Pupillary light reflex
Also called: Photopupillary reflex · Pupil reflex
Shine a light into one eye and both pupils shrink, even the eye in the dark. The wiring crosses in the midbrain so that the two pupils react together. You can watch it in a mirror.
How common
Does this happen to you?
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 is about 1.5 to 4 mm across4. In the dark it opens to about 3 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 both pupils react together, whichever eye the light falls on1.
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 is sending a weak signal. This is a Marcus Gunn pupil. It usually points to damage in that eye’s optic nerve or retina2. Emergency doctors check the reflex to test the brain stem, because brain stem damage can make it abnormal1.
Try it
- Stand at a mirror in a dim room. Wait a minute for your pupils to open up.
- Cover your left eye with your hand and keep it there for 20 seconds.
- Watch the pupil of your right eye in the mirror.
- Take your hand away quickly. Both pupils shrink together, even though only the left eye saw a change.
- 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.
Is it a problem?
No. The reflex itself is not a problem. It runs automatically in every sighted person and needs no attention1. Doctors rely on it precisely because it is so reliable, using it to check brain function in people who cannot respond in other ways1.
What is worth mentioning to a doctor is a change in the reflex itself:
- one pupil that stops reacting,
- two pupils of clearly different sizes,
- a pupil that reacts to near objects but not to light5.
Sources and further material
- Wikipedia Pupillary light reflex
- Wikipedia Relative afferent pupillary defect (Marcus Gunn pupil) The sign doctors look for when one optic nerve is damaged.
- Wikipedia Pupil
- Wikipedia Argyll Robertson pupil Pupils that react to near objects but not to light, called light-near dissociation.
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Related oddities
Accommodation reflex
When you look at something close, three things happen at once. The eyes turn inward, the pupils shrink, and the lens in each eye gets fatter. You do not decide any of it.
Voluntary pupil control
Changing the size of your pupils by will alone, without imagining a light or doing sums. One case has been studied with modern methods. Older reports exist, but most describe indirect tricks. Indirect control by thinking hard is possible for almost everyone.
Photic sneeze reflex
About one person in four sneezes when they step into bright sunlight. It runs in families, and the leading idea is a wiring mix-up between the nerve for the eyes and the nerve for the nose.
Afterimages
An image that stays in your vision after the thing you looked at is gone. Bright things leave a same-colour copy for a moment. Long stares leave a copy in opposite colours.