Depth Perception is One of the Brain's Greatest Tricks.

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Depth Perception is One of the Brain's Greatest Tricks.

The 3D Movie That Rewired a Brain

How depth perception works and the night that changed a man’s life

For as long as he could remember, Bruce Bridgeman’s world looked flat. As a Boy Scout, he struggled to spot birds his friends saw easily. When he drove, he often passed highway exits before he could finish reading the signs.

Then, in February 2012, something strange happened.

He went to see Hugo, a film about a boy living in a Paris train station, and against his instincts, paid extra for the 3D version his wife was eager to watch. When the show started, Bridgeman was shocked to see the images pop out at him.

More amazingly, when he left the theater, the outside world popped, too. It was like a switch had flipped in his brain.

“I saw a lamppost jumping out at me, and a person, and a car,” Bridgeman said. “It was so amazing I don’t remember what [my wife] said, or even the movie, because I was just so excited.”

Born with misaligned eyes, Bridgeman never developed the ability to perceive depth. Normally, each eye sees a slightly different image, which the brain combines into one.

For Bridgeman and about 10 percent of all people, this connection is never made, leaving them with a condition known as stereoblindness. People afflicted see different images with each eye, but they are so different that, rather than merge the two, the brain ignores one.

The scientific consensus is that in the first eight months of life people learn to coordinate their eyes in a way that develops the neural connections needed to merge these images. After that, the brain is hardwired. No matter what happens later, the hypothesis goes, people who miss this window won’t be able to perceive depth.

Yet Bridgeman, a psychobiology professor at University of California, Santa Cruz, has seen depth ever since he watched that 3D movie two years ago. Now, he’s part of a burgeoning group of neuroscientists, some driven by their own experiences, who are rethinking how people develop depth perception.


Optical illusions like this trick your brain.

The ability to see in three dimensions is one of the brain’s greatest tricks. Each eye separately detects light bouncing off of an object. Different layers of the eye bend the light until it reaches nerve cells, which convert it into electrical signals. These signals are passed to the brain, which then forms a single image.

All of this happens in a fraction of a second, which is fast, but not fast enough. To speed things up, the brain forms full images from partial pictures, filling in gaps based on past experiences, a tendency that optical illusions exploit.

The most revealing depth-perception experiments were conducted by David Hubel and Torsten Wiesel, neurophysiologists who met at Johns Hopkins University in 1958. By cutting an ocular nerve in several kittens, Hubel and Wiesel determined that depth perception required both eyes to work together.

In another study, the neurophysiologists sewed shut one eye of newborn kittens for three months. They later observed that the kittens responded to stimuli with one eye or the other, but never both.

The scientists concluded that in order to develop normal vision, the eyes must work as a pair in the first weeks of life. That’s when the visual cortex, the part of the brain that processes information from the eyes, is most pliable. It’s during that time that the brain learns to combine the images from each eye, according to Hubel and Wiesel, who won a Nobel Prize for their work.

“In humans, if you have normal vision for the first two months of life, you can see depth,” Wiesel said in an interview.

Among neuroscientists, this is often called the critical period.

The visual cortex combines information from each eye into a single image.

Susan Barry first learned about the kitten experiments in an undergraduate neurophysiology class. It was a revelation.

Born cross-eyed, she underwent three surgeries to correct her vision before the age of seven.

“I was told I had an unfocused, pensive look, probably because my eyes were looking at different places,” she says. “The surgeries made my eyes look straight and everyone assumed I was fine, so I thought I was fine.”

But while her eyes appeared normal, they rarely looked at the same object simultaneously. Barry tended to alternate between one eye and the other.
Understanding the secrets of depth perception was the first step to developing the RealSense technology capable of seeing in 3D.
Learn more about that effort


She began reading scientific journals, took every stereovision test she could find, and failed them all.

People with stereoblindness develop tricks to get by: they judge the relative size of objects; study shading and shadows; and fidget to create parallax, an effect that makes close objects appear to move more than distant ones.

Now that she understood her condition, Barry began to notice how it affected her life. Because road signs appeared jittery, she drove slowly, so slowly that other drivers often honked at her. To compensate, she arranged her life so that she wouldn’t have to drive much.

Once, when Barry, by then a biology professor at Mount Holyoke College in South Hadley, Massachusetts, taught a class of 90 students in a small lecture hall, a colleague asked why she never called on the students in the back.

“What students?” she said, surprised.

Although she knew she was long past the age where people developed depth perception, at the advice of her eye doctor, she tried vision therapy. She began daily exercises designed to develop the brain’s ability to control eye alignment, focusing ability and eye teamwork.

Within weeks, she began seeing depth. Her true moment of revelation came one morning while looking at herself in the mirror.

“I used to think that a spot on the mirror was on my clothes and I’d try to clean it up,” she said. “Then one morning, I saw that my reflection wasn’t on the surface. Holy Moly! I’m in a reflected space behind the mirror.”

Barry says that the ability to see depth has changed her life. Where she previously stayed away from crowded cities because it was hard to avoid bumping into people, she now visits Manhattan frequently to “feel like part of a wave” and to spot gargoyles at the tops of buildings.
Bruce Bridgeman saw depth for first time after watching a 3D movie.

Wiesel, whose work led to the critical-period hypothesis, believes people like Bridgeman and Barry who start seeing depth in adulthood must have had early childhood experiences with binocular vision.

“We sometimes store neural connections if they are not constantly practiced, like when the eyes are misaligned,” he said. “Then the connection is temporarily lost and the person doesn’t get the information, but these cases are extremely rare.”

Barry, however, says her parents told her she was cross-eyed at birth. Both she and Bridgeman now believe that the critical period is overstated and that the brain is pliable enough to learn how to process depth far later in life than most people imagined.
“
I think that means we are born with the ability to see depth but when the eyes aren’t aligned, they can’t capture the correct images.”

“I think that means we are born with the ability to see depth but when the eyes aren’t aligned, they can’t capture the correct images,” Bridgeman said. Once that’s fixed, he believes the mind automatically does its work.

But how could watching a 3D movie rewire part of Bridgeman’s brain?

Bridgeman himself isn’t sure. But he thinks the exaggerated images delivered through each lens of the 3D glasses forced his eyes to work together. And after being connected for two hours, his brain became used to merging different images and surprised him outside the theater.

Today, Bridgeman is looking for other people who have gained depth perception after watching 3D movies. So far, he’s found three people in Southern California with similar experiences.

He hopes it that it can become known as a treatment to help everyone see the world like he finally can — and in the process help raise awareness of the condition so that stereoblindness can be detected earlier.

“Wouldn’t it be great if an optometrist could say, ‘Can you watch this movie for two hours for me?’ to a child, rather than ‘Let’s stare at this squiggly line,’” Bridgeman said. “The movie would definitely be more enjoyable.”
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