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The retina, the filmy piece of tissue lining the back of the eyeball, is considered the best understood part of the brain. Intricate exchanges among the neurons of the retina allow much image construction to take place in the eye itself. Visual perception begins within cells called photoreceptors, where proteins embedded in cell membranes change shape when exposed to light. Layers of other retinal cells transfer signals from the photoreceptors to the optic nerve. The intervening layers process the signals--among other tasks, adjusting neural responses under different lighting conditions and increasingly distinguishing objects from their backgrounds. Vision researchers estimate that careful study of the anatomy and physiology of the vertebrate retina has clarified about half of all the interactions in this tissue.