For the most part, I tend not to do a lot of real science blogging, nor do I read science blogs all that much. Generally, the parts of my job that periodically fill me with wonder, fill everyone else with something more akin to the desire for suicide. But every once in a while an article comes around that is both way cool and relatively easy to explain why it is cool. Yesterday, a paper like that came along in Nature (subscription required):
Researchers have used gene therapy to restore colour vision in two adult monkeys that have been unable to distinguish between red and green hues since birth — raising the hope of curing colour blindness and other visual disorders in humans.
To me, the gene therapy part is not particularly interesting in this case. While many sick people are desperately interested in successful gene therapy approaches, what struck me more, is the implications this experiment has on our understanding of perception. For those of you that don’t spend the majority of your waking hours thinking about the nuances of biology and genetics, let’s go over a little bit of how we see and understand color.
Most animals that can see color have 4 kinds of light detectors in their eyes, one type called the “rod”, and 3 types called “cones”. Rod-opsin was the first gene shown to be responsible for detecting light and it is only expressed in the rods. Rods are our low-light (night-vision) sensors and mostly reside on the periphery of the retina. The 3 cones sit in the middle of our retina, provide most of our vision, and are responsible for detecting color by expressing 3 different opsins, each responsible for a particular range of light wavelengths. There is a short-wave detector (S, blue), medium wave detector (M, green), and a long wave detector (L, red).
Like a TV set, blending these three ranges of color allows us to integrate data into a color spectrum in our brains. To see color, it is essential to have at least 3 reference points, otherwise the brain can’t integrate color information. These genes are all on the X chromosome and not on the Y. If there is something wrong with one of the opsins, you get only 2-color (or sometimes 1-color) vision otherwise known as colorblindness. There is red-green colorblindness (M, or L damaged) or blue-yellow colorblindness (S or M damaged). This is more common in males because there are no “backup” copies on the Y chromosome if one color gene is damaged.
The monkeys in this study normally have 3 different color opsins, but only have only two of the three on any given X chromosome (we have all 3). The genetic result is that all males are color blind (2 on X none on Y), and females are often colorblind (same 2 on both X’s), but some females can actually have color vision (one copy is different). The researchers made a virus that, once it infects the photoreceptors of the retina, made those cells express the “missing” color gene (in this case L). In 20 weeks they showed monkeys could pass their colorblind tests.
The gene therapy angle is certainly cool, but the real ramifications of this go way beyond fixing colorblindness. A couple of years ago, there were studies that showed in mice if you added a 4th wavelength detector, you could actually expand color perception. There is also anecdotal evidence of a woman who carries an extra opsin with an intermediate sensitivity and her color matching abilities were far superior to the majority of those tested. The big difference in this new study is that it was done in adult mice, so their brains were already fully developed when they got their first taste of 3-color input. If you place patches over the eyes of any vertebrate at “birth”, they will reach an age where they are permanently blind. If the brain doesn’t get input from the eyes early during development, the connections are never made, and even if the eyes are working perfectly fine (they do), the brain never learns how to see. Perception of color appears to be different. What this means is that the brain is essentially able to process new wavelength inputs as soon as they are supplied and “sees” colors it couldn’t see before simply by adding differential input. The brain doesn’t have to grow into color vision, it is inherently capable of interpreting color input. Also the way the experiment was done, the monkeys don’t really have S, M, and L, but S, M, and M+L. So color perception relies on having 3 different inputs, but they don’t have to be 3 specific kinds of input.
The big punchline here is that we have (I think) all the scientific knowledge we need to be able to expand human perception. For example, some fish have 4-color vision. In some shallow water fish there is an opsin that can detect UV as well as RGB (let’s call it really short, RS). If you clone the RS fish opsin into this same gene therapy vector and inject it into a human retina (the vector used in this study is already cleared for human gene therapy trials), you could have a person with stable perception of color in the UV spectrum. If you find a willing subject, this could be done with current technology. I find this fascinating. Other opsins with a broad range of wavelength sensitivities are already known and more could be engineered. The possible variations are potentially huge.
What would 4-color vision be like? 5-color? Could you have artists that could make paintings that only 4-color “mods” could see? True night vision? Could you visualize illness in a person (fever, hypothermia)? The thing is, I am not in crazy dream territory anymore. It is easily possible in our lifetime, and perhaps within the decade that someone will do this.
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