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One of the most dramatic, although rare, human experiences is the acquisition of sight by blind individuals. A common misconception has been that light stimulation itself is sufficient to generate functionally meaningful vision. Historical cases of people regaining their sight after many years of blindness often ended in tragedy, with the great expectation that they would be able to see being crushed by the confusion of light that made no sense to the visually inexperienced brain. As a result, people gave up the attempt, reverted to a life of blindness and sank into deep depression.

How do we learn to see? This is an ancient debate that has intrigued scientists and philosophers for centuries. In An Essay Concerning Human Understanding (1690), John Locke famously posed the question of whether a man who was born blind and regained sight as an adult could distinguish between a cube and a sphere without touching them. It is now recognized that vision is not a spontaneous process that is 'hardwired' in the visual system — it also requires knowledge, experience and other complex sensory inputs.

In a Perspective article on page 71, Merabet and colleagues discuss the recent development of sophisticated micro-electronic devices that aim to restore vision in blind individuals. All these visual neuroprotheses are designed on the basis that focal electrical stimulation of intact visual structures evokes the sensation of discrete points of light. This approach has achieved limited success in some patients, who reported crude patterned perceptions, but restoration of truly functional vision is still beyond our reach. Further progress in the field will hinge largely on our understanding of the nature of visual development, how the brain changes and adapts to visual deprivation, and how well it can process new visual information.

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In This Issue. Nat Rev Neurosci 6, 1 (2005).

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