Pupillary light reflex

How it works

The pupil is a hole. Its size is set by two muscles in the iris. One is a ring that squeezes the hole smaller. The other is a set of spokes that pull it wider⁴. In bright light the pupil of an adult is about 2 to 4 mm across⁴. In the dark it opens to about 4 to 8 mm⁴.

The reflex starts in the retina. Light-sensing cells send a signal up the optic nerve. Some fibres peel off before the visual cortex and go to a small cluster of cells in the midbrain called the pretectal area³. From there the signal goes to both sides of the midbrain, not just one. Each side then sends an order down the oculomotor nerve to its own iris. Both pupils shrink¹.

That crossing is the key. Light in the left eye alone shrinks the right pupil too. This is called the consensual response¹. It means that the two pupils are always the same size, however the light falls.

Doctors use the reflex constantly. A torch swung from one eye to the other should keep both pupils small. If the pupils get bigger when the light moves to a certain eye, that eye’s optic nerve is sending a weak signal. This is a Marcus Gunn pupil, and it points to damage in that nerve². Pupils that do not react at all in an unconscious person are a serious sign, which is why paramedics check them at once¹.

Try it

1. Stand at a mirror in a dim room. Wait a minute for your pupils to open up.
2. Cover your left eye with your hand and keep it there for 20 seconds.
3. Watch the pupil of your right eye in the mirror.
4. Take your hand away quickly. Both pupils shrink together, even though only the left eye saw a change.
5. Now hold a phone torch beside your right eye, pointing at it from the side, while you watch your left eye in the mirror. The left pupil shrinks too.

Use a normal torch, not a laser and not anything painfully bright.