The possibility of an inhibition of the pupillary light reflex during the suppression phase in binocular rivalry was reported by Bárány and Halldén1 in 1948. In their study the rivalry patterns consisted of a black horizontal line presented to the left eye and a black vertical line presented to the right eye, the luminances of the target backgrounds being about 35 lux. The fovea of the left eye could be stimulated by a flash of light of 0.01-second duration. The intensity of the flash was so adjusted as to produce a recognizable pupillary reaction for about 30% to 70% of the flashes. The visual angle subtended by the flash stimulus was 55 minutes of arc. The room was dimly lighted during the experiment. The pupillary response of the subject was observed visually by the experimenter through telescopic spectacles. During the experiment the subject would report when he saw only