MANHASSET, N.Y. Researchers at the Keck School of Medicine of the University of Southern California have successfully demonstrated an electronic retinal prosthesis implant that gives sight to patients blinded by retinitis pigmentosa (RP).
The device works by impinging extra-cellular electric field upon the neurons behind the retinal receptors and thus manipulating that neuron's own electrochemical potential to spoof the sensation of sight.
Though retinal prostheses that use electrical stimulation are not a new concept, they have generally revolved around either brain implants, subcutaneous retinal implants, or wrapping implants around the optic nerve. "Though the idea has been around for some time, it's the actual realization and reproduction that's a first," said Dr. James Weiland, assistant professor of ophthalmology at Keck. "Also, this is the first epi-retinal implant to target RP."
According to Weiland, RP patients were chosen for a number of reasons, foremost of which is the fact that though the disease completely destroys the photo-receptor cells, post-mortem studies of donor eyes have shown that the other cells of the retina remain intact. These other cells include the bipolar cells, which typically gather the information from receptor cells in a healthy eye, as well as ganglia cells which in turn receive the information from the bipolar cells.
"So, for our device, that means that if we could put something within the eye and try and stimulate the retina's remaining cells [the bipolar and ganglia cells] and in turn give the sensation of light," said Weiland.
Other reasons RP patients are ideal are that sight is already completely gone, so the risk of damage is low. In addition, RP is a degenerative disease that starts in the 30s and 40s and gets worse over a period of 10 to 30 years.
"They've had vision at some stage and studies have shown that if someone loses their sight early in life the circuitry in the brain is not developed properly to process visual information," said Weiland. "If they lose it later, that circuitry is hardwired and is there."
The implant includes a grid of 16 electrodes that interfaces with the retina. A cable connects the grid to an external camera and processing engine driven by a Texas Instruments C54xx DSP running in the 10s of MHz. That DSP is executing algorithms that have been derived to provide the correct stimulation code sequence. The system gives a 16-pixel image that allowed the patient to see varying shades and the outline of a hand.
"We want to go as far as 1,000, but that'll take DSPs in the GHz range," said Weiland. "In the next 3 to 5 years, however we expect to prototype in the hundreds [of electrodes]."
Though size and power consumption are issues, eventually Weiland expects the implant to be small enough to be implanted within the eye, driven by a wireless connection to a camera on an eyeglass, and powered by induction.