The primate visual system is organized with remarkable spatial precision. In the central retina, which supports our finest spatial discriminations, signals from each of thousands of photoreceptors are faithfully transmitted into the central visual system along a series of synaptic relays which are so refined that they lose no significant information about spatial precision. This precise order arises during development as a result of visual experience; indeed, visual experience of clearly focused, stable, and binocularly aligned images seems to be required (for review, see Movshon and Kiorpes 1993). If visual experience in early life is abnormal, monkeys and humans commonly develop a disorder of spatial vision called amblyopia. Amblyopia has no obvious organic cause. It manifests itself as a deficit in visual acuity that cannot be corrected optically.