In patients blinded by degeneration of the retina's photoreceptor cells, the aim of visual prostheses is to restore useful vision (reading, mobility), which implies regaining the ability to detect light and discriminate.
Given the persistence, in conditions such as retinopathy pigmentosa and perhaps age-related macular degeneration, of the retina's inner neurons and especially their connection to the brain, the possibility of restoring visual sensation through electrical stimulation of the remaining neuronal circuits has long been envisaged.
Some twenty years ago, Mark Humayun demonstrated that electrical stimulation of the retina could trigger phosphene-like light sensations. This led, initially, in the wake of work on cochlear implants, to the proposal of limited-resolution implants, initially 16 electrodes, then more recently 50 to 60 electrodes, to restore basic visual sensations.
The systems used include a camera for image detection, computer processing and optimization, connection to a transmitter, and then an antenna sutured to the eye and connected by a cable comprising sixty or more fibers to an epiretinal chip. In pilot clinical trials, they have enabled a number of patients to demonstrate the recovery of certain visual functions, such as the ability to detect certain objects, sidewalks, edges and doors, and eventually to read large characters or even short words.
