However, both Nietz and Jacobson say they frequently receive calls from color-blind people searching for cures, and they hope the research can eventually be used in humans.

"It seems a trivial defect for those of us who are not color-blind, but it does close a lot of avenues," says Jacobson. People who are color-blind can't become commercial pilots, police officers, or firefighters, for example. "People tell me every day how they feel that they miss out because they don't have normal color vision," says Neitz. "You obviously don't want to risk other aspects of vision, but I think this could get to a point where this could be done relatively without risk."

The findings challenge existing notions about the visual system, which was thought to be hardwired early in development. This is supported, for instance, by the fact that cats deprived of vision in one eye early in life never gain normal use of that eye. "People had explored visual plasticity and development using deprivation in a lot of different ways," says Neitz. "But no one has been able to explore it by adding something that wasn't there."

That flexibility is also important for clinical applications of the technology. The fact that adult monkeys could use their novel sensory information suggests that corrective gene therapies for color blindness need not be delivered early in development, as some had feared. However, it's not yet clear whether color vision will be a unique example of plasticity in the adult visual system, or one of many.

Researchers hope the findings will prove applicable to other retinal diseases. Hundreds of mutations have already been identified that are linked to defects in the photoreceptors and other retinal cells, leading to diseases such as retinitis pigmentosa, a degenerative disease that can lead to blindness. However, unlike color blindness, in which the visual system is intact, save for the missing photopigment, many of these diseases trigger damage to the photoreceptor cells. "I think it's hard to know in what way it will extrapolate to more serious blinding disorders that involve more serious degeneration of retina," says Nathans.

The research also raises the possibility of adding new functionality to the visual system, which might be of particular interest to the military. "You might be able to take people with normal vision and give them a pigment for infrared," says Williams. "I'm sure a lot of soldiers would like to have their infrared camera built right into the retina."

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