How does the pupillary light reflex affect both eyes when light shines near one eye?

When light shines into one eye, the pupillary light reflex causes both pupils to constrict to nearly the same extent. This occurs because the reflex is bilateral.1

The underlying mechanism involves the pretectal nucleus in the midbrain, which serves as a relay center for pupillary responses. Fibers from the pretectal nerve cells undergo partial crossing-over, allowing signals to reach the Edinger-Westphal nucleus on both sides of the brain. Consequently, the motor neurons responsible for pupillary constriction are activated for both eyes simultaneously, even when only one eye is exposed to the stimulus.1