Can humans sense wetness?
It seems like a no-brainer that people can feel the rain during a storm or seawater the second they jump into the ocean. But can our bodies actually "sense" the water on our skin?
It turns out, the answer is no — at least not technically because our bodies don't have sensors specifically for detecting liquids. Rather, we rely on a conglomeration of other sensors to inform us when we're wet.
In the human body, a network of neurons known as the somatosensory system helps us process the things we touch. These specialized neurons, which include mechanoreceptors and thermoreceptors, are responsible for relaying tactile information from the skin to the brain, allowing us to recognize objects by feeling them and enabling us to detect different textures or temperatures.
However, humans lack hygroreceptors, which enable other animals, such as cockroaches and honeybees, to detect water or changes in the moisture and humidity of an environment. Instead, "we develop a guess about whether or not water is present on the surface in question, based off of the accumulation of available evidence, which is like this complicated, multidimensional thing," Jonathan Samir Matthis, an assistant professor of human movement neuroscience at Northeastern University in Boston, told Live Science.
Related: Do fish get thirsty?
To identify liquids, humans tap into visual stimuli and tactile sensations. For example, you see the smooth flow of water rushing over your hands in the sink and feel the sensation of droplets hitting your face during a shower. But the most important cue that humans use to sense water through the skin is temperature, according to a wide body of research.
"If you've got some moisture on the skin, chances are that it's going to evaporate from the skin and it's going to cool you," said Davide Filingeri, an environmental and sensory physiologist at the University of Southampton in the U.K., who has pioneered much of the research on how humans process wetness. As a result, humans might have been conditioned to associate the sensation of coolness with the presence of wetness, he said.
This may explain why it's difficult to tell when laundry on the line outside is dry if it is also cold, or why sitting on a cold metal chair can sometimes feel like sitting in a puddle. In a 2013 study, Filingeri put this confusing sensation to the test. The research team blindfolded nine individuals and ran through a series of trials in which they pressed a dry probe of varying cold temperatures to each participant's forearm. Five out of the nine participants perceived that the dry stimuli were wet if they were around 32 degrees Fahrenheit (0 degrees Celsius), which confirms that they were relying on just temperature to suss out whether something was moist.
"The brain doesn't have anything to distinguish between the illusion of wetness versus the real wetness," he told Live Science.
In 2014, Filingeri ran essentially the opposite experiment, in which he tested the effect of warm, wet stimuli in a group of blindfolded participants. He found that none of the individuals were able to perceive wetness during any of the simulations higher than 7.2 F (4 C) above their skin temperature.
Informed by these findings, Filingeri is currently working with clothing designers to create sportswear that is more breathable and can better handle sweat. Additionally, his lab is working with the Swiss Federal Institute of Technology Lausanne (EPFL) to restore thermal sensations in amputees with prosthetics so that they can trick the brain into "feeling" wetness in missing limbs.
"There's a lot of interesting clinical and nonclinical applications for some of this work," Filingeri said.
