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.