Why does water feel wet?


Ever had the strange feeling of being convinced you’re sitting on something wet, only to discover it’s just cold – not damp?  A new paper has just explained why this happens, by creating a new model of wetness perception.

Wetness perception has been something of a mystery.  Although there are clearly specified receptor cells in the skin for detecting a range of different attributes in the environment – thermoreceptors for sensing heat, mechanoreceptors for sensing pressure, and so on – there is no specific receptor for detecting how wet something is.  A mystery, that is, until a PhD student at Loughborough University began investigating it.

I like this paper because it’s an incredibly neat explanation of something so everyday that you don’t normally think about it.  I’m quite amazed that it appears wetness perception in humans has more or less escaped scientific scrutiny until now.

The model describes wetness perception as the result of “complex multisensory integration” of thermal and tactile inputs to the skin: if it’s cold and clammy, it’s probably wet.  So to demonstrate this, they altered thermal perception and mechanosensory perception in a few different ways, to work out how these inputs contribute to wetness perception.

The basis of their experiment was bringing a series of wet cotton stimuli, which varied in temperature, into contact with participants’ skin.   All the stimuli were carefully prepared to ensure they had exactly the same moisture content.

The first finding was that when they asked the participants to rate how wet it was, they reliably found that the cold, wet cotton was perceived to be wetter than more temperate wet cotton.  Secondly, when they were given the opportunity to have the cotton rubbed against their skin, providing mechanosensory stimulation from the movement as well as the thermal input, their ratings of wetness were more accurate than when the stimuli were held still against the skin.  So, very broadly, thermal and mechanosensory information are both important.

This gave them a nice theoretical starting point for how we combine different types of sensory inputs to create a sensation of wetness.  Next, they needed to work out the mechanics.  So the next stage was to delve further into the biology of it by a) seeing what happens when the activity of A-nerve fibres, which carry thermal and tactile information to the spinal cord, is suppressed; and b) comparing the wetness sensitivity of two different types of skin – the forearm, which is better at thermal sensitivity, and the fingerpad, which is better at tactile sensitivity.  From both of these methods, they determined that coldness was the biggest influence on overall wetness perception with mechanosensory information playing a supplementary role.

Based on this behavioural data, they propose a Bayesian neurophysiological model of wetness perception, in which activity from Aδ-nerve fibres (which respond to cold) and Aβ fibres (which respond to pressure) are combined to produce a rational estimate of wetness.

And, as they point out, it explains why you often don’t know that your nose is bleeding until you’ve touched it with your finger or looked in a mirror: if it’s too warm, you don’t sense the wetness.

References

Filingeri, D., Fournet, D., Hodder, S., & Havenith, G. (2014).  Why wet feels wet? A neurophysiological model of human cutaneous wetness sensitivity.  Journal of Neurophysiology, 112, 1457-1469.