If this experiment succeeds in monkeys—and assuming progress in human gene therapy, thus far mostly a failure—the Neitzes will try a similar technique on colorblind humans, attempting to give them a third, functioning cone. If that proves successful, the Neitzes hypothesize, you ought to be able to give people with normal sight a fourth cone, equipping us "tetrachromatic" vision.
What would tetrachromatic vision be like? Jay Neitz can only speculate based on the astounding difference between dichromatic and trichromatic vision. Because of the way the nervous system works, a fourth cone "would add two more entirely new colors, completely new and wonderful and unimaginable sensations. ... And it is not just those two new colors but the combinations of those colors with all the colors we already see. We would see almost everything differently."
There are weirder possibilities, too. In their first four-cone experiment, the Neitzes think they would engineer a photopigment sensitive in the visible light spectrum (probably in the gap between our current blue and green cones). But they could also make a cone receptive in the infrared zone. If the cones were to become too sensitive to infrared light, though, we would start "seeing" our own body heat, and that would blur our vision. But if you could engineer cones that were somewhat sensitive to the infrared spectrum, we might have extraordinary night vision—without goggles, street lights, or surgery.
Most human gene therapy trials have been unsuccessful. Until scientists puzzle out how to make implanted viral genes stick and not bite back, gene therapy for the eyes will remain only a hope.
Even if the Neitzes can make the gene therapy work, the brain may rebel. As with the cyborg eye, no one is certain the brain would accept the signals of a fourth cone, although Jay Neitz believes the mind is plastic enough to accept the strange new sights.
The Neitzes are in the final stages of preparation for their initial monkey trial.
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