Experts Show That a Blind Man's Eyes Can Help Him Navigate
Tuesday, December 23, 2008
After two strokes, he was completely blind, dependent on his cane and his wife's arm to safely walk down the street. But researchers had a hunch: They suspected that, unconsciously, the man might be sensing the world around him through his eyes better than anyone realized.
So the neuroscientists devised a simple experiment: They asked the man to walk down a long hallway unaided by his cane or anyone else -- without telling him they had turned the corridor into a makeshift maze by randomly placing boxes, chairs and other objects in his path.
To their astonishment, the man deftly maneuvered past every obstacle. Then he turned around and did it again, prompting the stunned researchers to burst into applause.
"We were so excited," said Beatrice de Gelder, a cognitive neuroscientist at Tilburg University in the Netherlands and Harvard Medical School, who reports the experiment today in the journal Current Biology. "It was really quite amazing to see."
The first-of-its-kind case is providing doctors with new insights into how vision works, suggesting that even when the brain's primary vision centers have been destroyed, signals entering the eyes are still registering. Although no one thinks the findings could help make the blind see, they do indicate some blind people may have hidden capabilities.
"It's sort of a crowning piece for a fairly long set of increasingly convincing reports that there is a form of vision which is not dependent . . . upon the primary areas responsible for processing inputs from the eyes," said Richard Held, a Massachusetts Institute of Technology neuroscientist. "This is the most convincing evidence for that."
On a practical level, the findings hint that such people could learn how to harness these unrecognized abilities to gain more independence.
"There could be training for these residual skills," de Gelder said. "One would hope it could have implications for dormant abilities in the brain that could be capitalized on, such as the ability to move around the house."
Other people previously have been shown to have "blind sight" (the ability to sense the movement and location of objects, and even identify emotions on others' faces). This ability surfaces when parts of the brain involved in interpreting vision are damaged but the eyes and optic nerves survive, allowing unaffected areas of the brain to still get input.
But the man in the hallway, a middle-aged physician identified only as TN, marks the first time any patient with such extensive brain damage has been documented to have this ability. TN's strokes had destroyed the visual cortex, the part of the brain primarily responsible for processing vision, on both sides of his brain.
"This was a radical case with complete loss of conscious vision," de Gelder said.
After confirming the extent of TN's brain damage with a series of scans and other tests in the Netherlands, the researchers concocted and then videotaped the hallway experiment.
Someone walked behind him in case he stumbled, but he never needed help. Because TN made no sounds while walking down the hallway, the researchers were confident he was not unconsciously using his ears for guidance, and saw no signs he was subtly feeling his way past obstacles with his feet.
"If you look at how his feet move, he doesn't do it by touching, and there's no sound location," de Gelder said.
While the researchers were thrilled, TN was baffled by their excitement.
"He was surprised. It didn't mean much to him. He didn't see what he was able to achieve," she said.
De Gelder said he must be using more-primitive parts of the brain: parts that prompt us to, for example, move away from a door that suddenly flies open, parts that enable animals with poor vision to navigate adroitly.
"They contribute more than we think they do for us to function in the world," she said.
"There is a form of relatively crude vision that survives the destruction of striate cortex," he said, using the scientific term for the primary visual cortex. "We really don't have an adequate understanding of what those other visual pathways are doing, where they are and how they are functioning."
Other researchers noted there could be alternative explanations for the findings. There might have been some activity remaining in TN's visual cortex that the brain scans missed. Or the person behind him may have inadvertently given him subtle cues.
"I worry a little bit that an observer followed the patient," said Jon H. Kaas, a Vanderbilt University psychologist. "But . . . I believe these findings."
The case provides yet more evidence that human actions may be guided by unconscious brain activity, he said.
"We have this impression that everything is in our conscious repertoire and we are operating accordingly," Kaas said. "But that might not be the case, and a lot of our behavior may be at an unconscious level. We may not know why we do them, but we do do them."
As for TN, his wife later told de Gelder that insight from the hallway experiment had enabled him to move about somewhat more easily.
"Consciously he is like a blind man. He walks like a blind man. He talks about himself as a blind man. With this, we can say, 'We know you can do it,' " she said. "We are planning to do some training with him again next year."