Pupil response to light is an indicator of which brain function?

• A. Cortex
• B. Hypothalamus
• C. Limbic
• D. Brainstem

Answer: (D)

The pupil light reflex is a test of brainstem function. The reflex arc is subcortical: light signals travel from the retina via the optic nerve to the pretectal area in the midbrain, then bilaterally activate the Edinger–Westphal nuclei. The oculomotor nerve carries the parasympathetic signal to constrict the pupil. Because this whole pathway resides in the brainstem, a normal pupil constriction in response to light indicates intact brainstem function, even if higher cortical (conscious) processing is impaired. The cortex, hypothalamus, or limbic system aren’t the primary controllers of this reflex, so their integrity isn’t what this test assesses. If the reflex is absent or absent in one eye, that can point to brainstem dysfunction.