The monkeys were presented with a touch screen filled with gray dots. Prompted by a tone, the monkeys were to touch any dots that had changed from gray to another color. Touching the colored dots earned a grape juice reward.
Whenever the colored dots were red or green, though, the color-blind monkeys acted frustrated—sometimes even shaking the display, Neitz said.
After the initial round of touch screen tests, the team injected a specially engineered virus behind the retinas of two of the color-blind monkeys.
The virus contained genes for red pigment in cone cells—cells in the eye that respond to light and color. The virus inserted the red-pigment genes into some of the monkey's green-sensitive cone cells, causing those cells to become red sensitive.
Within about 20 weeks, the two monkeys were able to point out red and green, according to the study, to be published tomorrow in the journal Nature.
To formerly color-blind monkeys, the change confers profound abilities, noted visual neurobiologist and color-vision expert Bevil Conway—for example, the ability to find fruit amid green leaves.
"Mind-blowing" Gene Therapy
Scientists have long thought that seeing in full color depends on a fabric of millions of specialized nerve cells that, in adult humans, are relatively fixed and inflexible.
So the idea that a single genetic intervention can cure color-blindness is "mind-blowing," said Conway, of Wellesley College in Massachusetts, who admitted he had initially "led the troop [that had been] saying there's no way this could work.
"When I think naively about evolution, I think, Oh wow, it must be hard to get all that circuitry up and running," said Conway, who was not involved in the study.
"And here they are basically showing you can get away with it with one simple little switch."
The new research suggests that the adult brain can adapt and accommodate a new system within its existing structure, which should force scientists to rethink some assumptions about the organ, co-author Neitz said.
The brain is much more flexible than many would expect, opening the door to treatments previously seen as impossible, Neitz said. And that, he added, "is huge."
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