Neuroscientists have found cells in the visual cortex, the part of the brain that processes vision, whose sole job is to respond to differences in the position of the images transmitted from each eye to the brain. These cells, called binocular neurons, are thought to be the key to seeing in three dimensions.
According to the Nobel Prize-winning research from David Hubel and Torsten Wiesel in the 1960s, the brain may only have a short window of opportunity in which to develop binocular vision, in which both eyes are used together. Their studies in cats, and many other studies since, suggest that if the developing brain isn’t exposed to overlapping images from the two eyes, it will never form the connections it needs to process a three dimensional scene, and that the binocular neurons in the visual cortex will never exist. These doors close early – at the end of childhood – after which people are locked into a two-dimensional world.
Depth of field
In Bridgeman’s case, he was left with a condition called alternating exotropic strabismus, often called “lazy eye”, in which both eyes independently have a tendency to drift outward. He could aim each eye individually at a scene, and swap back and forth between them, but he could never get both eyes to fix on a single point, and he couldn’t look through both eyes at once. So throughout his life he saw the world as a collection of flat panels.
Though stereovision is probably the most immediate, and certainly the most sensitive strategy that the brain has for acquiring information about depth, there are other cues that Bridgeman came to rely on heavily, like shading, perspective and occlusion (if you look at a forest while moving your head, the trees that are farthest away will blink in and out of view behind the ones that are closest).
Most of all, he used motion parallax – a visual phenomenon that you’ll notice if you ever drive down a wide-open road. Looking out the car window, you might see trees right near the road speeding by, jagged rocks a bit further in the distance moving more slowly, and big mountains way out toward the horizon standing still like set pieces bolted to the ground. This difference in the apparent motion of objects tells the brain how far away each one is from us. You don’t need a car to make it work; moving your head side-to-side achieves the same effect. It’s a trick Bridgeman used as he toured the cathedrals of Europe; walking up and down the aisles he viewed the jutting contours of the nave, converting his own motion into a sense of depth.
Our ability to adapt and use these other depth information sources means that people who are born stereoblind or with impaired stereovision often don’t find out about it until they reach adulthood. “It’s not included in any of the standard tests that optometrists do,” says Laurie Wilcox, a vision specialist at York University in Toronto. “And that’s a shame. It’s the sort of thing that’s useful to know.”
Nor does society require that we see in 3D. When handing out driver’s licenses, most states in the US don’t even give the kinds of eye tests that would detect poor stereovision, and when it is detected people have the opportunity to prove their competence in a road test. Bridgeman has been driving his entire adult life, though he says he’s always been aware of some edginess among family members when he gets behind the wheel.