Often when a person loses vision because injury or disease, the broken component is either the detection device -- the eyes -- or the wiring that carries images detected by the eyes to the brain's visual cortex. That large region in the brain's rear quarters is where raw images picked up by the eyes are interpreted and given meaning. And for at least two years after a person has suffered a loss of vision, that specialized region of the brain remains ready to detect and make sense of incoming messages from the eyes. If it gets none, the marvelously adaptable brain will begin to reassign some of those underutilized neurons to other tasks.
At the Massachusetts Institute of Technology's Cognitive and Brain Science Department, researchers hoping to restore sight have focused not on repairing the eyes that have ceased to function or the wiring that has frayed; instead, they have proposed to supply the electrical messages that the brain interprets as an "image" directly to the brain's visual cortex.
To show that such an approach could work, the researchers trained two male rhesus monkeys -- Hank and Malibu -- exhaustively in a visual task. Sitting in front of a large screen, the monkeys learned that whenever they saw two separate figures projected on the screen, they would get a reward -- a drop of apple juice -- if they identified the bigger or the brighter of the two by shifting their gaze toward it.
While each monkey practiced the new skill, researchers "listened in" on the distinct patterns of electrical messages passing among the cells of his visual cortex. They also recorded those patterns -- the distinctive "sight" of an image as it's being processed by the brain -- using microelectrodes threaded carefully into the monkey's striate cortex, which lies just beneath the skull.
What happened when the researchers "played back" those electrical patterns was remarkable: When a monkey's visual neurons were electrically stimulated with the "sight" of a circle, his eyes immediately went to it -- even when it was not really on the screen in front of him. When researchers electrically generated the image of a larger circle and put it next to a small circle projected in the screen in front of the monkey, Hank and Malibu demonstrated they could "see" that the new circle was larger by shifting their gazes directly to it. When the circle generated by electrical current was brighter than the one on the screen, the monkey demonstrated he could discern the difference in brightness by moving his eyes to the brighter one, as he had been taught.
The images created by electrical stimulation to the brain proved to be pretty "washed out," the researchers reported. But they still ranged in color from pink and purple to various shades of yellow. With an array of electrodes capable of delivering electricity to 256 brain cells in the visual cortex (128 in each of the brain's two hemispheres), images can be delivered to a narrow slice of the visual field; by stimulating more neurons, the researchers said, that visual field could be widened. By strengthening the power of the electrical current, the study suggested to researchers they could improve visual acuity.
In an interview, Peter H. Schiller, the lead author of the study, said that "this is just the very first step" in a lengthy process of building a neural prosthetic that might help blind humans see again. He said it could take "a good 10 years" to translate what can be learned from rhesus monkeys to humans, despite the fact that a monkeys' visual cortex is organized in much the same way as humans' brains. Learning how to translate complex visual images into electrical signals that convey the density of information a human expects -- not only the shapes and colors of objects but their distance from oneself -- will be an enormous task, Schiller added. And it will be a challenge to build electrode arrays powerful and resilient enough to feed information to the brain continuously without damaging delicate neurons, he said.