Voluntary pupil control

How it works

The pupil is a hole in the iris. Two smooth muscles set its size. A ring muscle, the sphincter, tightens to shrink it. A set of spoke-like fibres, the dilator, pull it wider³. Both are smooth muscles run by the autonomic nervous system¹. Smooth muscle does not take orders from the conscious brain. In theory, nobody should be able to change their pupils by will.

A few people can. The best-studied case is a 23-year-old German man known as D.W. He could widen his pupils by about 0.8 mm and shrink them by about 2.4 mm on command, with no light change and no mental trick². Brain scans while he did it showed activity in the dorsolateral prefrontal cortex and premotor areas, regions that normally plan movements of the skeletal muscles². He described it as focusing on the eye itself. How the signal reaches the smooth muscle is not known.

Indirect control is a different thing, and almost everyone can do it. Mental effort widens the pupil. Doing sums in your head produces a change of about 0.5 mm². Imagining a bright light makes the pupil shrink a little. These are normal responses to thought, not direct control.

Try it

This tests indirect control. Direct control cannot be learned by any known method.

1. Sit in front of a mirror in even, moderate light. Do not change the lighting during the test.
2. Look closely at one pupil and note its size.
3. Count backward from 300 in steps of 7. Keep watching. The pupil should widen slightly.
4. Stop counting. Imagine staring into the sun. The pupil should shrink slightly.
5. Expect a change of half a millimetre at most. A phone video helps you see it.