





Color Correction



The Neitzes wondered: Could gene therapy “cure” the male monkeys’ color blindness? To find out, the biologists developed a way to incorporate the gene for the human L-type photopigment into a small virus known as adeno-associated virus. Next they injected tens of trillions of viral particles into the monkeys’ eyes. Twenty weeks later up to one third of the M-type cones in the animals’ retinas had begun to express the L-type photopigment. In other words, the monkeys now had not two but three cone types: in addition to their original S-type and M-type cones, they had new M-type cones whose sensitivity had shifted toward the long-wavelength part of the spectrum.

The million-dollar question was whether the rest of the animals’ central nervous system could reprogram itself to make use of this additional information. Using a computer-administered color test, the Neitzes demonstrated that the treated monkeys, like the mice from the earlier experiment, did indeed discriminate among colors.

The monkeys’ new color awareness emerged as soon as the photopigments were expressed in their retinas. The lack of delay suggests that preexisting retinal and cortical circuitry can incorporate the additional information; no time-consuming rewiring was necessary. It also suggests how the evolutionary transition from two- to three-cone color vision might have come about.

From Monkeys to People
Two years after the Neitzes’ experiment their monkeys’ color vision remains transformed. Being the careful scientists they are, they do not take a stand on whether or not the monkeys see novel reddish hues. Yet I find no principled reason to deny it. The retinal machinery for trichromacy is present, and the monkeys’ behavior indicates that they experience these hues. Within a few years electrophysiological and functional imaging experiments will inform us whether the animals show increased processing in the regions of visual cortex dedicated to color perception. I would bet 100 to one that they do.

The virus used in this experiment is safe—it doesn’t replicate by itself, doesn’t cause disease and triggers only a mild immune response—and it has been approved for gene therapy in humans. So this technique could be adapted to help color-blind people see normally. The condition affects many millions in the U.S. alone. Provided that the risk-to-benefit ratio of gene therapy can be improved significantly, a potential cure could have a dramatic impact on the sensibilities of a large slice of humankind.

Jay Neitz believes that this operation will someday become as safe as refractive surgery such as Lasek. Methods such as the one the Neitzes have pioneered, as well as the optogenetic techniques discussed in my last column [see “Playing the Body Electric,” March/April 2010] may well, soon enough, make the (color)-blind see again.

Of course, there is little reason to stop there. Why not enhance visual experience to give the more adventuresome among us tetrachromacy? Or extend the window of visibility up into the ultraviolet or down into the infrared for superherolike vision? Thanks to cutting-edge molecular biology, we can see our way into a trans­human future.