Why Do Both Pupils Constrict When Light Enters One Eye?

When a small light is directed into one eye, that pupil constricts—and the pupil of the eye that was not illuminated usually constricts as well. The second eye has not somehow “seen” the light. Rather, the neural pathway for the pupillary light reflex sends signals to both sides of the midbrain.

After light reaches the retina, relevant signals travel along the optic nerve to the pretectal region. From there, the pathway communicates with parasympathetic control nuclei on both sides. Signals then pass through the oculomotor nerves and ciliary ganglia to the iris sphincter muscles. The illuminated eye therefore shows the direct light reflex, while the other eye shows the consensual light reflex.

This bilateral response is useful because the two eyes usually face similar ambient light, and coordinated pupils help regulate light entering both eyes. It is not a simple mechanical linkage: each side depends on an intact sequence involving the retina, optic nerve, midbrain pathways, oculomotor nerve and iris muscle. Clinicians shine a light into each eye in turn because the pattern of direct and consensual responses provides information about where signals still travel. This is an explanation of normal physiology, not a method for self-diagnosing eye or neurological conditions.

https://www.ncbi.nlm.nih.gov/books/NBK537180/