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Here’s Why Your Eyes Seem to Be Wired ‘Backward’

Light has to pass through nerve cells to get to the rods and cones, but that order is no mistake

(Ben Welsh/Design Pics/Corbis)
smithsonian.com

In a blink, light travels into our eyes; in minute fractions of a second, our brains decode and process images. Slow that remarkable process down, and it only becomes more amazing. 

The colors we see—all different wavelengths—move by microbes swarming on the surface of our eyesenter through the cornea and pass through the pupil. They bend through the lens and swim through the vitreous humor that keep the eye an orb. On the retina, the back of the eye, the light rays pass right through the nerve cells that will pass signals to the brain—but ignore them for now. They reach cones—that line the back of the eye and sense the differences in colors—and rods, which are color-blind but even more sensitive to light.

When you first learned this sequence (perhaps in middle school after dissecting a sheep’s eye) it seemed a little backward. Intuitively, you’d expect the rods and cones to stick up into the jelly-like vitreous, in order to snag passing light and pass it back to the nerve cells lurking behind them.

"This is a long-standing puzzle, even more so since the same structure, of neurons before light detectors, exists in all vertebrates, showing evolutionary stability," writes Erez Ribak, a physicist at Technion, Israel Institute of Technology, for The Conversation (via Scientific American). So there must be a good reason for the "backwards" structure, Ribak thought.

And there is. It helps us see in color better, Ribak and his colleagues reported at a meeting of the American Physical Society.

Another type of cell also lines that neuron-filled layer of the retina. They’re called glial cells, and they help support neurons. But in the eye they have a second role. They can guide light "just like fiber-optic cables." Ribak writes:

[M]y colleague Amichai Labin and I built a model of the retina, and showed that the directional of glial cells helps increase the clarity of human vision. But we also noticed something rather curious: the colours that best passed through the glial cells were green to red, which the eye needs most for daytime vision. The eye usually receives too much blue—and thus has fewer blue-sensitive cones.

Further computer simulations showed that green and red are concentrated five to ten times more by the glial cells, and into their respective cones, than blue light. Instead, excess blue light gets scattered to the surrounding rods.

The team then took a close look at how light was scattered in the retina of guinea pigs. Like humans, these little mammals are active during the day and so have similar need to see colors in daylight. Under the microscope the researchers saw that the glial cells did indeed create columns of concentrated light. Since cones aren’t as sensitive as rods, they appreciate this bit of extra illumination. And the scattered blue light was gathered up by rods.

"This optimization is such that color vision during the day is enhanced, while night-time vision suffers very little," Ribak writes.

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