wo researchers in the US have taken a huge step forward in developing technology to help blind people see: they have made an artificial retina that restored normal vision in blind mice. And they have already worked out a way to make a similar device for monkeys, which they hope to quickly redesign and test for human use.
Artificial retinas are not a new invention, however, the ones produced so far only produce rough visual fields where the user sees spots and edges of light to help them navigate.
But the one Sheila Nirenberg and Chethan Pandarinath at Weill Cornell Medical College in New York have developed allows animals to detect facial features and track moving images.
They report their breakthrough online in the 13 August issue of the Proceedings of the National Academy of Sciences (PNAS).
Unique Feature: Coded Neural Signals
Their artificial retina is different because it incorporates a unique feature: the neural code that the retina cells use to communicate the visual information to the brain. Combining the code with the ability to stimulate a large number of light-sensitive cells produces a system that gives the brain the correct amount and type of information in order to “see”.
Lead author Nirenberg, a computational neuroscientist at Weill Cornell, told the press she thinks one day blind people will be able to wear a visor, similar to the one Geordi La Forge wears on the television show Star Trek. The visor will have a camera that takes in light and a chip that turns that light into a code that the brain uses to recreate the image.
“It’s an exciting time. We can make blind mouse retinas see, and we’re moving as fast as we can to do the same in humans,” said Nirenberg, a professor in the Department of Physiology and Biophysics and in the Institute for Computational Biomedicine at Weill Cornell.
How the Retina Works
Scientists hope that artificial retinas could be used to treat human blindness within a decade.
Normal vision is where light enters the eye and falls on photosensitive cells that lie on the surface of the retina. The “circuits” in the retina convert the light into a series of coded electrical signals or neural pulses, and pass them onto output cells called ganglion cells that transmit the coded pulses to the brain via the optic nerve at the back of the eye.
The brain understands the stream of coded neural pulses and translates it into meaningful images.
A common cause of blindness is when the retina is damaged by diseases that kill the photoreceptors, and/or destroy the circuits that create the coded neural pulses. But often, these diseases don’t damage the output cells.
Why Current Prosthetics Can’t Do the Full Job
Current prosthetics work by using electrodes that are implanted into the blind patient’s eye, to drive the surviving cells: they stimulate the ganglion cells with electrical current.
But this method only produces very rough visual fields: the cells are stimulated, but they aren’t receiving the right signals, a sort of neural equivalent of “white noise”.
Scientists are working on various ways to improve on this approach. For instance one way is to have more stimulators in the implant, in the hope that with more stimulation, the image will improve.
Another approach that is being tested is using gene therapy to generate light-sensitive proteins in the retina to stimulate the ganglion cells.
But the invention that was “waiting to happen”, as Nirenberg explains, is one that not only stimulates large numbers of cells, but also stimulates them with the right code, the same one the retina uses to communicate with the brain.
How They Made the Discovery
Nirenberg had the idea that any pattern of light falling on the retina has to be converted into equivalent patterns of neural impulses via a general code or set of mathematical equations.
She said people have been trying to find the code for simple patterns. But she was convinced the code had to be generalizable for any type of stimulus, simple and complex, whether it be for faces, landscapes, anything that the eye looks at.
The actual “aha” moment came when she was working on the code for a different reason, said Nirenberg. She realized what she found could work on a prosthetic.
So she and Pandarinath put the equations they were working on onto an electronic chip, and combined it with a mini-projector.
The chip translates the light pattern (the image) that comes into the eye into coded electrical pulses, and the mini-projector converts them into light pulses.
The light pulses stimulate the light-sensitive proteins which have been inserted in the ganglion cells, and the result is the brain receives coded neural pulses.
They tested the method in mice. They made and compared two versions of the prosthetic: one without the code, and one with the code.
Nirenberg said the effect was dramatic. When they put in the code, the system’s performance “jumped” 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,” said Nirenberg.
They did some rigorous tests to establish that the patterns made with the help of the prosthetic in blind mice’s retinas matched the ones produced by retinas in seeing mice.
The study shows that the critical components for making a highly effective retinal prosthetic, the retina’s code and a high resolution method of ganglion cell stimulation, are now more or less in place, said Nirenberg.