Visual prostheses are a technological advancement for individuals with severe vision loss. These devices aim to restore a sense of sight by interacting directly with the visual system. This technology helps individuals perceive their environment.
What is a Visual Prosthesis?
A visual prosthesis, often called a bionic eye, is an implanted electronic device designed to restore functional vision. It bypasses damaged parts of the eye or visual pathway to stimulate remaining healthy cells or brain regions. This stimulation generates visual perceptions.
These devices are composed of an external system, often including a camera and a processor, and an implanted component with electrodes. The external camera captures visual information, which is then converted into electrical signals. These signals are transmitted to the implanted electrodes, which in turn stimulate the nervous tissue.
The goal of a visual prosthesis is to convert light information into electrical impulses that the brain can interpret as vision. This approach differs from corrective lenses or low-vision aids, which maximize existing vision rather than creating new visual input.
How Visual Prostheses Restore Sight
Visual prostheses use electrical stimulation to generate visual perceptions, known as phosphenes. Phosphenes are sensations of spots or patterns of light induced by electrical currents stimulating neural tissue. The goal is to generate consistent phosphenes that the brain can interpret as representations of the external world.
The process begins with an external camera, often mounted on glasses, capturing light from the environment. This visual information is then converted into electrical signals by a processing unit. These electrical signals are subsequently transmitted to an implanted array of electrodes.
The electrodes deliver electrical impulses to specific parts of the visual pathway, such as the retina, optic nerve, or visual cortex. These impulses excite remaining healthy neurons, leading to the perception of phosphenes. The brain then interprets these patterns of light, allowing the individual to perceive rudimentary visual information.
Different Approaches to Visual Prostheses
Retinal Prostheses
Retinal prostheses are designed for individuals whose vision loss stems from the degeneration of photoreceptor cells in the retina, such as in cases of retinitis pigmentosa or age-related macular degeneration. These devices work by replacing the function of the damaged photoreceptors.
There are two main types of retinal prostheses. Epiretinal prostheses are surgically placed on the inner surface of the retina, adjacent to the vitreous humor, and stimulate retinal ganglion cells. These devices require an external camera to capture visual information. Subretinal prostheses are implanted underneath the retina, often integrating the imaging sensor with the electrode array to directly activate bipolar cells, mimicking the function of natural photoreceptors.
Optic Nerve Prostheses
Optic nerve prostheses aim to stimulate the optic nerve directly, bypassing the retina entirely. This approach can be considered for patients with severe retinal illnesses or damage to the optic nerve itself, where retinal prostheses would not be effective.
These devices involve a spiral cuff electrode placed around the optic nerve, connected to a stimulator that receives signals from an external camera. While research has shown the ability to induce phosphenes through optic nerve stimulation, achieving selective and high-resolution perception remains an ongoing challenge.
Cortical Prostheses
Cortical prostheses represent an approach that bypasses both the eyes and the optic nerve, stimulating the visual cortex in the brain directly. This method is suitable for individuals with extensive damage to the retina, optic nerve, or other parts of the visual pathway upstream of the brain.
These devices involve implanting electrode arrays onto or into the occipital lobe, where the visual cortex is located. An external camera captures images, which are then processed and converted into patterns of electrical stimulation delivered to these electrodes. The goal is to generate phosphenes directly in the brain, allowing for a sense of vision even in cases of profound blindness.
The Vision Provided by Prostheses
The vision provided by current visual prostheses is not comparable to natural sight. Users perceive patterns of light, flashes, or simple shapes, often described as phosphenes. These perceptions are featureless white dots of light, which can appear circular.
The resolution of prosthetic vision is limited, and users may perceive objects as high-contrast outlines rather than detailed images. Color perception is absent, and the field of view can be narrow.
Despite these limitations, the functional benefits can be significant. This rudimentary vision can help individuals perceive light, identify large objects, detect movement, and navigate environments. While the vision may not allow for reading fine print, it can greatly enhance independence and interaction with the surroundings.