Bruce Bridgeman lived with a flat view of the world, until a trip to the cinema unexpectedly rewired his brain to see the world in 3D. The question is how it happened.
Good movies change people’s view of the world all the time, but how many can say a movie has fundamentally altered their vision forever? One person who can is Bruce Bridgeman. In terms of how he sees the world, there is life before Hugo, and life after Hugo.
On 16 February 2012, Bridgeman went to the theatre with his wife to see Martin Scorsese’s 3D family adventure. Like everyone else, he paid a surcharge for a pair of glasses, despite thinking they would be a complete waste of money. Bridgeman, a 67-year-old neuroscientist at the University of California in Santa Cruz, grew up nearly stereoblind, that is, without true perception of depth. “When we’d go out and people would look up and start discussing some bird in the tree, I would still be looking for the bird when they were finished,” he says. “For everybody else, the bird jumped out. But to me, it was just part of the background.”
All that changed when the lights went down and the previews finished. Almost as soon as he began to watch the film, the characters leapt from the screen in a way he had never experienced. “It was just literally like a whole new dimension of sight. Exciting,” says Bridgeman.
But this wasn’t just movie magic. When he stepped out of the cinema, the world looked different. For the first time, Bridgeman saw a lamppost standing out from the background. Trees, cars and people looked more alive and more vivid than ever. And, remarkably, he’s seen the world in 3D ever since that day. “Riding to work on my bike, I look into a forest beside the road and see a riot of depth, every tree standing out from all the others,” he says. Something had happened. Some part of his brain had awakened.
Conventional wisdom says that what happened to Bridgeman is impossible. Like many of the 5-10% of the population living with stereoblindness, he was resigned to seeing a world without depth. What Bridgeman experienced in the theatre has been observed in clinics previously – the most famous case being Sue Barry, or “Stereo Sue”, who according to the author and neurologist Oliver Sacks first experienced stereovision while she was undergoing vision therapy. Her visual epiphany came during the course of professional therapy in her late-forties. The question is why after several decadesof living in a flat, two-dimensional world did Bridgeman’s brain spontaneously begin to process 3D images?
For centuries, scientists have known that two eyes are better than one. The Roman physician Galen observed that images received by the two eyes are slightly different, as did Leonardo Da Vinci many centuries later. Open your left eye only and then switch to the right eye only and you’ll see how: the image looks the same but it has moved a little.
In the 1830s, the English scientist and inventor Charles Wheatstone discovered why: the differences between the two images allow the brain to generate a sensation of depth. He even designed ingenious devices called stereoscopes, in which two slightly different versions of the same image viewed together through the instrument transformed into a single three-dimensional drawing.
Somehow the brain fuses these images automatically, and it’s only in the last few decades that we have begun to understand the nerve signals underlying this stereovision. In much the same way that different cells in the tongue respond to different types of taste – bitter, sweet, salty and sour – so too there are cells in the eye and brain that respond to only one type of signal, for example, vertical or horizontal lines. The farther this signal travels into the brain, the more complex it becomes.
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 usedmotion 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.
So for years, Bridgeman believed that the depth cues from motion parallax, shading, and perspective provided decent substitutes for stereovision. Then, a year ago, he went to a talk by Suzanne McKee, a vision scientist at the Smith-Kettlewell Eye Research Institute in San Francisco, that challenged these notions.