Of the five senses, sight and hearing are often felt to be the most important. They allow us to interact with each other and our environment, and the loss of either sense can be devastating. Worldwide, an estimated 39 million people have severe vision loss and 360 million people have disabling hearing loss (1,2). Scientists have spent many decades studying the causes of vision and hearing loss, as well as working to understand how images and sounds are transmitted to and represented in the brain. After years of research, they are now creating technologies that can at least partially restore these senses. These technologies are called neuroprosthetics and take the form of devices that connect to brain cells to deliver information that the brain can no longer receive on its own, often due to injury or disease.
How do neuroprosthetics work?
The brain is comprised of specialized cells called neurons. One of the things that makes these cells unique is that they send information via electrical signals, which travel quickly through large networks of neurons to coordinate various brain functions. Scientists have taken advantage of this electrical signaling in designing neuroprosthetic devices (3). Electrodes can be used to deliver electrical current to neurons, and neurons will respond to that current similarly to how they respond to a signal from another cell. Therefore, scientists can create devices to replace damaged cells as long as they know how to replicate the electrical signals normally sent by those cells.
Scientists have been able to create neuroprosthetics for hearing and sight based on research investigating how auditory and visual neurons work. In the case of hearing, sound waves vibrate the eardrum, which transmits the vibrations to a chain of small bones inside the ear. Ultimately, those mechanical vibrations are turned into electrical signals by inner ear sensory cells called hair cells. Hair cells then deliver these signals to the auditory nerve, which transmits the message from the inner ear to the brain (4). The process is similar for vision, although in this case, sensory cells called photoreceptors located at the back of the eye in a tissue called the retina produce electrical signals when stimulated by light. They communicate with the optic nerve, which travels from the eye into the brain to pass on visual information (5). Scientists are studying how different sounds and images stimulate specific groups of hair cells and photoreceptors to produce particular patterns of electrical signals. Armed with this information, they have started making neuroprosthetic devices to help people whose hair cells or photoreceptors have been damaged.
A neuroprosthetic for hearing
The oldest form of neuroprosthetic is the cochlear implant, which was approved for use by the U.S. Food and Drug Administration (FDA) in the mid-1980’s (6). This device can be beneficial for people who are deaf, severely hard-of-hearing, or who have experienced profound hearing loss due to disease or injury, enabling them to once again hear sounds in their environment and carry on conversations. The implant is only recommended for people whose ears are so damaged that they are not helped by hearing aids, which work by making sounds louder. The reason the cochlear implant can help people with severe hearing impairment is that it bypasses the damaged area of the ear and directly stimulates the auditory nerve itself (7), which must be present and functional for the device to work (8).
Figure 1. This illustration depicts the components of a cochlear implant (7). The external machinery is located near the ear and sends electrical signals through the skin. These signals must travel to the electrode array positioned deep within the spiral-shaped cochlea.
The cochlear implant consists of an external component, containing a microphone, a speech processor, and a transmitter, and an internal component, comprised of a receiver/stimulator that sits just beneath the skin and sends signals to an electrode array positioned deep in the inner ear (Figure 1). Sound in the environment is picked up by the microphone, analyzed and converted to electrical signals by the processor, and sent through the skin by the transmitter. The receiver picks up these signals and sends them to the electrode array, which is positioned carefully so that it can deliver patterns of electrical activity to the auditory nerve, similar to those delivered by healthy hair cells (8). The implant allows people to regain some hearing; however, it does not restore completely normal hearing and requires that recipients spend time learning to interpret what they hear with the device. To date, over 200,000 people worldwide have received implants (7). Scientists continue to work on the technology of the external machinery and the design and positioning of the internal electrode to improve performance and provide more naturalistic sound (9).
A neuroprosthetic for vision
The newest advance in neuroprosthetics is a prosthetic for vision, the first artificial retina, approved by the FDA in February of this year (10). This device, called the Argus II, is the result of the U.S. Department of Energy’s (DOE’s) Artificial Retina Project. Six DOE national laboratories, four universities, and private industry worked together to develop the technology (11). The artificial retina works similarly to the cochlear implant, except this device uses a small camera attached to a pair of glasses to pick up images, and the device’s processor converts these images into light and dark pixels. The device’s receiver then turns this information into electrical signals and sends them to a sheet of photoreceptor-stimulating electrodes sitting on the retina. The photoreceptors finally send the information to the optic nerve and the brain (5, Figure 2).
Figure 2. The external hardware of the artificial retina looks like a pair of sunglasses with a miniature camera attached (top left). The receiver is on the eye and the processor is worn on a belt and not shown in this illustration. Signals are sent from the receiver to the implanted sheet of electrodes, which sits on the retina at the back of the eye and stimulates remaining photoreceptors (top right and bottom) (5).
At this time, the only people who are approved for this device are those who have lost their vision due to retinitis pigmentosa, a disorder in which the photoreceptors deteriorate (10). While their vision has largely been lost, they may still have some functional photoreceptors, which are necessary to receive the signal from the electrodes (5). Because this device only transmits information on light and dark regions, people do not get back normal vision, but are able to see things based on contrast, such as borders and outlines of objects or lights against a dark background. Due to the limited nature of what patients can see, this device has only been given to those with severe blindness. Even though it seems limited, people who have used the Argus II say they would rather see something than nothing and feel that the device helps them navigate the world more easily (10). Scientists are currently working on improving this technology and obtaining approval to use this system to treat people who have lost vision as a result of other causes.
The future of neuroprosthetics
The cochlear implant and artificial retina are only two of the many neuroprosthetics under development. Scientists are also beginning to work on technology that goes in the reverse direction, allowing electrical signals from the brains of disabled individuals to control external devices that can help them regain mobility and communication. Among these technologies are robotic arms controlled by electrodes implanted in the brains of paralyzed patients and robotic exoskeletons controlled by brain activity designed to help stroke survivors regain motion (3, 12). With the rapid progress in this field, scientists hope that it will soon be feasible for neuroprosthetics to improve the lives of people suffering from a variety of nervous system-related disorders.
Emily Lehrman is a Ph.D. candidate in the Program in Neuroscience at Harvard Medical School.
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