Researchers are developing a form of visual therapy that may help people blinded or partially blinded by stroke to regain at least some vision.
Krystel R. Huxlin of the University of Rochester Eye Institute hopes to exploit the phenomenon of blindsight, in which people who cannot see display an uncanny ability to guess what is going on in their visual field. A case study published in Current Biology last year, for example, described a South African man blinded by a stroke that destroyed both sides of his visual cortex. Yet, when researchers asked him to walk down an obstacle-filled hallway, he walked around them all. (See the video here.)
Vision relies on signals from the retina, at the back of each eye, traveling to the first visual area at the back of the brain, known as V1, and on to higher brain regions that detect motion, color and other aspects of vision. People with damage to “downstream” visual areas can still see, but they often display deficits. They may lose color vision, for example, or their ability to recognize familiar faces.
“V1 is kind of the bottleneck,” says Huxlin. “By damaging V1 you’re starving most of visual system of its primary visual input.”
In a paper published in the April 29 Journal of Neuroscience, Huxlin and her colleagues described how they tested seven patients who had suffered strokes to one side of the primary visual cortex, causing hemianopia, or vision loss in half of their visual field. The patients fixed their gaze on the middle of a computer screen and tried to determine whether flickering dots displayed within their damaged visual field were drifting right or left. When they guessed correctly a chime sounded.
At first patients succeeded only about 50 percent of the time—no better than guessing—but after practicing for about 30 minutes once or twice a day for 9 to 18 months, some developed the ability to correctly detect and discriminate the direction of motion of the dots 80 to 90 percent of the time.
“Some of the patients are fully aware of changes and improvement in their vision,” Huxlin says. “Some develop an actual sense of seeing, but it doesn’t reach consciousness in everyone.”
Exercises Strengthen Visual Pathways
Huxlin believes that patients with damage to V1 detect motion thanks to visual signals bound for V1 that are normally diverted to higher visual areas as they travel through the thalamus, a relay station for sensory information. Strengthening those pathways may provide some ability to see.
“There’s significant plasticity in the adult brain,” Huxlin says. “If you retrain the system … then you can actually improve function over time, and this is accompanied by changes in particular areas of the brain.”
Huxlin and her colleagues hope to develop a type of therapy that will improve visual function in people with damage to the visual cortex, and they recently received a patent for the idea of retraining vision with the type of exercises described in her paper.
Some vision experts believe this may lead to the recovery of useful vision in people whose visual cortex has been damaged.
“I think this is a very motivating paper,” says Melissa Saenz, a researcher at the California Institute of Technology who studies the relationship between brain activity and consciousness. “It motivates researchers to develop training methods, and it must be very motivating for someone with a significant blind spot to learn that some visual ability is available and can be improved with training.”
Some Remain Skeptical
Others are not so confident.
“I wouldn’t say I’m pessimistic; I’m realistic,” says Eli Peli, a senior scientist at the Schepens Eye Research Institute and a professor of ophthalmology at Harvard Medical School.
Peli points out that research on blindsight usually involves people like Huxlin’s subjects, who suffer from hemianopia due to a stroke on one side of the brain. Peli says they may be able to respond correctly to visual stimuli presented to their blind spot by using vision cues received by the functional part of their visual field. People with extensive damage to V1 on both sides of the brain cannot see because the visual cortex is no longer capable of processing signals, much less sending them on to higher regions of the visual system, Peli adds.
“I will start to believe in blindsight when they show me a few totally blind people who can perform these tasks,” he says. He does not believe that the man sidestepping obstacles in his path is demonstrating genuine blindsight.
“That patient simply sees,” Peli says. “I work with low-vision patients, and they often say, ‘I have no vision in that eye,’ but they actually have some vision when tested carefully.”
But Huxlin points out that some signals from the retina are diverted to higher visual areas before they reach V1. “These are the pathways we are trying to target and strengthen with our form of visual motion discrimination training,” she says.
Also, training decreases the size of the blind area, which means some blindsight is transformed into sight. “Not fully normal sight,” Huxlin says, “but conscious and usable sight.”