HR), deep thigh temperature, rating of perceived exertion (RPE), and thermal comfort (TC) and thermal sensation (TS) were measured. Mean power output at 2nd half was significantly greater (p < 0.05) in VEST (3rd trial: 589 ± 58 W, 4th trial: 584 ± 58 W) than in CON (3rd trial: 561 ± 53 W, 4th trial: 561 ± 53 W). HR were significantly lower in VEST during HT and higher in VEST at the last maximal pedaling (p < 0.05). At the end of HT, neck skin temperature and mean skin temperature were significantly lower in VEST (32.04 ± 1.47°C, 33.76 ± 1.08°C, respectively) than in CON (36.69 ± 0.78°C, 36.14 ± 0.67°C, respectively) (p < 0.05). During 2nd half, TS, TC, and RPE were significantly lower in VEST than in CON (p < 0.05). There was no significant difference in Tre and deep thigh temperature throughout each conditions. These results indicate that wearing a new type of cooling vest during HT significantly improves intermittent exercise performance in the heat with decreased neck and mean skin temperature and improved subjective responses.

Normative surface skin temperature changes due to blood redistribution: A prospective study

The continuing development and manufacture of infrared devices, together with improvements in thermal body mapping techniques have simplified surface skin thermography which is being used more extensively than ever before. Normative thermography data, however, remains incomplete. A normative blood redistribution range of skin temperatures was established for use as a reference for laboratory infrared thermography (IT), thermal body mapping, and mass fever screenings. 500 healthy volunteers participated in this prospective study. To determine the maximum range of the skin temperature changes due to the posture-related physiological blood redistribution, the volunteers were asked to keep one extremity up and another extremity down whilst lying, sitting, and standing. We obtained 6000 hand and 400 foot temperature readings. The normal temperature was 29.1 ± 0.6 °C for the middle fingers and 27.8 ± 0.7 °C for the toes. The physiological temperature change during body position changes ranged from 4 to 6 °C (fingers: 27-31 °C; toes: 26-32 °C). At normal room temperature, the surface skin temperature may vary within this range due to blood redistribution. These changes reflect the individual variability of vasomotor activity. This physiological range of temperatures should be taken into account during IT and other thermography-involved investigations.

Determination of car seat contact area for personalised thermal sensation modelling

A lot of daily activities are conducted in a sedentary posture. This includes a thermal interaction between the human and the seat that has implications on thermal perception and comfort. These interactions are investigated by simulating heat and mass transfer, thus, reducing a need for costly and time demanding subject studies. However, it is not clear, from the available literature, what portion of the body surface area is actually affected by the seat with respect to human anthropometry. The aim of this study was to develop a predicting function of the seat contact area based on anthropometric parameters. The results showed strong linear correlation between the contact area obtained by printing a body silhouette on paper placed at the seat and body weight, height, body surface area, and body mass index. The body surface area and the body weight were identified as the best predictors for the contact area.

Self-paced exercise performance in the heat with neck cooling, menthol application, and abdominal cooling

OBJECTIVES

To investigate whether the exercise performance benefits with neck cooling in the heat are attributable to neck-specific cooling, general body cooling, a cooler site-specific thermal perception or a combination of the above.

DESIGN

Counter-balanced crossover design.

METHODS

Twelve healthy participants cycled in the heat (34°C, 30% relative humidity), at a power output (PO) self-selected to maintain a fixed rating of perceived exertion (RPE) of 16. Each participant underwent four experimental trials: no cooling (CON), neck cooling (NEC), abdominal cooling (ABD), or neck cooling with menthol (MEN). Participants cycled for 90min or until their workload reduced by <70% of their initial PO. Changes in PO, rectal temperature (T_re), mean skin temperature (T_sk), whole-body thermal sensation (TS_wb) and thermal sensation of the neck (TS_neck) were recorded throughout.

RESULTS

The mean reduction in PO throughout exercise was similar (p=0.431) for CON (175±10W), NEC (176 ±12W), ABD (172±13W) and MEN (174±12W). The ΔT_re at the end of exercise was similar (p=0.874) for CON (0.83±0.5°C), NEC (0.85±0.5°C), ABD (0.82±0.5°C) and MEN (0.81±0.5°C). TS_wb was cooler (p<0.013) in MEN (125±8mm) compared to CON (146±19mm), NEC (135±11mm) and ABD (141±16mm).

CONCLUSIONS

No differences in exercise performance or thermal strain were observed in any of the cooling trials compared to the CON trial, despite significantly cooler TS_wb values in the MEN and NEC trials compared to the CON trial. These findings differ from previous observations and highlight that the benefit of neck cooling may be situation dependent.

Thermosensory micromapping of warm and cold sensitivity across glabrous and hairy skin of male and female hands and feet

The ability of 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 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 hands and feet in male (M; 30.2 ± 5.8 yr) and female (F; 27.7 ± 5.1 yr) adults matched for body surface area (M: 1.77 ± 0.2 m2; F: 1.64 ± 0.1 m2; P = 0.155). Findings indicated that warm and cold thermosensitivity varies by fivefold across glabrous and hairy skin of hands and feet and that hands (warm/cold sensitivity: 1.25/2.14 vote/°C) are twice as sensitive as the feet (warm/cold sensitivity: 0.51/0.99 vote/°C). 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 and 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 men and women 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 young adults available to date. These maps offer a window into peripheral and central mechanisms of thermosensory integration in humans and will help guide future developments in smart skin and sensory neuroprostheses, in wearable, energy-efficient personal comfort systems, and in sport and protective clothing.

NEW & NOTEWORTHY We provide the most detailed thermosensitivity maps across glabrous and hairy skin of hands and feet in men and women available to date. Our maps show that thermosensitivity varies by fivefold across hands and feet, distal regions (e.g., fingers, toes) are less sensitive than proximal (e.g., palm, sole), hands are twice as sensitive as feet, and men and women present small thermosensitivity differences. These findings will help guide developments in sensory neuroprostheses, wearable comfort systems, and sport/protective clothing.

Peripheral and central determinants of skin wetness sensing in humans

Evolutionarily, our ability to sense skin wetness and humidity (i.e., hygroreception) could have developed as a way of helping to maintain thermal homeostasis, as much as it is the case for the role of temperature sensation and thermoreception. Humans are not provided with a specific skin hygroreceptor, and recent studies have indicated that skin wetness is likely to be centrally processed as a result of the multisensory integration of peripheral inputs from skin thermoreceptors and mechanoreceptors coding the biophysical interactions between skin and moisture. The existence of a specific hygrosensation strategy for human wetness perception has been proposed and the first neurophysiologic model of skin wetness sensing has been recently developed. However, while these recent findings have shed light on some of the peripheral and central neural mechanisms underlying wetness sensing, our understanding of how the brain processes the thermal and mechanical inputs that give rise to one of our "most worn" skin sensory experiences is still far from being conclusive. Understanding these neural mechanisms is clinically relevant in the context of those neurologic conditions that are accompanied by somatosensory abnormalities. The present chapter will present the current knowledge on the peripheral and central determinants of skin wetness sensing in humans.

The biology of skin wetness perception and its implications in manual function and for reproducing complex somatosensory signals in neuroprosthetics

Our perception of skin wetness is generated readily, yet humans have no known receptor (hygroreceptor) to signal this directly. It is easy to imagine the sensation of water running over our hands or the feel of rain on our skin. The synthetic sensation of wetness is thought to be produced from a combination of specific skin thermal and tactile inputs, registered through thermoreceptors and mechanoreceptors, respectively. The present review explores how thermal and tactile afference from the periphery can generate the percept of wetness centrally. We propose that the main signals include information about skin cooling, signaled primarily by thinly myelinated thermoreceptors, and rapid changes in touch, through fast-conducting, myelinated mechanoreceptors. Potential central sites for integration of these signals, and thus the perception of skin wetness, include the primary and secondary somatosensory cortices and the insula cortex. The interactions underlying these processes can also be modeled to aid in understanding and engineering the mechanisms. Furthermore, we discuss the role that sensing wetness could play in precision grip and the dexterous manipulation of objects. We expand on these lines of inquiry to the application of the knowledge in designing and creating skin sensory feedback in prosthetics. The addition of real-time, complex sensory signals would mark a significant advance in the use and incorporation of prosthetic body parts for amputees in everyday life.NEW & NOTEWORTHY Little is known about the underlying mechanisms that generate the perception of skin wetness. Humans have no specific hygroreceptor, and thus temperature and touch information combine to produce wetness sensations. The present review covers the potential mechanisms leading to the perception of wetness, both peripherally and centrally, along with their implications for manual function. These insights are relevant to inform the design of neuroengineering interfaces, such as sensory prostheses for amputees.

Tactile cues significantly modulate the perception of sweat-induced skin wetness independently of the level of physical skin wetness

Humans sense the wetness of a wet surface through the somatosensory integration of thermal and tactile inputs generated by the interaction between skin and moisture. However, little is known on how wetness is sensed when moisture is produced via sweating. We tested the hypothesis that, in the absence of skin cooling, intermittent tactile cues, as coded by low-threshold skin mechanoreceptors, modulate the perception of sweat-induced skin wetness, independently of the level of physical wetness. Ten males (22 yr old) performed an incremental exercise protocol during two trials designed to induce the same physical skin wetness but to induce lower (TIGHT-FIT) and higher (LOOSE-FIT) wetness perception. In the TIGHT-FIT, a tight-fitting clothing ensemble limited intermittent skin-sweat-clothing tactile interactions. In the LOOSE-FIT, a loose-fitting ensemble allowed free skin-sweat-clothing interactions. Heart rate, core and skin temperature, galvanic skin conductance (GSC), and physical (w(body)) and perceived skin wetness were recorded. Exercise-induced sweat production and physical wetness increased significantly [GSC: 3.1 μS, SD 0.3 to 18.8 μS, SD 1.3, P < 0.01; w(body): 0.26 no-dimension units (nd), SD 0.02, to 0.92 nd, SD 0.01, P < 0.01], with no differences between TIGHT-FIT and LOOSE-FIT (P > 0.05). However, the limited intermittent tactile inputs generated by the TIGHT-FIT ensemble reduced significantly whole-body and regional wetness perception (P < 0.01). This reduction was more pronounced when between 40 and 80% of the body was covered in sweat. We conclude that the central integration of intermittent mechanical interactions between skin, sweat, and clothing, as coded by low-threshold skin mechanoreceptors, significantly contributes to the ability to sense sweat-induced skin wetness.

Human skin wetness perception: psychophysical and neurophysiological bases

The ability to perceive thermal changes in the surrounding environment is critical for survival. However, sensing temperature is not the only factor among the cutaneous sensations to contribute to thermoregulatory responses in humans. Sensing skin wetness (i.e. hygrosensation) is also critical both for behavioral and autonomic adaptations. Although much has been done to define the biophysical role of skin wetness in contributing to thermal homeostasis, little is known on the neurophysiological mechanisms underpinning the ability to sense skin wetness. Humans are not provided with skin humidity receptors (i.e., hygroreceptors) and psychophysical studies have identified potential sensory cues (i.e. thermal and mechanosensory) which could contribute to sensing wetness. Recently, a neurophysiological model of human wetness sensitivity has been developed. In helping clarifying the peripheral and central neural mechanisms involved in sensing skin wetness, this model has provided evidence for the existence of a specific human hygrosensation strategy, which is underpinned by perceptual learning via sensory experience. Remarkably, this strategy seems to be shared by other hygroreceptor-lacking animals. However, questions remain on whether these sensory mechanisms are underpinned by specific neuromolecular pathways in humans. Although the first study on human wetness perception dates back to more than 100 years, it is surprising that the neurophysiological bases of such an important sensory feature have only recently started to be unveiled. Hence, to provide an overview of the current knowledge on human hygrosensation, along with potential directions for future research, this review will examine the psychophysical and neurophysiological bases of human skin wetness perception.