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©2013 UpToDate ® Print Email Left afferent pupillary defect The left pupil is the same size as the right under all conditions of illumination because the efferent pathways are intact, but both pupils are smaller when light is directed at the right eye than when it is directed at the left eye, because light is detected better by the right eye. Alternate swinging of the light between the two eyes therefore produces dilation each time the light is directed to the left eye. With permission from: Gelb DJ. The Neurologic Examination. In: Introduction to Clinical Neurology. Woburn, MA, Butterworth-Heinemann 2000.