Key Concepts
Sensory nervous system, temperature, heat

Introduction

Have you ever tried to guess the temperature of the water in the pool? On a hot day, the water might feel chilly at first, but once you’re in, you don’t notice the temperature as much. On a cool day, though, the pool water that is the same temperature often feels quite comfortable. Is it all relative? Is our body equipped to tell absolute temperature?

The question might make you curious about how our body collects information about our environment, processes it and forms our perception of the world. Do this activity, and the next time you jump in the pool on a hot summer day, you will be able to understand why you’re about to feel so cold!”! 

This activity is not appropriate for use as a science fair project. Good science fair projects have a stronger focus on controlling variables, taking accurate measurements, and analyzing data. To find a science fair project that is just right for you, browse our library of over 1,200 Science Fair Project Ideas or use the Topic Selection Wizard to get a personalized project recommendation.

Background

Our hands, especially our fingertips, are well-equipped to collect sensory information from the environment surrounding them. They contain an overwhelming amount of sensory receptors. External circumstances, like temperature, texture and touch prompt these receptors to produce electrical signals. The signals travel through a sensory nerve along the arm to the brain, where they are processed, then compared to past experiences and finally they are labeled.

Each receptor is triggered by a specific stimulus. Thermoreceptors detect temperature changes. We are equipped with thermoreceptors that are activated by cold conditions and others that are activated by heat. Warm receptors will turn up their signal rate when they feel a warming or heat transfer into the body. Cooling or heat transfer out of the body results in a decreased signal rate.  Cold receptors, on the other hand, increase their firing rate during cooling and decrease it during warming. 

Something interesting happens when your expose receptors to a specific sensation like heat for a long time; receptors start to tire out, decrease their activity and you will no longer notice the sensation as much.

Could this desensitization also alter our sensitivity to a subsequent stimulus? Do this activity and found out!

Materials

  • Three pots, big enough in which to emerge two hands
  • Warm water, do not make it too hot
  • Room-temperature water
  • Ice-cold water or ice cubes
  • Towel to protect your work surface
  • A clock to time yourself

Preparation

Safety Tip: Be sure not to make the warm water too hot. If you experience any discomfort from the warm or cold water, remove your hands from the pots, let them adjust to room temperature and start again with water that is at less extreme temperatures.

  1. Prepare a work surface that can get a little wet by either laying down a towel, or removing any objects that should not get wet.
  2. Fill one pot with ice-cold water. You can also use room-temperature water and add a couple of ice cubes to cool the water in this pot.
  3. Fill a second pot with room-temperature water.
  4. Fill a third pot with warm water. Be sure not to make the water too warm; you need to be able to comfortably have your hands in this water for a little while.

Procedure

  1. Put your right hand in the pot with ice-cold water. Does this water feel hot, warm, lukewarm, cold or very cold?
  2. Put your left hand in the pot with warm water. How does this water feel?
  3. After having you hands in the pots for about a minute or two, pay attention to the temperature of the water in each pot again. Does the cold water still feel as cold as it initially did? What about the warm water?  If it feels differently, do you think the actual temperature of the water in the pots changed considerably during this short time, or has your perception of the temperature changed?
  4. Now, remove your hands from the pots with ice-cold and warm water and place both hands in the pot with room-temperature water. How would you label the temperature of the water in the pot? Does it feel hot, warm, lukewarm, cold or very cold? If it is hard to say, pay attention to what you would say if you felt only with your right hand, and what would you say if you felt only with your left hand? Do your hands agree or disagree about the temperature of the water?

EXTRA:  Instead of using two hands, give your index finger a warm bath and your middle finger of the same hand a cold bath. The sensory signals created by the thermoreceptor in this test run along the same sensory nerve up your arm to the brain. Would you still be able to say one finger feels cold and the other finger feels warm? Would you still get confusing messages when after a minute, you put both fingers in water at room temperature? Now try with a fingertip touching an ice cube and a warm cloth at the same time. Are you still able to say that half of the tip is warm and the other half is cold? Are you still confused when you put the fingertip on a room-temperature object?

EXTRA:  In this experiment, the water in the pots is at different temperatures. What if you put your hand in contact with objects that feel cold or warm, but are at the same temperature, like a metal door knob or pot and the carpet or a wool sweater? These objects are both at room temperature, but appear to be different in temperature because they conduct heat differently. Let your whole hands touch these objects. Does the difference in temperature perception reduce over time in this case? Do you still get confusing messages if, after a while, you put your hands in contact with a third material, like glass?  

Observations and Results

Did the right hand feel as though the water was hot, while the left hand experienced it as chilly?

When you initially placed your right hand in the cold water, cold thermoreceptors in your hand fired creating signals, which, after being processed in the brain, enabled you to label the water as “cold.” As the left hand was put in hot water, warm thermoreceptors initiated signals, allowing you to identify the water in this pot as “warm.”

After a while, the thermoreceptors in your hands wore out. They got desensitized and the water in the pots did not feel as cold or as warm anymore.

When you placed both hands in a pot of room-temperature water, your brain got confused. Your right hand entered with desensitized cold thermoreceptors and active warm thermoreceptors. The heat flow into the cold hand fired the warm thermoreceptors. Your brain interprets these as coming from a warm environment. You perceived the water with your right hand as warmer than it really was. A similar process happened in your left hand, which entered with desensitized warm thermoreceptors and experienced heat flow from the warm hand to the room-temperature water. Your left hand felt as though the water was colder than it really was.

As your hands perceived the water in the room-temperature pot differently, you got confused. Your brain returned conflicting information about the temperature of the water in the room-temperature pot. This experience shows that your perception of temperature is influenced by the previous environment.

Cleanup

  1. Dispose of the water and clean the pots and any spilled water.

More to Explore

Credits

Sabine De Brabandere, PhD, Science Buddies

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Key Concepts
Sensory nervous system, temperature, heat
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