How we sense temperature
3 water tight containers, each with a capacity of between 2-5 litres, and big enough for somebody to submerge their hand in
2. A couple of cupfuls of ice
3. 1-2 litres of cold tap water
4. 2-5 litres of hot water (bath temperature, about 40oC)
5. 2-5 litres of tepid water (cold water with a dash of warm added in, about 22oC)
6. You, with your sleeves rolled up.
Set up your containers:
Add ice plus cold water to the left hand side container
Add room temperature water to the middle container
Add bath temperature water to the right hand side container
Plunge your left hand into the container holding the ice, and your right hand into the container holding the bath temperature water. Make sure the water goes up to at least the base of the fingers -up to the 1st knuckles, also called the major knuckles. Leave your hands in the water for about 2 minutes.
Then simultaneously move both of your hands into the middle container.
What do you feel?
You will probably be experiencing something quite peculiar - a mismatch, or difference in temperature sensation, between the two hands. Even though both hands are now in the same container, and experiencing the same temperature, the left hand should feel hot, whilst the right hand should find the water pretty chilly.
Which part of the brain is responsible?
The somatosensory cortex. This brain region is band of tissue running along the top of your brain, from the back of one ear to your other ear and processes all of the sensory information. The somatosensory cortex sits below where your headphones rest on your head.
Why is this happening?
You are experiencing something called a sensory adaptation - a phenomena that hands are particularly prone to.
Our hands, especially our fingertips, are well evolved to help us collect information to help explore the world around us, providing us with information about temperature, texture and shape. Human fingertips contain some of the densest areas of nerve endings on the body - there's about 25, 000 nerve receptors per square cm! It is this abundance of nerve endings that allow them to collect information to such a fine degree and send signals to the brain to process this information.
Your hands, and fingertips, are key components of the so called somatosensory system which provide us with our physical sensation of the world. This also includes our skin, muscles, the heart, joints, bones and heart.
Sensation detecting nerves found in this system are called sensory nerves and are activated by different sensations, be it temperature, pain, or tactile sense (touch). On the end of each sensory nerve there are many different receptors which detect different feelings. For example, thermoreceptors specifically detect temperature. Some thermoreceptors detect cold conditions whilst other thermoreceptors are activated by warmth.
In this experiment when the left hand is placed in ice cold water the cold sensitive thermoreceptors are activated causing an electrical pulse which passes down the sensory nerve in the fingertips and hands to the brain.
On the other side, when the right hand is placed in the bath warm water is having its heat thermoreceptors activated, causing a different type of electrical pulse to propagate down the warm sensory nerve in the fingertips and hands to the brain.
Electrical information from the thermoreceptor activation is passed from your hands, along your arms, up through the top part of your spinal cord and into the brain through the sensory nerves. The information is then processed in the region of the brain called the somatosensory cortex, see above.
If your hand is exposed to heat for a long time then the hot sensitive receptors, will, much like muscles after a long workout, start to get tired. They become less sensitive to the stimulus and dampen down their activity and decrease the electrical signal which is sent to the somatosensory area of your brain.
The same things happen to the cold receptors; if your hand is exposed to the cold for a long time then the nerve endings become less sensitive to cold.
You desensitised your cold nerve endings on your left hand by exposing them to ice cold water. When you then moved their hand to a warmer environment the cold sensitive receptors had adapted and dampened down their activity, but the warm receptors had not, and comparatively had high potential levels of activity meaning your left hand perceived the middle container to be warmer than it really was.
On the right side, you effectively wore out your hot sensitive nerve endings by exposing them to warm water. When you moved your hand to a colder environment the hot sensitive receptors had adapted and dampened down their activity, but the cold receptors had not, so the right hand perceived the middle container to be colder than it really was.
This process of adaptation of the thermoreceptors and sensory nerves explains why you experienced such a mismatch of temperature sensation when your hands were in the middle container. Sensitivity to temperature had altered based on your previous environment.
The same process explains why when you first jump into the sea on a really warm day the sea feels rather chilly at first, but then you don't notice the temperature so much. But if you jump into the sea on a colder day the sea doesn't seem to be that nippy. It's all relative!
Similarly when you first rest your hand on a table you notice the texture and temperature of the table but after a while you don't feel it. Take your hand away for a while and then put it back and you will start to notice the sense of the table again. This is also due to sensory adaptation.
Why do scientists study this?
There's lots of research being done on temperature sensation. Scientists study earthworms, zebra fish and fruit flies to understand it! Why? Well it helps to give us a better handle on how our nervous system takes in information about the world around it, and processes it to provide our perception of the world.
One study has found that people with a condition called major depressive disorder are less sensitive to really cold temperatures. We have no idea why this might be! Is the depression causing a lower response to pain, or the other way round? Or are the temperature pain and depression circuits in the brain linked somehow? Flies, earthworms and zebrafish don't have the answer for us on this just yet but may well one day soon.
The Risky Part: what to be aware of and how to keep the science safe:
Risk to Audience
Risk to Presenter
Residual Risk to Audience
Residual Risk to Presenter
Temperature Sensation and Adaptation
Temperature - ice and hot
Risk to Volunteer - make sure that the warm water is not too hot. It should be the same temperature as a comfortable bath. If the volunteer experiences any discomfort from the warm or cold water they should remove their hands from the containers.
Ensure that the bath warm water is not too hot.
Molecular mechanism for trimetric G protein-coupled thermosensation and synaptic regulation in the temperature response circuit of Caenorhabditis elegans, Neurosci Res. 2013 Mar 28. pii: S0168-0102(13)00085-0. doi: 10.1016/j.neures.2013.03.008.
The zebrafish ortholog of TRPV1 is required for heat-induced locomotion, J. Neurosci. 2013 Mar 20;33(12):5249-60. doi: 10.1523/JNEUROSCI.5403-12.2013.
Dopaminergic neurons encode a distributed, asymmetric representation of temperature in Drosophila. J. Neurosci. 2013 Jan 30;33(5):2166-76a. doi: 10.1523/JNEUROSCI.3933-12.2013.
Increased cold-pain thresholds in major depression. J. Pain. 2010 Mar;11(3):287-90. doi: 10.1016/j.jpain.2009.07.012.