WHY FEET FEELS WET : UNDERSTANDING THE ILLUSION OF WET
Human sensitivity to
wetness plays a role in many aspects of daily life. Whether feeling humidity,
sweat or a damp towel, we often encounter stimuli that feel wet. Though it
seems simple, feeling that something is wet is quite a feat because our skin
does not have receptors that sense wetness. The concept of wetness, in fact,
may be more of a "perceptual illusion" that our brain evokes based on
our prior experiences with stimuli that we have learned are wet.
So how would a person
know if he has sat on a wet seat or walked through a puddle? Researchers at
Loughborough University and Oxylane Research proposed that wetness perception
is intertwined with our ability to sense cold temperature and tactile sensations
such as pressure and texture. They also observed the role of A-nerve fibers --
sensory nerves that carry temperature and tactile information from the skin to
the brain -- and the effect of reduced nerve activity on wetness perception.
Lastly, they hypothesized that because hairy skin is more sensitive to thermal
stimuli, it would be more perceptive to wetness than glabrous skin (e.g., palms
of the hands, soles of the feet), which is more sensitive to tactile stimuli.
Davide Filingeri et
al. exposed 13 healthy male college students to warm, neutral and cold wet
stimuli. They tested sites on the subjects' forearms (hairy skin) and
fingertips (glabrous skin). The researchers also performed the wet stimulus
test with and without a nerve block. The nerve block was achieved by using an
inflatable compression (blood pressure) cuff to attain enough pressure to
dampen A-nerve sensitivity.
They found that wet
perception increased as temperature decreased, meaning subjects were much more
likely to sense cold wet stimuli than warm or neutral wet stimuli. The research
team also found that the subjects were less sensitive to wetness when the
A-nerve activity was blocked and that hairy skin is more sensitive to wetness
than glabrous skin. These results contribute to the understanding of how humans
interpret wetness and present a new model for how the brain processes this
sensation.
"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," the research team wrote. "Our results provide evidence
for the existence of a specific information processing model that underpins the
neural representation of a typical wet stimulus."
The article "Whys
wet feels wet? A neurophysiological model of human cutaneous wetness
sensitivity" is published in the Journal of Neurophysiology.
It is highlighted as one of this month's "best of the best" as part
of the American Physiological Society's APSselect program.
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