Even when vision has failed, the mind's eye can see, given just the right voltage to just the right place in
, says a study published Monday in the Proceedings of the National Academy of Sciences. The
, which used rhesus monkeys with intact vision, demonstrated one way to restore something akin to sight in the blind. It showed that direct stimulation to the brain's visual cortex can create the perception of shapes, colors and contrasts -- even when the eyes cannot see them.
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
. 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
will begin to reassign some of those underutilized neurons to other tasks.
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.