When the name and the ink colour are different, most people slow down.

When you try to say the ink colour, you cannot avoid reading the word. If the two bits of information conflict, your brain struggles to work out what the correct answer is, and it takes longer.

This test is very sensitive to subtle changes in brain function. Lack of sleep, fatigue, minor brain injury and high altitudes will all increase the time it takes to do the test. The test has even been used on Everest expeditions to see how altitudes are affecting different people.

This is called ‘The Stroop effect’ and was discovered in 1935 by John Ridley Stroop.