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 this year, 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 decades of 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 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.
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.
“It turns out that the one-eyed view of the world is deceptive,” says McKee. If you ask people with normal stereovision to close one eye and judge the position of objects along the line of sight, they are terribly imprecise, even if they shake their heads to create motion parallax. “I compared their one-eyed judgments to their two-eyed judgments and found that their two-eyed judgments were five to 10 times better,” says McKee.
Bridgeman was struck by the work. According to McKee, he was missing something. Perhaps depth could feel deeper.
The same was true in the case of Stereo Sue. She was born cross-eyed, and when Sue met Oliver Sacks at a dinner party she told him she hadn’t missed much by not being able to see the world in 3D. Fast forward several years, though, and it became a different story.
Sue sought therapy after experiencing side effects from her vision problems. Most of it involved training her eyes to converge on a single point, as this is the first, crucial step to forming stereoscopic images. Then, it was a matter of practicing, staring through polarized glasses at 3D targets, hoping they’ll take shape. “Your brain has to make a choice,” says Leonard Press, a vision therapist in Fair Lawn, New Jersey. “Either you have a lot of eye-strain or instability and you stop watching – you quit. Or, if you’re going to continue watching and you don’t suppress or tune out one eye, the sense of 3D begins to emerge.”
In Stereo Sue’s case, the 3D just popped. She was driving home from the clinic when she first felt it. Space yawned open and the steering wheel just started to hover out in front of her.
Given the overwhelming evidence for a critical period in visual development, most experts assume that Sue – and Bridgeman, by extension – had at least one moment of binocular vision while young, an experience that primed them for full stereovision later in life by initiating the necessary neuronal growth. And, in fact, Bridgeman does remember one such experience. It was a gimmick on a cereal box that let you fold pieces of cardboard into a stereoscopic image. For a moment, as he stood over it, the printed figures rose up from the box.
This one brief encounter with the three-dimensional world may have been sufficient to establish the synaptic connections needed for stereovision, like setting up telephone lines and then waiting thirty years to turn them on.
All of which lends further evidence to the idea that the brain is more malleable than scientists first believed. “This is the way a lot of neuroscience is going,” says Wilcox, of York University. “The amount of plasticity is greater than we had thought. It is really fascinating that you may just be able to have some really limited neural substrate that’s binocular and be able to build on that.”
In which case, turning the phone lines on becomes the trickiest part. It took Sue Barry thousands of trials and months of effort just to sense the few inches between her steering wheel and her dashboard. But it took Bridgeman a mere two hours to be able to see everything before him. Why? “This idea of the ability being there, but people not accessing it, I think, is a really interesting one,” says Wilcox, who has witnessed breakthrough moments in her own research. In her experience, it usually comes when the person finally figures out what to look for.
Last summer, a study by Dennis Levi, professor of optometry at the University of California, Berkeley, reported the experience of five adults who learned to see in 3D after growing up either stereoblind or with impaired stereovision. In his experiment, Levi found that his subjects were most likely to have a breakthrough if the stereoscopic images were reinforced by monocular cues like relative size and shading. This could explain why Bridgeman’s experience was so dramatic.
Unlike most clinical settings, which primarily push the patient to recognise a disparity between the images coming from the two eyes, 3D movies use all kinds of stimuli, delivering depth in every way possible. When Levi layered monocular cues into his training exercises, he got results. 3D movies may work the same way. “These monocular cues, and many others, certainly exist in 3D movies. The best 3D movies really use all of the depth cues to enhance the perception of depth,” says Levi.
For their part, 3D cinematographers think about these things all the time, even though they’re concerned about entertainment, not therapy. “It's intuitive that monocular cues, which partially stereoblind people rely on every day are essential to the quality of their 3D experience. My mantra is to incorporate monocular cues wherever possible,” says Barry Sandrew, a stereographer who worked on the 3D effects for both the Shrek and The Pirates Of The Caribbean franchises.
There’s another big lesson in all this: Make therapy fun. Press, the vision therapist in New Jersey, thinks that one of the reasons Hugo changed Bridgeman’s vision was because it was able to hold his attention continuously for more than two hours. In the clinic, that can be a huge challenge. “One of the problems with vision therapy is that it’s like physical therapy, in the sense where you have to do repetitive activities and it gets boring,” says Press.
Doctors do the best they can, but therapy techniques rely on simple geometric shapes that lack the artistry that keeps an audience engaged. “I think it’s going to be the next frontier,” says Press. “ Just like teachers are figuring out how to use iPad technology better to keep the kids engaged, we can and should be doing a little better job giving people vision therapy targets that keep them stereoscopically engaged.” That could mean designing better visual exercises or just sending them out to the movies.
As for Bridgeman, the surcharge he paid for his glasses went much farther than he ever expected. He is still viewing the world as though it were a movie with a freshness that holds lessons for the stereoblind as well as those with perfect 3D vision.
“I enjoy looking out at the world and seeing some things in front of others and looking at the forest and the trees,” he says. “A tree becomes a big three-dimensional sculpture rather than a pattern. That’s a treat.”