How Our Vision Works
By Maria Popova
The news that scientists have discovered a new part of the human eye only demonstrates that no matter how painstakingly mapped and thoroughly understood we might think the human body may be today, there’s much yet to know. Vision is particularly intriguing for, as Virginia Woolf put it in her meditation on moving images, “the eye licks it all up instantaneously” and thus our experience of the world emerges.
But how does the human eye work, exactly? In The Forever Fix: Gene Therapy and the Boy Who Saved It (public library) — the remarkable story of an eight-year-old boy nearly blind from a rare hereditary disorder and the groundbreaking medical procedure that made him fully sighted within a few days — geneticist and journalist Ricky Lewis takes us on “a trip through an eyeball, from the pupil, where light enters, to the back of the eye,” bringing to life the familiar metaphor of the eye as a camera to illustrate how our vision works:
[T]he retina is the layer of the eye’s wall that includes the photoreceptor cells — the rods and cones — that capture light energy and change it into the electrical language of the nervous system. The rod cells provide black-and-white vision and detect motion, and the cone cells send signals for color. The retina also has cell layers that transmit the light signals to the optic nerve, which sends the information to the part of the brain that interprets the input as a visual image. The comparison of the human eye to an old-fashioned camera is apt — the back of the retina is like a sheet of photographic film. … Each eye has 100 million of these long, skinny cells, and each has about two thousand translucent discs that fold inward from the surrounding cell membrane, making the rod look a little like an electric toothbrush. The aligned discs resemble toothbrush bristles at one end, and a neural connection at the other end of the cell that goes to the brain corresponds to the part of the toothbrush that plugs into a power outlet.
Embedded in the rod’s folded discs are many molecules of a pigment called rhodopsin, which actually provides vision. Each rhodopsin molecule is built of a protein part called opsin and another, smaller part made from vitamin A, called retinal. A flash of light lasting mere trillionths of a second changes the shape of the retinal, which in turn changes the shape of the opsin. The change in opsin triggers chemical reactions that signal the nearby optic nerve, which stimulates the visual cortex in the brain. In this way, each of the 100 million rods and 3 million cones of a human eye contributes a tiny glimpse of a scene, which the brain then integrates into an image.
To fully appreciate the astounding machinery of the eye, we need only consider the magnificent mechanics through which it transmutes the fragmented glimpses of the world into our fluid visual experience of it:
To see the world as a continuous panorama, rather than a series of disconnected snapshots, rhodopsin must quickly reform after it changes in response to light. In dim light this happens slowly, and the rhodopsin is recycled inside the eye. But in very bright light, rhodopsin contorts too fast to fully recover. This is why we are temporarily blinded when walking out of a dark theater, to which our rhodopsin has adapted, into bright sunshine. It is also why we tell children to eat their carrots, because vitamin A deficiency causes night blindness. Cones work in a similar way, but instead of rhodopsin, they use three other visual pigments that are sensitive to different wavelengths of light, and interpret the hues of red, green, and blue that color our world. Mammals other than humans and our primate cousins have only two types of cones, which restricts the color palette available to them.
Rare vision defects, like that of Corey, the book’s protagonist who was diagnosed with a form of Leber congenial amaurosis, expose just how intricate, how perfectly wired yet delicate, our vision is. Take, for instance, the retinal pigment epithelium, or RPE:
The RPE is a caretaker of sorts for the rods and cones, removing wastes while absorbing stray light rays that might otherwise bounce around the eyeball, creating meaningless flashes. In a layer of cells next to [the rods and cones] called the retinal pigment epithelium (RPE) is a caretaker of sorts for the rods and cones, removing wastes while absorbing stray light rays that might otherwise bounce around the eyeball, creating meaningless flashes. The RPE’s most important job is to store vitamin A. It uses a protein, called RPE65, to activate the vitamin, forming the retinal essential for black-and-white vision.
The Forever Fix goes on to tell the fascinating story of how gene therapy helped Corey, whose eyes were unable to make the protein variant RPE65 and who was thus genetically doomed to have his rods and cones shrivel to nothing and result in eventual blindness, had his sight restored and life changed, alongside a handful of complementary cases that explore the allure and promise of genetic science.
Public domain images via Flickr Commons
Published June 13, 2013