Binocular interactions in the entrainment and phase shifting of locomotor activity rhythms in Syrian hamsters

To assess binocular interactions and possible ocular dominance in entrainment of circadian rhythms, Syrian hamsters maintained in LL were subjected for several weeks to schedules of eye occlusion with opaque contact lenses. In separate groups, the opaque lens was inserted into the left or right eye for 12 h at the same clock time each day. The left and right eyes of other groups were alternately occluded for 12 h each day, with initial occlusion of either the left or right eye for different groups. A majority of hamsters entrained their locomotor activity rhythm when 1 eye was occluded for 12 h. The modified visual input imposed by covering 1 eye is sufficient to induce entrainment. Locomotor rhythms of most animals in which the 2 eyes were alternately occluded for 12 h each day phasedelayed onset of activity during the 1st few days of the lensing procedure; activity onset then free ran with tau < 24 h for several weeks until entraining with tau of 24 h regardless of whether the left or right eye was initially occluded. Entrainment eventually occurred when activity onset coincided with occlusion of the eye contralateral to the one that was first lensed. Photic and nonphotic explanations for eventual entrainment of locomotor rhythms are discussed, and evidence for asymmetrical photic input from the 2 eyes to the SCN is considered.