BioEd Online

A pair of neuroscientists have built an advanced prosthetic that can partially restore sight to blind mice. The device could eventually be developed into a system that can be used in humans.

Between 20 and 25 million people worldwide suffer from blindness due to the degeneration of their retina, the thin tissue at the back of the eye that turns light into a neural signal.  Few treatments exist, and to date, there is only one medically approved prosthetic—an array of surgically implanted electrodes that can directly stimulate the optic nerve. It allows patients to discern edges and letters, but its users still cannot recognize faces or perform many everyday tasks.

Sheila Nirenberg of the Weill Medical College of Cornell University in New York City believes that the problem is at least partially down to coding. Even though the retina is as thin as tissue paper, it contains several layers of nerves that appear to do preliminary signal processing. "The thing is, nobody knew the code," says she says.  Without it, Nirenberg believes that current and future visual prosthetics will never be able to create images that the brain can easily recognize.

Nirenberg has come up with a replacement code, and now she and her student, Chethan Pandarinath, have developed a prosthetic that uses it to restore some measure of sight in blinded mice. The duo began by injecting the eyes of their mice with a genetically engineered virus. The virus carried a genetic code to the nerve cells at the back of the eye, causing them to produce a protein normally found in algae. The protein is light-sensitive and caused the nerve cells to send a signal to the brain when a beam of light was shown into the eye. Earlier efforts stopped there, but Nirenberg and Pandarinath went a step further. Rather than feeding visual signals directly into the eye, they processed them using a code they had developed by watching a healthy retina's response to stimuli. According to their analysis, the new input allowed the mice to track moving stripes, something they hadn't been able to do before. A separate analysis of a signal travelling through the blinded retina showed that the encoder image was more recognizable than the non-encoded one (see image). The work appears in today's issue of the Proceedings of the National Academy of Sciences¹.

The importance of encoding has been debated amongst scientists working on visual prosthetics, says James Weiland, an ophthalmologist at the University of Southern California in San Diego. Some believe it will be crucial while others think the brain can adapt to an unprocessed signal. The new paper shows that encoding provides a real advantage, he says. But its full effectiveness won't be known until its tried by actual blind people. "You can't say for sure until you have the patient telling you, 'yes I see it. It's better when you do that,'" he says.

Nirenberg says she hopes to put her new system to the test soon. The encoding is simple enough to be done by a microchip, which, together with a small video camera could fit on a pair of glasses. The camera would record a signal and the encoder could then flash it directly into the eye onto the genetically treated nerve cells. If it works, the technique is simple enough that it could be done in a doctor's office. She hopes trials can begin soon. "We would like to be in patients in the next one or two years," she says.