The pupillary reactions during the alternating light test of Levatin consist of bilateral pupillary constriction followed by redilatation each time the light moves from one eye to the other. The pupils constrict less and redilate more in an eye with an optic nerve lesion than in the unaffected fellow eye. The amplitude of pupillary responses and the average pupil size decrease when the light is alternated more rapidly. Placing neutral density filters over the better eye has the effect of decreasing the amplitude of pupillary responses when that eye is stimulated while increasing responses of the diseased eye. Because of contraction anisocoria and physiologic anisocoria, the most reliable pupillographic and clinical criterion for diagnosing a relative afferent pupillary defect using the alternating light test is an initial pupillary constriction in the suspected eye that is smaller with direct than with consensual stimulation.