Researchers say they're close to a retinal implant that could restore normal vision to the blind.
Weill Cornell Medical College scientists say they've deciphered a mouse's retina's neural code and used it to create the device.
They've also cracked the code for a monkey retina - which is essentially identical to that of a human - and say they will soon design and test a prosthetic for blind people.
Unlike current prosthetics, which provide blind users with spots and edges of light to help them navigate, the new device provides the code to restore normal vision. It's so accurate, says the team, that facial features can be recognized and moving images tracked.
"This is the first prosthetic that has the potential to provide normal or near-normal vision because it incorporates the code," says computational neuroscientist Dr Sheila Nirenberg.
Normal vision occurs when light falls on photoreceptors in the surface of the retina. The retinal circuitry then processes the signals from the photoreceptors and converts them into a code of neural impulses. Output cells, or ganglion cells, send these impulses to the brain, which translates them into meaningful images.
Typically, blindness spares the retina's output cells, and current prosthetics generally work by driving these surviving cells. Electrodes are implanted into a blind patient's eye, and stimulate the ganglion cells with current - but this only produces rough visual fields.
But Nirenberg reasoned that any pattern of light falling on to the ret
ina had to be converted into a general code - a set of equations - that turns light patterns into patterns of electrical pulses.
"People have been trying to find the code that does this for simple stimuli, but we knew it had to be generalizable, so that it could work for anything - faces, landscapes, anything that a person sees," she says.
To create the prosthetic, she and her team implemented the mathematical equations on an encoder 'chip' and combined it with a mini-projector. The chip converts images that come into the eye into streams of electrical impulses, which the mini-projector then converts into light impulses. These light pulses then drive the light-sensitive proteins, which have been inserted into the ganglion cells, to send the code on up to the brain.
"Incorporating the code had a dramatic impact. It jumped the system's performance up to near-normal levels - that is, there was enough information in the system's output to reconstruct images of faces, animals - basically anything we attempted," says Nirenberg.
"What these findings show is that the critical ingredients for building a highly-effective retinal prosthetic - the retina's code and a high resolution stimulating method - are now, to a large extent, in place," says Nirenberg.
The retinal prosthetic will need to undergo human clinical trials, especially to test the safety of the gene therapy component, which delivers the light-sensitive protein. But it's expected to be safe, since similar gene therapy vectors have been successfully tested for other retinal diseases.