"People like me or like Bruce who are cross-eyed, have strabismus, probably have the capacity to see in stereo if you can change your visual habits so as to point the two eyes at the same place at the same time, and bring in correlated input into the brain," said Barry, who has corresponded with Bridgeman but not studied him directly.
Several scientific studies have shown that, with training, people can recover stereo vision even if they've never had it, said Suzanne McKee, senior scientist at the The Smith-Kettlewell Eye Research Institute in San Francisco.
This is a recent shift in thinking. For decades, scientists believed that stereo vision could not be recovered, once lost.
"It turns out that, in fact, with a lot of practice, like many other things you can recover capabilities, which is exciting," McKee said.
Similarly, scientists once thought that there was a "critical period" of the first few years of life, during which one would either be able to see in three dimensions or not. Barry was 48 when she started to perceive three dimensions, and at first kept her awareness secret from other scientists out of concern that "they would think I'm hysterical."
So what about that movie?
In his academic life, Bridgeman's specialty is -- guess what! -- the visual system. He's working on such projects as eliminating sickness in driving simulators and exploring illusions such as why people overestimate the slopes of hills.
After viewing "Hugo," Bridgeman read up on research on the neuroscience of vision to come up with a possible explanation of what happened.
David H. Hubel and Torsten N. Wiesel, who shared the 1981 Nobel Prize in Physiology or Medicine, had done research on kittens that gave Bridgeman ideas about what could be going on in his own brain.
The researchers studied the eyes and visual systems of young cats with misaligned eyes, showing that most cells in the brain's visual cortex would be activated by one eye or the other. But a small group of these cells also responded to both eyes. Bridgeman's hypothesis is that the analogous cells in himself got awakened during the movie, becoming newly connected to allow stereoscopic vision.
"It was sort of serendipitous that I had spent my lifetime studying vision, and then this experience happened, so I could talk about it and maybe understand it in ways that most people wouldn't be able to," he said.
The act of watching a movie isn't going to fix someone who has an eye turned abnormally, but if someone was already at the cusp of this happening, it's possible, Harris said.
In Harris' practice, patients with intermittent exotropia -- eyes that point outward much of the time -- typically go through a six-month process of training, involving exercises to help them focus both eyes together.
Harris has seen patients have overpowering emotional experiences after therapy sessions, saying, "it's the first time I've seen the space between things."
It makes sense to Harris that a 3-D movie could have had this effect on Bridgeman. Ordinary movies do not reward the brain for bringing an outward-deviating eye in, but a 3-D movie does.
"The moment he brought his eye in subconsciously, he was like, 'Oh wow, this looks cool,' and then probably maintained the effort," Harris said.
The movie probably had the effect of training not the muscles of Bridgeman's eyes per se, Harris said, but rather "the software he used in terms of how he looked at the world."
Researchers have suggested that creating video games with three-dimensional information could be used to help children and adults with eye misalignment problems achieve improved stereo vision, McKee said.
Looking for more insights
Bridgeman's vision still isn't perfect. A few months after viewing "Hugo," he went to an optometrist in Santa Cruz who administered a test for stereo threshold, the same one Bridgeman had taken at Berkeley in the 1980s. The test showed his stereo vision had improved dramatically since his earlier test, but still wasn't within normal range.
There still remain many unanswered questions. Just how common is it for an adult to acquire stereo vision? Which specific visual anomalies might respond best to watching a 3-D movie? Bridgeman wants to know.
He's in touch with Dennis Levi at the University of California, Berkeley, who led a study of five adults who went from being stereo-blind to seeing depth again through training.
Since a BBC article appeared about Bridgeman last year, he's gotten numerous e-mails from others with stereo vision problems. One man told him he had an experience similar to Bridgeman's.
"When it happens, it's very sudden and pleasant, to see things in 3-D that have been flat for all your life," Bridgeman said.
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