A test used to detect a relative afferent pupillary defect or Marcus Gunn pupil. Direct light is shone into the normal eye; both pupils constrict equally due to the consensual response. Light is quickly swung over to the contralateral pupil, which appears to dilate as a result of a relative decrease in the optic nerve fiber function in the affected eye. This response is seen in asymmetrical optic nerve diseases such as glaucoma, optic neuropathy, and optic neuritis.