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

Abstract

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. 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.

Effect of Fiber Type, Water Content, and Velocity on Wetness Perception by the Volar Forearm Test: Threshold Detection Test

“…In the predictive model of wetness threshold, the MIU of fabrics giving an explained variance of 59% was followed by wetting time, giving an additional explained variance of 11%, indicating that at the initial detection of moisture on the skin, the fabric is rather dry, and the surface texture of the fabric is the main influencing factor for the skin to sense wetness. This observation agrees with the previous finding that mechanical signal may contribute more to the perception of wetness when a cool stimulus is absent (Filingeri & Ackerley, 2017). In a recent study (Raccuglia, Sales, et al., 2018), the factors determining clothing comfort was investigated during physical exercise.…”

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

“…In humans, skin wetness sensing, i.e., hygrosensation, contributes to the awareness of the surrounding thermal environment (24), it plays a key role in behavioral thermoregulation (e.g., sweat-induced skin wetness drives thermal discomfort and heat avoidance; 48), and it is involved in the sensorimotor control of fine manipulation (e.g., sensing surface wetness and slippage mediates adjustments in hand grip; 18,31,44).…”

User‐Interactive Thermotherapeutic Electronic Skin Based on Stretchable Thermochromic Strain Sensor

“…Human skin cannot recognize absolute temperature and adapts easily to persistent heat. [ 22 ] The long‐term application of heat to human skin causes low‐temperature burns: 44 °C (6 h), 45 °C (3 h), 48 °C (15 min), and 52 °C (1 min). [ 23 ] Consequently, the development of an easily controlled thermotherapeutic visualization device in a stretchable form is highly desirable.…”