A blind participant in the study traced the letter ‘N’ on a touch screen after the implanted electrodes simulated a pattern on his visual cortex. (Image Credit: M. Beauchamp et al. ./Cell 2020)
Researchers from the Baylor College of Medicine in Houston have developed a brain implant called visual prosthetics that allows both sighted and blind participants to see the shapes of letters. Even though it’s still in the early stages of development, this technology could stimulate the brain and restore people’s vision in the future. The team presented their findings in a paper published in the journal Cell on May 14th, 2020.
A sequence of electrical signals was delivered to the implants placed on the visual cortex, which simulated a pattern to trace shapes that participants were able to “see.” These small jolts of electricity produced tiny pinpricks of light called phospenes. Vision restoration attempts in the past involved producing multiple phospenes simultaneously, similar to light bulbs on movie marquees. However, those signals were difficult to interpret, forming smidges of light or a blotch of coalesced lights.
“When we used electrical stimulation to dynamically trace letters directly on patients’ brains, they were able to ‘see’ the intended letter shapes and could correctly identify different letters,” said Dr. Daniel Yoshor, professor and chair of neurosurgery. “They described seeing glowing spots or lines forming the letters, like skywriting.”
See image: https://imgshare.io/image/Nry2aH
This figure shows how simulation to the visual cortex allows participants to see shapes. (Image Credit: M. Beauchamp et al. ./Cell 2020)
The researchers placed an array of electrodes over the visual cortices of six participants, four with sight and two blind. Specifically, the electrodes were placed over the V1 region of the brain, where information from the retinas gets channeled for early processing. Sighted participants were already having surgery done to have electrodes implanted in their brains to treat epilepsy by monitoring seizures. Meanwhile, the blind individuals participated in a separate study examining visual prosthetics, and at the time, they had the electrodes implanted.
The team discovered that if they switched on one electrode at a time, the participants were able to see a phsophene in its predicted zone. If several electrodes were activated at the same time, the phosphenes still appeared but didn’t form any clear shapes. Afterward, the researchers attempted a different approach: they speculated that by sweeping an electrical current across numerous electrodes, they would be able to trace patterns on the brain’s surface and, as a result, produce identifiable shapes.
To create a letter shape, they produced phosphenes between the areas of two individual electrodes, which connected the dots between them. By using this technique, the team was able to draw letter shapes, like “W,” “S” and “Z” on the V1’s surface. Each shape was drawn upside-down and backward, which is how information reaches the visual cortex from the eyes. Ultimately, the participants were able to see the traced shapes and recreated them on a touch screen.
In this study, the researchers were only able to test simple shapes, but in the future, outlines of common objects, like houses, faces or cars could be traced using the same technique. Additionally, visual prosthetics in the future will most likely consist of thousands of electrodes, while the ones used in this study only had a few dozen. These electrodes could be designed to enter the cortex so that each electrode tip is closer to the neurons that lay several hundred microns underneath the cortical surface.
“The primary visual cortex, where the electrodes were implanted, contains half a billion neurons. In this study, we stimulated only a small fraction of these neurons with a handful of electrodes,” said Dr. Michael Beauchamp, professor and in neurosurgery, director of the Core for Advanced MRI. “An important next step will be to work with neuroengineers to develop electrode arrays with thousands of electrodes, allowing us to stimulate more precisely. Together with new hardware, improved stimulation algorithms will help realize the dream of delivering useful visual information to blind people.”
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