Circadian modulation of the Limulus eye for day and night vision

The horseshoe crab has been an outstanding model for vision and circadian research. Its lateral eyes are probably the best understood neural structure in the animal kingdom. Fundamental principles of image processing were first gleaned from the animal, including phototransduction, light adaptation, and lateral inhibition. A circadian clock in the brain modulates anatomical and physiological properties of the eyes to increase visual sensitivity each night. As a result, horseshoe crabs can use their eyes to locate mates equally well day and night. This chapter summarizes studies that have localized the clock output neurons that regulate vision, characterized their patterns of electrical activity, and identified octopamine as their primary circadian neurotransmitter. We are at a stage now where we can begin to model the response of each photoreceptor and each optic nerve fiber to natural visual scenes day and night.