Which statement best describes the effect of light on the pupils?

• A. More light causes dilation
• B. More light causes constriction
• C. Less light causes dilation
• D. Light has no effect

Answer: (B)

In bright light the pupils constrict to limit the amount of light reaching the retina and protect vision. This reflex is driven by the parasympathetic system: light information goes from the retina to the pretectal area, then to the Edinger–Westphal nucleus, and fibers traveling with the oculomotor nerve cause the iris sphincter muscle to tighten, narrowing the pupil. In dim light, the opposite occurs—the sympathetic system stimulates the iris dilator muscle, enlarging the pupil to let in more light. So the best statement is that more light causes constriction; less light leads to dilation. Light clearly affects pupil size, so saying there’s no effect isn’t correct.