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Science corner

Sophie Duncan plays tricks on the brain

When does hot feel like cold? How can you make a white wall look even brighter? This week's science corner explores how to trick the brain.

Take three bowls of water and label them one to three. In the first bowl put some cold water and some ice. In the second put water at room temperature, and in the third put hot water. (Take care not to make this water too hot. You need to be able to hold your hand in it for 30 seconds without feeling uncomfortable or burning yourself.) To do the experiment, place your right hand in bowl one and your left hand in bowl three. Wait for 30 seconds. Now place both hands in bowl two and observe how hot the water feels over the next 30 seconds.

You should find that the water in bowl 2 feels hotter to your left hand than to your right hand. This is because the receptors that are sensitive to temperature respond to changes to the temperature they have got used to.

The right hand experienced the water at room temperature as warm in comparison to the cold water in bowl one, whereas the left hand experienced it as cold in comparison with the water in bowl three. This mismatch was adjusted as both hands got used to the water at room temperature.

The second experiment is slightly different. Take a piece of A4 white paper and roll it into a tube lengthways. Stand in front of a white wall and hold the tube in front of one eye.

Keeping both eyes open look at the wall. You should find that the wall observed through the tube looks lighter than the wall observed by the other eye.

This is because our eyes are sensitive to the amount of light they receive.

When the receptors on the back of the eye are stimulated by a lot of light, they send a signal to the brain which in turn causes the sensitivity of surrounding receptors to be turned down.

The wall of the tube makes a dark ring, so the brain does not send a signal to turn down the sensitivity of the receptors, and the hole looks bright.

Sophie Duncan is project manager for science at the BBC

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