In 1939 one of us (Lowenstein1) described a pupillary reflex which he called the "reflex to darkness." In order to elicit this reflex both eyes of the subject tested are adapted to a constant level of illumination in which darkness is a periodic stimulus. Two small lamps separated from each other by a screen (so that the right eye cannot see the left lamp and vice versa) burn continuously for a period of twelve minutes, which is the time necessary for adaptation. The light of one or the other of these lamps is periodically interrupted by a motor interrupter for one second.2 During the test the patient fixes on a nonluminous point in the distance. The resulting movements of the pupils are recorded by pupillography.
The reflex to darkness is manifested both directly and consensually. The pupillographic representation of a normal reflex to darkness is shown schematically in
LOWENSTEIN O, GIVNER I. PUPILLARY REFLEX TO DARKNESS. Arch Ophthalmol. 1943;30(5):603–609. doi:10.1001/archopht.1943.00880230035004
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