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Closing one eye rewired the brain
In the seventies and eighties, David Hubel and Torsten Wiesel recorded electrical activity in the visual cortex, first at Johns Hopkins and then at Harvard Medical School. Hubel was a physician, and he wanted to know what happens when a child is born with a cataract or a lazy eye.
To mimic that, they closed one eyelid early in development. The visual brain responded entirely to the open eye, as if it had taken over the territory of the closed one. When they removed all visual input by closing both eyes for a short stretch, nothing changed.
Their work showed that the brain is a customized map, and that every input competes for space on it: one eye or the other, this finger or that one. Experiments in other labs pointed the same way. When two fingers were taped together early in development, their representations fused in the brain, and the person could no longer tell the movements and sensations of the two fingers apart.
Because inputs compete, an adult who wants a new connection has to give something up. A new connection cannot be added without removing something else.
Why does the open eye win the territory?
Neurons in the visual cortex strengthen the connections that are active together and weaken the ones that are silent. When one eye sends little signal, its inputs lose ground to the other eye's inputs in the same region. Hubel and Wiesel found that cortical cells organized into columns that favor one eye or the other, called ocular dominance columns. Columns for the closed eye shrank while those for the open eye widened.
Why did closing both eyes change nothing?
The competition happens between the two eyes, so with both closed there is no winner to take over. The cells that had not received input from either eye simply stayed in their original arrangement for that short period. This is one reason the experiment pointed to competition and not to damage from the lack of light. Hubel and Wiesel shared the 1981 Nobel Prize in Physiology or Medicine for this line of work on visual processing.
How does a lazy eye relate to this?
A lazy eye, known as amblyopia, develops when the brain favors one eye during childhood and the weaker eye's connections fade. The closed eyelid experiments gave researchers a controlled version of the same thing. Doctors later learned that the early years are a sensitive period when the balance between the eyes is most easily shifted. This is why vision problems in young children have long been screened for early.
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