It's a cold winter afternoon and you've just brought the washing in. Suddenly, you pause. Are these clothes a bit damp?
Authors
- Kate Poole
Associate Professor in Physiology, UNSW
- Felix Aplin
Lecturer, Department of Physiology and Translational Neuroscience Facility, School of Biomedical Sciences, UNSW
In the laundry basket, your hand can clearly distinguish the textures of silky shirts, woollen jumpers, cotton bed sheets. So why is it so hard to tell whether the washing is cold and damp, or just cold?
A sensible guess might be that our hands are numb from the cold, making it harder to detect wetness. The less intuitive reality is that we humans completely lack any specialised sensors that respond to "wetness" in the first place.
That's right - your skin alone can't actually detect if something is damp.
How do we tell if something is wet?
Our ability to sense information about our external environment starts at nerve cell endings in our skin.
There are specialised molecules on the surface of these nerve cells called sensory receptors. They convert environmental information into a biological signal, which is amplified into an electrical impulse and transmitted along nerve cells to the brain.
There are numerous different sensory receptors in nerve cells that are activated by cold, warmth or damaging heat. There are also sensory receptors that respond to very precise tactile information such as vibration, friction or stretch.
When you put your hand in that laundry basket, all the information about temperature and texture comes from the relevant sensory receptors in your hand.
The electrical signals triggered by these multiple inputs can also be integrated together in the central nervous system, or the brain. In this way, we can infer an extraordinarily nuanced sense of our environment.
Our experience of wetness arises in this way. It comes from the combined input of cold and friction receptors, refined by association with previous experience.
This means our brain can detect wetness without needing a specific sensory receptor activated by moisture. All we need is a set of related sensory inputs that, taken together, are consistent with our concept of wetness.
This integration of information even extends to include our other senses (the look, smell and sound of the material), and draws on related context (such as "is it currently raining?") to inform our subconscious assessment of whether something is wet.
It's not a perfect system
Sensory integration is only as good as the quality of input it receives. It can be hard to feel if clothes are damp in wintry conditions, because both cold and wet things activate our cold receptors.
Likewise, in hot weather it's much easier to tell when our skin or clothes are damp, as evaporation leads to both local cooling and a slight change in pressure on the skin.
However, we can experience similar challenges in sensing wetness in a warm environment: try closing your eyes in a warm bath and lying still. Without evaporation or cooling from the contact of water against your submerged skin, it can become difficult to consciously perceive that those parts are wet - at least until you lift your arm out of the water and the skin cools as water evaporates.
Our lack of specific wet receptors doesn't mean our perception of wetness is an illusion generated by our brain. Sensory integration allows us to sense a consistent and tangible quality of objects.
While this sense may sometimes be unreliable, our perception of wetness is not totally untrustworthy. On the contrary, the precision with which we navigate our environment by combining information from different sensory receptors is, frankly, awe-inspiring.
There's a way to help your brain
Next time you're trying to figure out whether your washing is damp or just cold, you can try and use some of these insights.
Help your brain out by adding additional context to the signals you're receiving from the nerve endings in your hand. Squeezing or rubbing the clothes to generate more friction can enhance the mechanical information that influences our ability to detect wetness.
You can also try using visual information, such as variations in colour across the fabric when lightly stretched, to add fresh sensory context.
Or, shifting from cold and damp to warm and damp by heating a small part of material with a hairdryer (or your own breath) will make it much easier to tell if the clothes are dry, or … if you need to hang them all out again.
Kate Poole receives funding from the Australian Research Council and Development and has previously received funding from the National Health and Medical Research Council.
Felix Aplin receives funding from the Australian and New Zealand College of Anaesthetists and has previously received funding from the National Health and Medical Research Council.
