In many animals the brain has some control over its own sensory input. The horseshoe crab, Limulus, provides a rather simple visual system demonstrating that what the brain tells the eye can influence what the eye tells the brain. The incoming visual information is called the afferent signal, and the central regulatory signal to the eye is called the efferent signal. There is a day-night, or circadian, rhythm in the brain that transmits efferent nerve impulses to the eyes during the night, inducing multiple changes in the retina of the eye that produce a million-fold increase in sensitivity of the eye, for working in the dark. This project will use anatomy, physiology, behavior, and computational neuroscience approaches to study how the retinal rhythms of sensitivity relate to changes in the visual system and the animal's behavior. Long- term physiological recordings will be made with precise control of efferent signal to the retina. The retina of this compound eye contains about 1000 nerve cells, and is the largest network of neurons for which a quantitative description exists. Its properties will be modeled computationally on the massively parallel Connection Machine, and the resulting properties will be related to the animal's visual behavior, particularly the ability to see and respond to the presence of other animals at night underwater. Results from this unique study, from molecules to behavior, will have an impact on work in visual neuroscience, biological rhythms, efferent control of sensory systems, feedback regulation of engineering systems, and on understanding more about the neural basis of behavior.
