Accommodation reflex
Also called: Near reflex · Accommodation-convergence 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.
How common
Does this happen to you?
How it works
A camera focuses by moving its lens back and forth. The eye focuses by changing the shape of its lens. The lens hangs inside the eye from a ring of fine fibres. Around that ring is the ciliary muscle3. When the muscle is relaxed, the fibres are tight and pull the lens flat. A flat lens focuses on far objects. When the muscle contracts, the ring gets smaller, the fibres go slack, and the lens springs into a rounder shape. A round lens focuses on near objects2.
This lens change is one part of a set of three. When you shift your gaze from far to near, the eyes also rotate inward so that both point at the same spot, and the pupils shrink to sharpen the image1. All three run together and are known as the near triad. The signal comes from the oculomotor nerve, the third cranial nerve. Its parasympathetic fibres work the lens and the pupil, and its motor fibres turn the eyes inward1.
Blurred vision is the trigger. The visual cortex notices that the image is out of focus and passes the message down to the brainstem, which adjusts the lens until the image is sharp1. It happens on its own, and you never notice.
The lens stiffens with age. It can no longer round up as far, so the closest point you can focus on drifts away from your face. A child can focus on an object about 10 centimetres away2. By the mid-forties most people hold a book at arm’s length and need reading glasses. This is presbyopia and it is universal4.
Is it a problem?
No. The near reflex is a normal, automatic part of vision. Its one common side effect is presbyopia, the drop in near focus that most people notice after age 404. A pupil that shrinks for near objects but does not shrink in bright light is a different matter. This pattern, called light-near dissociation, points to a nerve problem and should be checked by a doctor5.
History and culture
Doctors test the reflex by asking you to look at a far wall and then at a pen held close. They watch for the pupils to shrink and the eyes to turn in. A pupil that shrinks for near objects but not for bright light is a classic sign in neurology, first described in the 1860s by Douglas Argyll Robertson5.
Sources and further material
- Wikipedia Accommodation reflex
- Wikipedia Accommodation (vertebrate eye)
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Related oddities
Pupillary light 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.
Crossing your eyes
Turning both eyes inward toward the nose on purpose. Most people with normal two-eyed vision can do it. Your eyes will not get stuck that way.
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.
Purkinje images
A candle held near someone's eye produces four tiny reflections, one from each surface of the cornea and lens. Eye trackers and eye doctors use them. In the right conditions you can see two of them inside your own eye.