By Rebecca Salowe
Scheie Vision Annual Report 2020
A recent publication in Cell reported that a visual cortical prosthetic that employs dynamic brain stimulation allows blind and sighted patients to “see” shapes. This study was led by visual neuroscientist Daniel Yoshor, MD, who recently joined the University of Pennsylvania (UPenn) as the Chair of the Department of Neurosurgery and Vice President of Clinical Integration and Innovation.
This report builds on decades of research on the development of a bionic, prosthetic eye. Like previous models, the design relies on a camera to directly send visual information to the visual cortex in the brain. This process bypasses the optic nerve, which is damaged in many adults with vision loss.
Visual information from the camera is delivered to the brain through electrodes placed on the visual cortex. A small electrical pulse then stimulates the brain to create a phosphene, which is a spec of light that floats through the field of vision. In recent years, bioengineering advances have led to the development of devices with dozens of electrodes that are wirelessly powered and controlled.
However, this progress leaves important questions unanswered. What happens when multiple areas of the visual cortex are stimulated? Will the resultant phosphenes combine into a single coherent image?
Unfortunately, recent research shows that phosphenes created by multiple electrodes do not behave like “pixels in a screen” that readily merge into one full image. Recognizing this gap, Dr. Yoshor and colleagues sought to develop new stimulation paradigms that could lead to a clinically usable device.
“Instead of stimulating multiple points on the brain all at the same time, we stimulate the brain dynamically, using electrical stimulation to paint an image on the brain,” Dr. Yoshor explained.
In addition to this dynamic stimulation, Dr. Yoshor and colleagues also used a technique called current steering to stimulate areas of the brain located in between electrodes. By passing a current between two adjacent electrodes, a virtual electrode is created at an intervening location. The proportion of current from each electrode can be varied, changing the location of the virtual electrode. In other words, researchers can manipulate the current to create phosphenes at additional intermediate locations.
How does this help to create a full image for the patient? By varying the current dynamically on a rapid timescale, the goal is to create the perception of a phosphene moving on a continuous line. This is in contrast to the simultaneous appearance of many phosphenes that do not create a coherent image.
To test this idea, dynamic stimulation sequences were developed to correspond to different letter-like forms. Four sighted patients and two blind patients were included in the study. The sighted patients already had electrodes implanted in their brains to monitor epilepsy.
After the electrodes were stimulated, the participants were asked to reproduce the letter shapes on a touchscreen. Over many trials, both sighted and blind participants could reliably draw, name, and discriminate each letter.
“It was remarkable how intuitive this was for patients,” said Dr. Yoshor. “With very little training, they were able to recognize letters produced with dynamic stimulation.”
Dr. Yoshor and colleagues compared this method to traditional static stimulation of electrodes. With static stimulation, patients could not identify the letters. Dr. Yoshor hypothesizes that the phosphenes may coalesce into a single phosphene, explaining this outcome.
Though this study only examined outlines of letters, future studies could expand to include common objects such as houses, faces, or bodies.
These results have important implications for visual cortical prosthetics. In the future, it may be possible for these devices to create coherent, full images for blind patients, which could have life-changing consequences.
“While there is still a lot of work to be done, the ability to generate a percept of a visual form is an important step towards the development of a visual prosthetic that can restore useful vision to the blind,” said Dr. Yoshor. “I look forward to working with my new colleagues at UPenn in neuroscience, bioengineering, and ophthalmology, to make further advances that can help patients with irreversible blindness.”
Dr. Yoshor’s co-authors William Bosking, Denise Oswalt, and Michael Beauchamp have also moved to UPenn to continue this research.