A Marcus Gunn pupil is said to occur when there is a unilateral lesion in the afferent visual pathway anterior to the chiasm.
The integrity of the afferent pathway may be ascertained by rapidly stimulating each eye in succession with a torch and observing the direct and consensual light response in each. The normal light response is pupillary constriction. In a Marcus Gunn pupil, there is reduced afferent input and the pupils fail to constrict fully. Stimulation of the normal eye produces full constriction in both pupils. Immediate subsequent stimulus of the affected eye produces an apparent dilation in both pupils since the stimulus carried through that optic nerve is weaker.
In severe optic nerve lesions, the eye is amaurotic but a a consensual light response can still be demonstrated.