Groups in Germany and the US have been testing electronic implants aimed at restoring vision to people with retinal dystrophy. The condition is hereditary or age-related, and causes degeneration of the photoreceptors – light-sensitive cells in the retina – leading to blindness. It affects 15 million people worldwide.
Eberthart Zrenner and colleagues at the University of Tübingen in Germany have developed a microchip carrying 1500 photosensitive diodes that slides into the retina where the photoreceptors would normally be. The diodes respond to light, and when connected to an outside power source through a wire into the eye, can stimulate the nearby nerves that normally pass signals to the brain, mimicking healthy photoreceptors.
The team reports that their first three volunteers could all locate bright objects. One could recognise normal objects and read large words.
Nerves in the eye normally adapt to visual input and stop transmitting signals after a short time. Tiny movements of the eye overcome this by constantly projecting the image back and forth between neighbouring nerve cells so that each has time to recover and resume transmitting signals. Because the implant is inside the eye, this mechanism worked normally in the trials. Another device being tested sends images from a head-mounted camera to ocular nerves, but as the image forms outside the eye the tiny movements cannot maintain it and patients must rapidly shake their head instead.
In this first pilot study, the team removed the devices after a few weeks as a safety precaution. But they have designed a new system which can be implanted permanently when approved.
Journal reference: Proceedings of the Royal Society B, DOI: 10.1098/rspb.2010.1747