Thermal Sensibility

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.

Thermal Perception for Information Transmission: Theoretical Analysis, Device Design, and Experimental Verification

Information transmission is a fundamental issue for human-computer interaction (HCI). Traditional interaction methods through visual, voice, and force haptics are very mature. However, the thermal perception (TP) for HCI is not studied in depth. This work proposes the TP-based information transmission framework. Firstly, we investigated the human hand-object contact heat transfer model and the temperature perception resolution of the hand and verified the feasibility of spatiotemporal temperature stimulation for information transmission by simulations. Then, a thermal device was designed, which utilized a 7×5 Peltier array, a water cooler, temperature sensors, and a control module to realize various static and dynamic spatiotemporal temperature patterns stimulation. Finally, we implemented a device prototype and recruited 20 subjects for experimental studies. The results show that the device can display various temperature patterns and provide thermal stimulations with high precision and speed. Furthermore, the subjects can accurately recognize different temperature values, icons, codes, and waveforms with their palm and fingers after a few times of training, which validates the TP-based information transmission method. Therefore, people can apply this method to interact with machines for information feedback, virtual reality, augmented reality, etc.

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.

Non-Contact Thermo-Visual Augmentation by IR-RGB Projection

This paper presents an approach for non-contact haptic augmentation with spatial augmented reality (SAR). We construct a thermo-visual projection system by combining a standard RGB projector and a fabricated infrared (IR) projector. The primary contribution of this paper is that we conduct thorough psychophysical experiments to investigate a design guideline for spatiotemporal projection patterns for both RGB and IR projectors to render a warm object with high presence. We develop application systems to evaluate the validity of the proposed system and design guideline. The evaluation results demonstrate that the proposed system can render warm objects with significantly higher presence than a standard SAR system.

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.

Local body cooling to improve sleep quality and thermal comfort in a hot environment

The effects of local body cooling on thermal comfort and sleep quality in a hot environment were investigated in an experiment with 16 male subjects. Sleep quality was evaluated subjectively, using questionnaires completed in the morning, and objectively, by analysis of electroencephalogram (EEG) signals that were continuously monitored during the sleeping period. Compared with no cooling, the largest improvement in thermal comfort and sleep quality was observed when the back and head (neck) were both cooled at a room temperature of 32°C. Back cooling alone also improved thermal comfort and sleep quality, although the effects were less than when cooling both back and head (neck). Mean sleep efficiency was improved from 84.6% in the no cooling condition to 95.3% and 92.8%, respectively, in these conditions, indicating good sleep quality. Head (neck) cooling alone slightly improved thermal comfort and subjective sleep quality and increased Stage N3 sleep, but did not otherwise improve sleep quality. The results show that local cooling applied to large body sections (back and head) could effectively maintain good sleep and improve thermal comfort in a hot environment.

Effects of transcranial direct current stimulation on temperature and pain perception

Transcranial direct current stimulation modifies cortical excitability and in consequence some cerebral functions. In the present study we aimed to elucidate whether tDCS could affect temperature and pain perceptions in healthy subjects testing different stimulation parameters. A total of 20 healthy subjects were studied by means of quantitative sensory testing. Two different experiments were performed. First, we studied the effects of 15 minutes 2 mA anodal transcranial direct current stimulation applied over left M1 and parietal cortex in two separated sessions. Then, we tested the effects of 5 minutes tDCS over M1 by means of a sham controlled design to optimize the possibility to study minimal effects of tDCS using different polarities (cathodal and anodal) and intensities (1 and 2 mA). 2 mA anodal tDCS, when applied for both 15 and 5 minutes over the motor cortex, increased cold perception threshold. Conversely, motor cortex cathodal tDCS modulated cold perception threshold only when 1 mA intensity was used. M1-tDCS can modify the temperature perception; these effects are polarity and intensity dependent. As stimulation intensity seems critical to determine the effects, we suggest that for clinical application strong anodal tDCS (>1 mA) or weak cathodal tDCS (<2 mA) should be used for pain control.

Characteristics of the local cutaneous sensory thermoneutral zone

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 2 determined 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.NEW & NOTEWORTHY Contrary to a widespread concept in human thermoreception, we show that local sensory thermoneutrality is achievable outside the 32-34°C skin temperature range. We propose that sensory adaption underlies a new mechanism of temperature integration. Also, we have developed from vision research a new quantitative test addressing the balance in activity of cutaneous cold and warm thermoreceptors. This could have important clinical (assessment of somatosensory abnormalities in neurological disease) and applied (design of personal comfort systems) implications.

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.

Enzyme replacement therapy improves function of C-, Adelta-, and Abeta-nerve fibers in Fabry neuropathy

BACKGROUND

Peripheral neuropathy in Fabry disease predominantly involves small nerve fibers. Recently, enzyme replacement therapy (ERT) with recombinant human alpha-galactosidase A has become available.

OBJECTIVE

To evaluate whether ERT improves Fabry neuropathy.

METHODS

In 22 Fabry patients (age 27.9 +/- 8.0 years) undergoing ERT with recombinant human alpha-galactosidase A (agalsidase beta) for 18 (n = 11) or 23 (n = 11) months and in 25 control subjects (age 29.0 +/- 10.4 years), the authors performed quantitative sensory testing using the 4, 2, and 1 stepping algorithm (CASE IV). Detection thresholds of vibration (VDT) on the first toe were assessed; cold detection thresholds (CDT), heat-pain onset (HP 0.5), and intermediate heat-pain (HP 5.0) assessments were made on the dorsum of the feet. Patient values above mean + 2.5 SD of control values were considered abnormal.

RESULTS

Before ERT, VDT, CDT, HP 0.5, and HP 5.0 were higher in patients than control subjects (p < 0.05). Following ERT, patients developed lower thresholds than prior to ERT for VDT (15.5 +/- 3.5 vs 14.3 +/- 4.1; p < 0.05), HP 0.5 (22.3 +/- 6.7 vs 19.4 +/- 1.3; p < 0.01), and HP 5.0 (27.3 +/- 5.6 vs 22.5 +/- 2.3; p < 0.01). Moreover, fewer patients had abnormal results of VDT (2 vs 4), CDT (7 vs 12), HP 0.5 (0 vs 9), and HP 5.0 (4 vs 20) after than before ERT.

CONCLUSIONS

ERT therapy with agalsidase beta significantly improves function of C-, Adelta-, and Abeta-nerve fibers and intradermal vibration receptors in Fabry neuropathy. Lack of recovery in some patients with abnormal cold or heat-pain perception suggests the need for early ERT, prior to irreversible nerve fiber loss.