Although Virgil, MM and others like them certainly possess a rudimentary form of vision, decades of visual deprivation may never be completely redeemable. The human brain has an amazing capacity for plasticity, but there are some things that it cannot do. MM will likely never see the way that we see.

MM's visual capacities continue to improve, but he also remains somewhat uncomfortable with his new sense. As a blind person, MM became extremely proficient at skiing, with the help of a guide to give him oral directions. After his eyesight was restored, skiing frightened him. The trees, snow, slopes, people -- everything whizzed by him, chaotic and uninterpretable. After much practice, he is now a moderate sighted skiier -- but when he really wants to go fast and feel confident, he closes his eyes.

There is a window of opportunity in youth, often called a critical period, during which the brain can best form neural connections that correspond both to retinal images and to practical experience. During the critical period for the visual cortex, normal visual input is required to wire everything correctly. If input is missing during this period, the brain's links will probably not be built correctly. In fact, brain tissue ordinarily used in visual processing might even be taken over by other systems, perhaps tactile or olfactory systems. Some of MM's visual abilities lend further support to the theory that he missed a critical period of visual development. He is quite good at visual tasks that involve motion. Tasks that stumped him at first often became solvable if motion was incorporated into them. He became able to detect the circular patterns in random noise if the patterns were moving. And he began to see the "square with lines" as a cube if the lines moved, and the cube appeared to be rotating. At the end of their evaluations, the researchers saw some patterns emerging in MM's visual abilities and deficiencies. His ability to detect and identify simple form, color, and motion is essentially normal. His ability to detect and identify complex, three-dimensional forms, objects, and faces is severely impaired. The researchers have a tentative explanation for these variations in visual skill. Motion processing develops very early in infancy compared with form processing. By the time MM lost his eyesight in the accident, the motion centers in his brain were probably nearly complete. So when he regained some eyesight in his forties, those connections in the brain were ready to go. The parts of the brain that process complex shapes, however, do not develop until later in childhood, so MM's brain likely missed its chance to establish those particular brain connections. The authors also propose that our brains may retain the ability to modify and refine complex form identifications throughout life, not just throughout childhood. New objects and faces are continually encountered throughout life, and our visual processing centers must be able to adapt and learn to see new shapes and forms. MM's brain never had the chance to learn.

his problems didn't seem to be vision deficiencies so much as visual interpretation deficiencies. And deficiencies of this sort lie not with the retina's ability to perceive light and color, but with the brain's ability to process the retina's signals correctly. We usually do not think of the above problems as involving interpretation, because we have performed these interpretations so many times, and from such a young age. But since MM lost his sight at an early stage of development, since he had no visual input into his brain after age three, the researchers suspect that the visual centers in his brain did not develop normally -- and now, they likely never will.

By far the most difficult tasks for MM involve three-dimensional interpretation of his environment. When an image is projected onto the retina, it is two dimensional, because the retina is essentially flat. When we are very young, our brains learn to use depth cues, such as shadows and line perspective, to see the three-dimensional world. Eventually, incorporating these cues into a coherent picture of the world becomes involuntary. Our ability to judge size correctly is one example of the brain's reinterpretation of two-dimensonal images. When a person walks away from us, the image of her becomes smaller and smaller on our retina. We know that people do not actually shrink as they move away, however. The brain combines the shrinking retinal image with perspective and depth cues from the surroundings, and we "decide" that the person is moving away. When MM lost his sight when he was three years old, his brain probably had not yet constructed the connections that incorporate separate perceptions into one combined perception. When a person walks away from MM, he has to remind himself that the person is not actually shrinking in size!