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. Spoke-like fibres, the dilator, pull it wider³. Both run on 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.

At least one person can. In 2021 a German team published a single case, a man who changed his pupil size on command⁴. He widened his pupils by about 0.8 mm and shrank them by about 2.4 mm, with no light change and no mental trick⁴. The team tested the obvious cheats, such as focusing on something near or raising mental effort, and none of them explained what he did⁴. Brain scans taken while he did it showed activity in the dorsolateral prefrontal cortex and nearby premotor areas, regions that make and pass on voluntary commands⁴. How the signal reaches the smooth muscle is still not known⁴.

Indirect control is a different thing, and almost everyone can do it. Mental effort widens the pupil a little². Imagining a bright light makes it shrink a little. These are normal responses to thought, not direct control.

How common it is

This is a count of studied people, not a rate. One case of direct pupil control has been studied with modern methods, in a paper published in 2021⁴. Older reports describe more cases, including one from 1895⁴. Most of those were probably indirect tricks⁴. Nobody has screened an ordinary group of people for the ability, so nobody knows how many can do it. The ability is easy to miss unless someone tests for it. A screening study of a few hundred people would change the picture. Indirect widening through mental effort is different and works in almost everyone².

Try it

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

1. Sit at a mirror in even, moderate light. Do not change the lighting.
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 very small change, easy to miss. A phone video helps you see it.

Is it a problem?

No. Direct control, and the indirect control almost everyone has, say nothing about your health¹⁴. It is a curiosity, not a sign of anything wrong with the eyes or the nerves.

See a doctor if:

• a pupil changes size on its own,
• a pupil stays unusually large or small,
• the two pupils become different sizes when they were not before.

Those changes are unrelated to this trick and are worth checking¹.