The principle behind retinal implants, or as they are called implantable intraocular retinal prosthesis, relies upon the way the human retina works. The retina consists of tiny cells called rods and cones which receive light from the lens and transmits the images via electrical and chemical signals down the optic nerve to the brain. It is in the brain that the signals are translated to what we actually “see.”
The cells in the outer layer of the retina are called photoreceptor cells. People with eye diseases such as Macular Degeneration and Retinitis Pigmentosa have had their photoreceptor cells damaged. However the deeper layer of cells in the retina which act as neural pathways and are responsible for transmitting signals from the photoreceptor cells to the brain are left undamaged. The implantable intraocular retinal prosthesis takes over for the photoreceptor cells, stimulating the retina cells in the deeper layer and thus restoring vision.
Research into these types of retina implants is taking place in the United States, Great Britain, Germany, and other countries. Typical of these experimental implants are the ones being worked on at the Doheny Eye Institute at the University of Southern California. In its current form the retinal implant is a 4 mm x 5 mm device studded with 16 electrodes in a 4 x 4 array. It is implanted in the retina where the photoreceptor rods and cones are, and it transmits electrical impulses to the intact neural paths in the deper layer of the patient’s retina, which in turn transmit the signals to the brain, allowing for sight. The patient wears a pair of special eye glasses with a digital camera that transmits the images it captures to the implant.
Six patients with advanced Retinitis Pigmentosa have been given this implant and can now see on a limited basis. Patients have been able to experience light and, in later tests, make out some shapes, like a foot high letter at a distance of a few feet. They can also tell the difference between certain objects, such a plate or knife. The patients cannot tell detail, however. That improvement has to await more sophisticated implants now under development. Even so, the first implant was done about three and a half years ago and still functions very well.
Other implants being studied can be implanted on the optic nerve and even in the brain cortex itself. Each type of implant has its own advantages and disadvantages.
Research is also ongoing at University Eye Hospital in Cologne, the University of Bonn, MIT, North Carolina State University, and Glasgow University's Department of Physics among other places. Human trials have shown similar results as has been achieved at the Doheny Eye Institute.