Scientists have created a prosthetic device capable of restoring normal sight in people suffering from neurological forms of blindness. The device translates what the eye perceives into neural code that is deciphered by the brain, something other prosthetics do not do.
In neurodegenerative diseases such as macular degeneration or retinitis pigmentosa, blindness occurs because light-sensitive cells or photoreceptors on the retina in the back of the eye fail, and so the retina is no longer able to send signals to the brain for processing. But typically the retina's ganglion cells - which transmit images to the brain - are spared. Scientists use these surviving cells to build prosthetic devices, implanting electrodes in patients' eyes to stimulate what remains of the visual circuitry. The result, says neurophysiologist Sheila Nirenberg, is a rough, poor visual field.
But Nirenberg and her student, Chethan Pandarinath, at Weill Medical College at Cornell University in New York have added what they consider to be the missing link in the visual pathway. They've created a prosthetic device with an encoder that mimics the neural language or pulses that are normally sent from the retina's ganglion cells to the brain where it is deciphered and perceived as imagery.
"We basically captured that code and built it into a prosthetic," said Nirenberg. "If you don't do this, there's a ceiling, sort of, on how well these prosthetics can ever do. And if you add the code, you suddenly have a critical ingredient for making this work."
To demonstrate the prosthetic's effectiveness, the investigators showed a movie to healthy mice and recorded the pattern of cell firings or spikes to the animals' brain as they watched it. Then, they exposed blind mice to the same movie. Nirenberg says those rodents had no functioning photoreceptors, and no response to the film. Then the mice were fitted with the encoder which created pulses of visual information to the retina.
"We made it come back to life," said Nirenberg. "So, we know if it can produce the same pattern of activity as the normal retina, then it will be sending the same signals up to the brain. So it will be sending signals to the brain that the brain can understand."
Nirenberg says the prosthetic comprises two components: the encoder, which would be built into a pair of dark glasses or a sun visor, and gene therapy, which introduces a protein that can transmit the coded visual pulses from the retina's ganglion cells to the brain.
The researchers say they also have the code for monkey retina, which is very similar to that of humans.
An estimated 20- to 25-million people around the world suffer from blindness as a result of diseased retinas. Researchers hope to develop an artificial retina prototype and begin clinical trials in the next year or two. Eventually, Nirenberg says, scientists could develop implants that encode neural information to the brain.
An article on the creation of a new visual prosthetic is published in Proceedings of the National Academy of Sciences