Imagine a world where the earth wavered with every step, where you couldn’t tell up from down in a dark room, or where walking on a soft carpet threw you off balance. That’s the reality for people who have lost function in the vestibular system, part of the inner ear that controls balance.
Neuroscientists at the Massachusetts Eye and Ear Infirmary, in Boston, are gearing up to test a new prosthesis that might help. In a month, Dan Merfeld will flip the switch on an experimental device implanted into a rhesus monkey whose own vestibular system has been disabled. Merfeld and his collaborator Richard Lewis hope that the device will do for balance what the cochlear implant has done for hearing. “If we can show we can improve balance in monkeys, that would be a stimulus for moving into clinical trials,” says Lewis, a scientist and otoneurologist (a neurologist who specializes in ear diseases).
The cochlear implant–a surgically implanted electronic device that gives deaf people a sense of sound–has been the most successful neural prosthesis to date. Merfeld, Lewis, and others are leveraging technologies developed for that implant to create a similar prosthesis for the vestibular system.
The inner ear functions like a gyroscope. Three orthogonally oriented structures, called the semicircular canals, sense the orientation of the head via movement of fluid within the canals. Nerves connected to these structures send a train of neural signals to the brain, which integrates that information with visual signals and other cues to maintain balance and stabilize vision–for example, to keep our eyes focused on one point as we walk, eliminating the jittery, handheld-camera effect we might otherwise perceive. When the vestibular system is wiped out, serious balance issues can result. Such a disorder is sometimes a side effect of antibiotics. It can also be caused by trauma, infection, and some diseases. For example, more than 500,000 individuals in the United States suffer from Meniere’s disease, a particularly debilitating disease of the inner ear.
“Patients can remain with symptoms of imbalance, which is sometimes crippling, forever,” says Timothy E. Hullar, an otolaryngologist at the Washington University School of Medicine, in St. Louis. “As a clinician with a number of patients with bilateral vestibular loss, I’m very excited to think that in a few years prostheses might be a treatment option.”
The relative simplicity of the vestibular system makes it an ideal target for prosthesis. The horizontally oriented canal, for example, detects left-right motion, such as a negative shake of the head. Neurons that connect to this canal send electrical pulses to the brain at a high rate when the head turns to the left, and a low rate when it turns to the right. Merfeld’s prosthesis mimics this signaling system: a motion sensor on the head measures rotation, sending that information to a microprocessor that converts it into electrical impulses, which are transferred to an electrode implanted into the inner ear.
Previous research has shown that the device can help squirrel monkeys that had part of their vestibular system rendered dysfunctional. When the device was turned on, the monkeys’ vestibuloccular reflex improved, meaning that they could better keep their eyes stable while their heads moved. (So far, the researchers have targeted only one of the canals with their prosthesis. Other scientists working in the field have targeted all three canals in rodents.)
The team now wants to determine how well the device can treat other symptoms of vestibular disorders, such as balance and perception. (The brain uses information from the vestibular system to control both the muscles that move our eyes and the postural muscles that keep us upright.)
Measuring these senses has proved hard to do, even in humans. We manage our vestibular system–unlike vision or hearing–largely unconsciously, making it difficult for people to quantitatively report what they perceive, says Christopher Platt, who oversees balance and vestibular research at the National Institute on Deafness and Other Communication Disorders. So Merfeld and Lewis will test their prosthesis in the rhesus monkey, which can be trained to perform complex tests. To test balance, for example, the animals are taught to stand with one limb on each of four small platforms that individually move around, giving the illusion of an earthquake. Then the researchers measure the animal’s ability to maintain balance in response to the movements. To measure perception, animals are taught to turn a steering wheel to vertically orient a line on a computer screen. Without other visual cues, a monkey or person without vestibular function will orient the line at the same angle as the head.
“That’s very important because it means they can test the monkeys with exactly the same tests they give to humans, and get a better estimate of how well their device is doing, with the hope it can be transferable in humans,” says Platt.
If all goes well in initial experiments, the researchers hope to increase the complexity of the device, targeting all three canals of the inner ear, and eventually other structures. Neither Merfeld’s group nor others working in the field have yet targeted a second set of vestibular structures, the otolithic organs, which sense the head’s linear acceleration.
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