The Blind Can See, with the First Commercially Available Retinal Implant
In February, The FDA approved the Argus II, the first retinal implant to enable blind patients with retinitis pigmentosa to see. The implant was produced by Second Sight, a medical products company, which has been implanted in about seventy blind patients so far. Second Sight was founded by Alfred Mann, a biomedical entrepreneur who also founded Advanced Bionics, a company specializing in cochlear implants for deaf patients.
Retinitis pigmentosa includes most genetic diseases of the retina that lead to blindness for about 1.5 million people worldwide. In a healthy eye, rods and cones called photoreceptors convert light into electrochemical impulses. These are sent to the brain, which converts them back into images. This is similar to the mechanism for hearing in a healthy ear, which involves receptors that convert vibrations into a signal. PAtients with retinitis pigmentosa have damage to the rods and cones in the retina.
Cochlear implants use a microphone to record sound, which is converted into a signal and sent to the auditory nervous system. Likewise, the Argus II retinal implant uses a visual processing unit to convert an image from a camera into an electrical signal that can be interpreted by the brain.
A handful of academic groups have been researching retinal implants since the early 1980s, but Second Sight is the first company to release a product. A company called Visus Technologies is currently raising capital to fund product development.
At first, the brain typically doesn’t know how to interpret these foreign signals. When Lisa Jordan’s audiologist turned her cochlear implant on for the first time after surgery in September, she heard loud and overwhelming noise that sounded like a helicopter landing.
“The first day was absolutely awful,” Lisa Jordan said. “Even though they tell you not to have high hopes, I had high hopes of being able to understand some speech right away… And that first day, I could understand essentially nothing.”
“I think the experience [of a retinal implant] is generally similar,” said Brian Mech, vice president of business development for Second Sight. “There’s an initial period where things don’t quite make a lot of sense, and over time they learn how to interpret what’s going on.”
This improvement can be attributed to the patient’s neuroplasticity and adaptation and to a procedure called electrode mapping, Mech explained.
“We can address each electrode individually and that and that allows us the ability to customize the implant. We’ll measure the threshold and dynamic range on each electrode for every patient, and we’ll also determine the frequency of stimulation that’s optimal for each patient,” Mech said.
Electrodes in retinal implants also must be adjusted for each patient. “Each person is unique and so with [electrode mapping] you’re trying to create a specific program for that person.” said Dr. Edie Gibson, Au. D, an audiologist with Second Sight.
It’s largely trial and error, so communication with the patient is important, she said.
The whirring helicopter noise Jordan heard settled down over the course of the electrode mapping process. She was finally able to discern environmental sounds and understand speech.
However, she still couldn’t distinguish a man’s voice from a woman’s, or a toilet flushing from a “‘Transformers’ movie with electronic sound effects,” she said.
As her brain adapted, Jordan’s hearing became more clear. She’s now able to match pitch perfectly with all notes besides E flat and D flat, which she says are about half of a note sharp. Most of her improvement didn’t come from electrode mapping, but from learning to make better sense of the electrical impulses over a period of months.
Dr. Gibson’s job used to be adjusting, or mapping, the electrodes the day after surgery, but now she focuses on long-term adaptation.
“It's a series of exercises, starting from the very beginning of identifying sounds and working through discrimination, all the way up to the complicated tasks like the telephone and music,” Gibson said.
Results for both cochlear and retinal implants will vary by patient according to Mech. Many patients with cochlear implants are unable to understand music or talk on the phone after years, whereas Jordon and other patients walk out of the room speaking on a cellphone once electrode mapping is complete.
“There are times when you turn someone on and they’re hearing very well... but it’s not as common,” Dr. Gibson said. “It’s much more typical that it’s a gradual process.”
The best results come from a combination of the right electrode settings and the brain’s adaptation through the learning process. “They go hand in hand,” she said. “There’s the adjusting of the electrodes and there’s giving the brain time to adjust... They both must happen.
Dr. Miguel Nicholelis and his team at Duke University implanted functional infrared light sensors into rats’ brains. Much like Lisa Jordan, and the patients receiving the Argus II implant, the rats were at first disoriented. They initially responded by scratching their faces, perhaps since the implant was in the part of brain associated with touch. After about a month, every rat was able to navigate through infrared light just as well as the visible spectrum.
Nicholelis says his studies on novel sensory implants may shed some light on the learning process involved with cochlear and retinal implants. “The mechanisms by which animals learn to use this new information source will also be an interesting avenue for future research, as such research should suggest how to accelerate sensory prosthetic acquisition,” Nicholelis wrote in Nature.
This science news brief was written under the guidance of JYI Science Writing Mentor Jake Berkowitz.