A breakthrough in understanding how the eye sends visual information to the brain could soon lead to "bionic" implants that restore almost perfect vision to millions of blind people, scientists have announced.
Researchers have cracked the neural "code" used to shuttle images from the eye's retina to the visual centres of the brain and have incorporated this code into a microchip that can be inserted into the eye.
Tests on the retinas of blind mice have radically improved their vision compared to existing microchips. The scientists said they have also cracked the code for monkey vision which is essentially the same as that used in humans.
The researchers envisage being able to construct futuristic visors for the blind, similar to those used in Star Trek, to enhance the visual abilities of the 25 million people in the world suffering from conditions such as macular degeneration and retinitis pigmentosa, which cause the loss of light-sensitive cells in the retina.
Sheila Nirenberg, a neuroscientist at Weill Cornell Medical College in New York, said that the advance is a radical improvement on existing attempts to insert bionic eye implants which have only had limited success in restoring vision to the blind.
"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. This is the first prosthetic that has the potential to provide normal or near-normal vision, because it incorporates the code," Dr Nirenberg said.
Existing prosthetic devices used to enhance vision are based on tiny light-sensitive electrodes that simulate nerve cells within the eye to compensate for the loss of the natural light-sensitive cells of the retina, the cones and rods. However, these prototype devices, when tested on patients, only manage to produce spots of light or high-contrast edges. Patients are unable to discern the details of a face, for instance.
Scientists have tried to compensate for this technical limitation by increasing the density of electrodes in the implant. But Dr Nirenberg's team used an additional approach by incorporating an intelligent "encoder" that sits between the incoming light and electrode stimulators.
It is this encoder that can modify the stimulation of the nerves leading from the retina to the brain in a way that accurately reflects the natural visual process of the retina, the scientists say in their study published in the journal Proceedings of the National Academy of Sciences.
The key to the success was the discovery that the light-sensitive cells of the retina use a type of code, or set of equations, to convert light into the electrical pulses sent to the brain via nerves cells, or ganglia, within the retina, Dr Nirenberg explained.
"Not only is it necessary to stimulate large numbers of cells, but they also have to be stimulated with the right code – the code the retina normally uses to communicate with the brain," she said. "People had been trying to find the code that does this for simple stimuli, but we knew it had to be generalizable, so it would work for anything – faces, landscapes – anything a person sees."
The encoder consists of a microchip that converts incoming images into streams of electrical pulses. A mini "projector" within the encoder than converts these electrical pulses back into a pattern of light impulses that are used to stimulate light-sensitive proteins within the ganglia cells of the retina.
A gene therapy technique is used to insert these light-sensitive proteins in to the mouse ganglia, which would also need to be used if human patients are to benefit from the technique, the scientists said.
To test the idea, the scientists built two prosthetic devices attached to mouse retinas, one with the code and one without. The results, and those combined with experiments on laboratory mice, show that the bionic implant enabled blind mice to see visual details, said Dr Nirenberg.
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