Monkeys Cured of Color Blindness

Two male squirrel monkeys now see the world in an entire new way in full color.
Female squirrel monkeys can see in color, but male squirrel monkeys are normally red-green colorblind because they lack pigments in the retina that notice those wavelengths of light.

Now, researchers have performed gene therapy that permitted two male squirrel monkeys named Sam and Dalton to produce proteins that detect red light. As soon as the red-light-harvesting protein was made in the monkeys' eyes, the animals were able to distinguish between red and green spots in color vision tests, Jay Neitz of the University of Washington in Seattle and his collaborators report online September 17 in Nature.

The experiment wasn't supposed to work, Neitz says. People born with cataracts don't develop nerve relations that help the brain make sense of messages sent by the eye. If the defect isn't corrected early, these people stay essentially blind even if their eyes return to full function later.

Because there was no reason to assume color vision was different from other types of vision, the team had assumed it would not be possible to reverse the deficit in an adult animal.

Neitz polled experts in the vision field on whether they thought producing photoreceptors in colorblind adult monkeys could give color vision. "Every single person said, "absolutely not.'" But the researchers decided to move forward with the experiment to see if they could get the pigment protein to be made in the eye.
Male monkeys lacking the red photoreceptor protein were given injections of a virus carrying a gene for the protein.

Levels of the protein slowly rose in some retinal cells. After 20 weeks, Neitz and his colleagues started to see differences in the way Sam and Dalton performed on daily color vision tests. Around that time, protein production levels peaked and the monkeys have maintained stable color vision for two years since treatment.

Still, the monkeys' actual sensation of color what it looks like to them remains a mystery?

"The achievement is technically amazing and conceptually very cool," says Melissa Saenz, a neuroscientist at Caltech in Pasadena, Calif. But even though the monkeys can distinguish some new wavelengths of light, "there's no proof that the monkeys recognize a new dimension of color," she says. For example, the monkeys may now perceive red and green as different shades of yellow and blue, colors the animals already knew.

"If it doesn't involve experiencing new sensations of color, it would not dramatically change the experience of colorblind people if the treatment were valid to humans," Saenz says.