From the literature review the following conclusions can be made:
1. As a physical variable, skin wetness is a fundamental parameter for the body’s thermal homeostasis due to its role in facilitating heat losses via evaporation of sweat from the skin.
2. As a perceptual variable, skin wetness is an important determinant of autonomic and behavioural responses.
3. Although much is known on the biophysical role of skin wetness in contributing to thermal homeostasis, surprisingly little has been done to elucidate how humans sense wetness on their skin and how the level of physical skin wetness relates to the level of perceived skin wetness.
4. In contrast with insects, in which humidity receptors sub-serving hygrosensation have been identified and widely described, humans’ largest sensory organ i.e. the skin seems not to be provided with specific receptors for the sensation of wetness.
5. As human beings, we seem to 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 skin friction) inputs generated by the interaction between skin, moisture and (if donned) clothing.
6. What remains unclear is the individual role of thermal and tactile cues and how these are integrated peripherally as well as centrally by our nervous system when experiencing the perception of skin wetness.
5. The first scientist who has attempted to explain the basis of this perception was Bentley, who in 1900 proposed a sensory-blending hypothesis which suggests the blend of pressure and coldness as responsible for evoking the perception of wetness.
CHAPTER 1 – INTRODUCTION AND REVIEW OF THE LITERATURE Page 53 6. Since Bentley’s study, a number of researchers have investigated the perceptual responses to either: a) skin’s contact with external (dry or wet) stimuli; b) the active production of sweat.
7. Studies that have investigated the perceptual responses to skin’s contact with external (dry or wet) stimuli using Quantitative Sensory Testing (QST) with a
discrimination paradigm have shown that humans readily discriminate between
higher and lower wetness levels. However, these studies have provided limited evidence on the potential sensory mechanisms underpinning the ability to sense and discriminate skin wetness.
8. Studies that have investigated the perceptual responses to skin’s contact with external (dry or wet) stimuli using QST with a magnitude estimation paradigms have shown that thermal (cold) sensory inputs play a primary role in driving the perception of skin wetness.
9. It has been proposed that we tend to associate the cold sensations evoked by the drop in skin temperature occurring during the evaporation of moisture from the skin, as a signal of the presence of moisture, and thus wetness, on the skin surface.
10. Cold stimuli able to reproduce such skin cooling rates are suggested to suffice in evoking the perception of wetness. However, limited evidence is available in support of this hypothesis.
11. Although indications of the key role of thermal cues in the perception of skin wetness have emerged, limited mechanistic evidence has been provided on the potential link between the biophysical effects of the stimuli applied (e.g. variations in skin temperature), the resulting physiologically responses (afferent sensory inputs) and the way these were used by the participants to characterize their perception of skin wetness.
12. Only one study has provided evidence on how thermal (cold) and tactile sensory cues could be integrated to aid the discrimination of skin wetness during the contact with an external (dry or wet) stimulus. However, as well as for previous studies,
CHAPTER 1 – INTRODUCTION AND REVIEW OF THE LITERATURE Page 54 limited physiological measurements were performed, eventually limiting the possibility to define a sensory model for skin wetness perception.
13. Only few studies have investigated how the level of physical skin wetness relates to the level of perceived skin wetness under conditions of sweat-induced whole-body skin wetness.
14. In all these studies, skin temperature was observed to significantly increase during the exercise protocols, thus indicating that participants were able to sense as well as to regionally discriminate skin wetness despite no cold sensations were experienced.
15. It could be hypothesised that in conditions of sweat-induced skin wetness, individuals rely more on tactile (i.e. stickiness) than on thermal inputs (i.e. thermal sensations) to characterise their skin wetness perception. However, to date, this hypothesis remains purely speculative.
16. Only few studies have investigated whether regional variations in wetness perception across the body exist. These studies have provided initial insights about the hairy regions of the body on which skin wetness might be perceived to a larger extent (e.g. the torso).
17. Only one study has investigated whether skin wetness perception varies between hairy and glabrous skin and found no differences in skin wetness sensitivity between these types of skin. However, due to the presence of a body of literature which indicates the existence of somatosensory differences (i.e. discriminative and pleasant touch, temperature and pain) between hairy and glabrous skin, it seems reasonable to hypothesise that the same would apply to skin wetness perception.
18. Overall, no studies have been found to specifically endorse a mechanistic approach (i.e. combining psychophysical and neurophysiological methods) to the investigation of the neural bases of human skin wetness perception. As that, the knowledge on how humans sense warm, neutral and cold wetness on their skin is still lacking.
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