When you put your hand in water, you know it's wet. When you step outside on a humid day, you feel wetness against your skin. When you sweat, it feels wet running down your face. But how exactly do you know what wet feels like?
A team of scientists from Loughborough University and Oxylane Research recently decided to find out why, exactly, wet feels wet.
The interesting thing about the sensation of wetness is that our skin does not have receptors for actually detecting and sensing wetness. In fact, it's possible that wetness is a sort of illusion that our brain works out because of prior experiences with things we've learned are wet, such as water and sweat.
Researchers took a group of 13 healthy male volunteers, all college students, and exposed them to various wet stimuli, including things that were warm, room temperature and cold. They tested each of these stimuli on various parts of the volunteers' body, including forearm, (which contain more hair) and fingertips (which contain no hair). Researchers also tested wetness with and without blocking specific nerve receptors by using a blood pressure cuff to block off specific nerve sensitivity.
The results were fascinating. It seemed that volunteers responded more to wet stimuli when it was at colder temperatures. They also discovered that when they expanded the area of the blood pressure cuff, which numbed nerves, volunteers were less sensitive to wetness. Even more interesting was that hairier parts of the body are more sensitive to wetness than those parts without hair.
To sum up, the results showed that wetness is more about feeling texture and temperature, rather than actual wetness. It is also connected to how the brain perceives wetness. This is why jumping into the cold ocean feels more wet than jumping into a heated pool. The brain knows that it associates water with wetness, and in both situations, it triggers the nerves to create a wet sensation. If those nerves are blocked, however, the sensation of wetness is lessened.
"Based on a concept of perceptual learning and Bayesian perceptual inference, we developed the first neurophysiological model of cutaneous wetness sensitivity centered on the multisensory integration of cold-sensitive and mechanosensitive skin afferents," writes the research team. "Our results provide evidence for the existence of a specific information processing model that underpins the neural representation of a typical wet stimulus."
