The three blind mice immortalized in nursery rhyme could have had entirely different fates today, thanks to a breakthrough in retinal implants that has restored sight in blind mice. Researchers at Weill Cornell Medical College have cracked the code for both a mouse and a monkey retina's neural code, which has enabled them to develop a working artificial retina for mice that they hope will pave the way for a similar device that will restore sight in humans.
When caused by diseases of the retina, blindness results when photoreceptors on the surface of the retina and the retinal circuitry that processes the signals from the photoreceptors and converts them into a code of neural impulses are damaged or destroyed. Current prosthetics are designed to stimulate ganglion cells in the damaged tissue, which are often spared during the course of the disease, by applying current. However, this approach yields only rough visual fields, according to the researchers.
To improve prosthetics and the image quality they produce, researchers are exploring the effects of increasing stimulators as well as the use of light-sensitive proteins, introduced by gene therapy, as an alternate method of cell stimulation. But these efforts may be overlooking one important piece of the puzzle, according to lead researcher Sheila Nirenberg, a computational neuroscientist at Weill Cornell. "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."
Putting this theory to the test, the researchers developed a retinal prosthetic consisting of an encoder and a mini projector. Using a set of mathematical equations, the encoder converts images that enter the eye into electrical impulses, which, in turn, are converted into light impulses by the mini projector. In response to these light impulses, light-sensitive proteins that have been introduced into the ganglion cells subsequently communicate the code to the brain.
"The reason this system works is two-fold," Nirenberg explains. "The encoder is able to mimic retinal transformations for a broad range of stimuli, including natural scenes, and thus produce normal patterns of electrical pulses, and the stimulator is able to send those pulses on up to the brain."
This approach, according to the researchers, is so effective that it enables the user to discern facial features and allows animals to track moving images; the prosthetic provides normal or near-normal vision. Building on this success with restoring vision in animals, the researchers plan to advance their sight-restoration technology and model it for human use.
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