Marcus Gunn pupil sign [Robert Marcus Gunn, English ophthalmologist, 1850–1909] , paradoxic dilation of the pupils in an ophthalmological examination in response to afferent visual stimuli. In a dark room a beam of light is moved from one eye to the other. Normal miosis is caused by the consensual pupil reaction when the normal eye is illuminated, but as the light is moved to the opposite, abnormal eye, the direct reaction to light is weaker than the consensual reaction; hence both pupils dilate.
