The retina, which is the light-sensing neuronal tissue in the back of the eye, processes images through several steps. Cells called photoreceptors detect light. They pass their information to other neurons for an intermediate processing step, and then that newly coded information is passed to retinal ganglion cells. Retinal ganglion cells connect straight into the brain. Voila, vision.
In diseases of blindness, photoreceptors die but those other cells are fine and well. Prosthetics take advantage of that by stimulating retinal ganglion cells based on what an external camera sees. In the lab is an alternative to the implantable devices currently used: a blue-light sensing protein is hooked onto retinal ganglion cells. Flashes of blue light corresponding to the world around stimulate the retinal ganglion cells. In either case, retinal ganglion cells replace dead photoreceptors. The problem is that retinal ganglion cells (and the brain) aren’t equipped to handle raw information like that. They are supposed to get the final message. Firing retinal ganglion cells in response to light doesn’t send a very detailed message to the brain.
The solution is to create an “encoder.” The new prosthetic works like this: a camera takes in the scene, as before. Then, a computer algorithm turns images into electrical pulses. The computer does the intermediate processing normally done by other cells in the retina. The algorithm is based on how normal mice process visual information. A blue LED sends out flashes of light to match the encoder’s electrical pulses. This stimulates the blue-light detecting protein that’s been engineered onto retinal ganglion cells. The end result? Retinal ganglion cells in blind mice fire in about the same pattern as retinal ganglion cells in a mouse that’s not blind. Voila, vision.
Reference: Retinal prosthetic strategy with the capacity to restore normal vision. Sheila Nirenberg and Chentan Pandarinath. PNAS epub Aug 13, 2012. doi: 10.1073/pnas.1207035109
Also, a Nature News article by Geoff Brumfiel. “Prosthetic Retina helps restore sight in mice.” Nature, Aug 13, 2012.
Image is a schematic of a traditional prosthetic retina. Credit: http://www.nsf.gov/od/lpa/news/03/pr03115_images.htm