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Scientists at the Netherlands Institute for Neuroscience (NIN) have recently developed high-resolution brain implants to stimulate shape perception in monkeys, demonstrating the possibility of restoring functional, life-enhancing vision in the blind, a new study reports.
This is, however, not the first time visual prosthesis was used to generate artificial vision. Electrical stimulation of the visual cortex of both humans and animals had shown to “elicit the perception of a dot of light, known as a phosphene” more than 70 years ago.
Since then, numerous efforts had been made to generate artificial vision by using this technology. However, early attempts with the visual prosthetics were unsuccessful in producing useful percepts due to the technological limitations.
The study, published in Science on December 3, 2020, stated that until now only a limited number of electrodes were used and positioned on the surface of the brain, which activated several millimeters of cortex, producing large phosphenes, or crude images.
With the advancements in implantation technology and cutting-edge materials over the decades, researchers at NIN are using a large number of intra-cortical electrodes to investigate the generation of artificial visual percepts. For the study, they installed 1024 electrodes in the visual cortex of two macaque monkeys to create percepts of letters, orientation, and motion.
According to the researchers, “Microstimulation by intracortical electrodes require currents that are two orders of magnitude lower, activates neurons located within a few hundred micrometers of the electrode tip, and potentially yields higher-resolution phosphene percepts.”
The monkeys had previously been trained to identify letters and visually-presented dots on a computer with an eye movement. Later, when the letters were directly stimulated in the cortex without any visual stimuli, the monkeys could still recognize the letters.
According to the findings, simultaneous stimulation of multiple electrodes in the cortex gives rise to the perception of shape, and successive stimulation gives rise to the perception of motion, providing proof of concept for the use of electrical microstimulation to create a form of artificial vision in the blind.
A typical visual prosthesis system consists of a camera that the user wears on a pair of glasses and a portable processor that transforms camera footage into instructions for electrical stimulation of the visual cortex.
The researchers, however, said that it would be a few years before a visual cortical prosthesis for humans is available. They still need to overcome a few technical challenges. One of the hurdles is to covera large enough region of the visual field with a sufficient density of phosphenes to generate interpretable percepts. And, they also need “a wireless technology with high channel counts to develop long-lasting, biocompatible electrodes that minimize the risk of gliosis, tissue trauma, and encapsulation.”
While letter recognition is possible with a few hundred electrodes, thousands of electrodes are required for fluent reading and object recognition in crowded scenes, they added. “By implanting more than 1000 electrodes across a large area of the visual cortex, we could generate numerous phosphenes across a contiguous region of the visual field and elicit recognizable shapes.”
The present demonstration of artificial vision, and with these developments, the scientists think there will be finally a light at the end of the tunnel for around 40 million people across the world who are living with blindness.
Reference:
Chen, X., Wang, F., Fernandez, E., & Roelfsema, P. R. (2020). Shape perception via a high-channel-count neuroprosthesis in monkey visual cortex. Science, 370(6521), 1191–1196. https://doi.org/10.1126/science.abd7435
