Artificial vision devices are presently very crude: grids of electrodes embedded in the retina that can stimulate retinal cells to create the appearance of a pattern of glowing dots based on what a camera sees. This is enough to pick out letters, navigate a room, or distinguish faces with practice, which is a big step up from being absolutely blind. These are still prototypes, however, steps on the way to better things. Researchers are laying the groundwork for more a subtle integration between microelectronic devices and retinal cells:
Just 20 years ago, bionic vision was more a science fiction cliché than a realistic medical goal. But in the past few years, the first artificial vision technology has come on the market in the United States and Western Europe, allowing people who've been blinded by retinitis pigmentosa to regain some of their sight. While remarkable, the technology has its limits. "It's very exciting for someone who may not have seen anything for 20-30 years. It's a big deal. On the other hand, it's a long way from natural vision."
Although much of visual processing occurs within the brain, some processing is accomplished by retinal ganglion cells. There are 1 to 1.5 million retinal ganglion cells inside the retina, in at least 20 varieties. Natural vision - including the ability to see details in shape, color, depth and motion - requires activating the right cells at the right time. The new study shows that patterned electrical stimulation can do just that in isolated retinal tissue. In laboratory tests, researchers [focused] their efforts on a type of retinal ganglion cell called parasol cells. These cells are known to be important for detecting movement, and its direction and speed, within a visual scene. When a moving object passes through visual space, the cells are activated in waves across the retina.
The researchers placed patches of retina on a 61-electrode grid. Then they sent out pulses at each of the electrodes and listened for cells to respond, almost like sonar. This enabled them to identify parasol cells, which have distinct responses from other retinal ganglion cells. It also established the amount of stimulation required to activate each of the cells. Next, the researchers recorded the cells' responses to a simple moving image - a white bar passing over a gray background. Finally, they electrically stimulated the cells in this same pattern, at the required strengths. They were able to reproduce the same waves of parasol cell activity that they observed with the moving image.
"There is a long way to go between these results and making a device that produces meaningful, patterned activity over a large region of the retina in a human patient. But if we can handle the many technical hurdles ahead, we may be able to speak to the nervous system in its own language, and precisely reproduce its normal function."