Retinal prostheses such as the Argus II, Bio-Retina and the Retina Implant AG microchip all work – more or less – by stimulating the retina’s ganglion cells with light-induced electrical signals. The images produced in the patient’s visual cortex tend to be quite rudimentary, however. This is partially because the rate at which the signals are sent isn’t the same as the rate of neural impulses normally produced by a retina. Now, researchers have deciphered the neural code used by mouse ganglion cells, and used it to create a prosthesis that reportedly restores normal vision to blind mice. They have additionally deciphered the neural code of monkeys, which is close to that used by humans, so a device for use by blind people could also be on the way.
The prosthesis was created by a team led by Dr. Sheila Nirenberg, a computational neuroscientist at New York City’s Weill Cornell Medical College. She noted that other approaches were looking mainly at ways in which more cells could be stimulated, either through electrodes or light-sensitive proteins. “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.
While researching the mouse neural code for another project, it occurred to her that it could be applied to a vision-restoring device. Working with a student, she programmed this code into a chip, which was subsequently combined with a mini projector. The chip converts images entering the eye into properly-coded electrical impulses, which are in turn converted into light impulses by the projector. Light-sensitive proteins placed on the mouse’s ganglion cells using gene therapy react to those light pulses, sending electrical code to the brain.
Tests indicated that blind mice using the prosthesis regained almost normal vision – they were able to discern faces, animals, natural scenes and various other images to about the same degree as would be possible with a regular, natural retina. They could also track moving images. When the same device was tested without the code, its performance wasn’t nearly as good.
“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,” said Nirenberg. “I can't wait to get started on bringing this approach to patients.”
A patent application for the technology has been filed, and human trials are being planned. Nirenberg envisions the human version of the device as being somewhat like the visor worn by Geordi LaForge on the TV series Star Trek: The Next Generation.
Source: Weill Cornell Medical College
