It's possible to restore some rough semblance of sight in the blind with artificial retinas. New research suggests that varying the length of the electrical pulses used to let a blind eye "see" could enable much higher-resolution retinal implants, so that blind people can better navigate their environment with confidence.
Retinitis pigmentosa (RP) is a hereditary disease that attacks the light-sensitive cells in the eye's retina (photoreceptors). As those cells deteriorate over time, the person gradually goes blind. Implanted artificial retinas, while far from conferring perfectly restored vision, can help patients detect motion, for instance, or sense large objects. Andrew Weitz, assistant professor of research opthalmology at the University of Southern California, and lead author on the new study, compares the difference to a near-sighted person trying to read a distant neon sign with and without their glasses.
The retinal implant used in the USC study was an Argus I version, dubbed "the bionic eye". (There is a later version, Argus II, as well.) Unlike the EnChroma glasses, designed to correct colour blindness, these implants can restore partial sight to those who are completely blind, although the colour resolution is pretty rough. "Most patients see yellowish-whitish light," Weitz told Gizmodo via email.
The implant procedure involves laying a tiny grid of electrodes onto the patient's retina, with each electrode being akin to a pixel in an image. When the patient dons a special pair of sunglasses outfitted with a tiny camera, those images are sent wirelessly to the array, electrically stimulating whichever cells correspond to the pattern of the image. For example, looking at the letter E would send electrical pulses to whichever retinal cells would produce the same E-shaped pattern.
"At least that's how it was intended to work," said Weitz. "But the ability of the electrode array to stimulate precise patterns of cells in the retina was poor — until now." He and his USC colleagues found that you could get much more precise patterns in the cells via electrical stimulation by using longer pulses. They performed experiments on both rats and a human subject, and reported their findings in a paper in Science Translational Medicine.
The rats didn't get actual working implants. Instead, their retinas were removed and stimulated with Weitz's homemade version of the same type of electrode array used in the Argus I retinal implant. "By keeping the retinas in heating, oxygenated saline that mimics the saline inside the eye, you can keep retinas alive this way for many hours," he explained.
Weitz tested the retinas of both regular rats, and rats crossbred to develop a genetic form of blindness similar to RP in humans.
First he injected a virus into the eye (*shudder*), implanting a gene that encodes for a protein that makes the cells light up when they're zapped with an electrical current. It took a couple of weeks before that protein became actively expressed, at which point the retina was removed and placed on the array under a microscope.
Then Weitz zapped the retina with electrical pulses of varying duration to see how the cells responded to the stimulus. The results: pulses of longer duration produced much more focused spots of light compared to shorter pulses, correlating to high resolution and (ultimately) better artificial vision.
Rats are one thing; getting this to work in human beings is another. So Weitz performed a similar experiment on a human subject who had the Argus I implant, testing the response to short and long pulses. This is admittedly the smallest possible sample size (N = 1), but the Argus II hardware won't allow for sufficiently long pulses, making it impossible to use those subjects right now. Since Argus I is an older technology, the available subjects were pretty rare.
"There is certainly a possibility of individual differences among humans, since patients have different forms of RP at different stages of the disease," Weitz said. But he still thinks the basic finding — short and long pulses stimulate different types of cells in the retina — will hold up across many human subjects.
Now Weitz is looking to modify the hardware for the Argus II implant so it can handle longer electrical pulses, with an eye toward conducting tests on more human subjects. He'll also be looking into alternative materials for the electrode arrays, since the longer pulses deliver more electrical charge to eyes, which may or may not be safe, depending on the patient. If all goes well, patients with artificial retinas will be able to see the light even more clearly.
Weitz, Andrew et al. (2015) "Improving the spatial resolution of epiretinal implants by increasing stimulus pulse duration," Science Translational Medicine 7(318): 318ra203.
Images: (top) Cells in a rat retina activated by electrical pulses. A. Weitz et al./Journal of Neurophysiology (April 2013). (bottom) Short pulses of current vs long pulses in a rat retina. A. Weitz et al.