Electrically stimulating the vestibular nerve can influence human movement – and may create better virtual-reality devices and prosthetics.
It’s a disturbing thought: being able to remotely control the way a person moves at the push of a button. But scientists have already managed to do just that – although not with the same repertoire of complex movements as, say, a practiced nine-year-old controlling a toy race car.
Scientists in Australia, Japan, and the United States are trying to develop more refined ways of stimulating the brain’s balance organ – not just to influence movement, but also to create more realistic virtual reality simulations, as well as medical prosthetics to help people with balance disorders.
The devices work by stimulating the vestibular system – a set of tiny structures just behind the ear that keep the head upright and make the visual world appear steady, even when a person is walking and looking around. Three fluid-filled canals, known as the circular canals, sense rotation of the head, while another structure, the otoliths, sense the direction of gravity. Signals from the vestibular nerve travel to the brain; for example, a greater frequency of signals from one ear signals that the head is moving in that direction.
Scientists can stimulate the vestibular system with a small jolt of electricity delivered just behind the ear from a small external device, sending the normal vestibular signals out of whack. Last summer, Japanese scientists from Nippon Telegraph and Telephone Communication Science Laboratories demonstrated such a device at a technical conference in Los Angeles. Volunteers put on an odd-looking set of headphones and a blindfold, while someone else pushed buttons on the remote controller, making the blindfolded subject weave awkwardly around the room.
But the device doesn’t work as well as a remote-controlled car. A quick jolt to one side of the head makes people feel like they’re falling over, so they correct their balance by moving to one side or the other, creating a swaying type of walk. And the person must be blindfolded for the device to work, otherwise visual signals will correct for the apparent mismatch in head position. “It makes you feel like you’re moving in a certain direction, but it’s not really that specific,” says Steven Moore, a scientist at the Mount Sinai School of Medicine, who studies this stimulation, known as galvanic vestibular stimulation. “That’s the big limiting factor.” (Click here for a video of Moore walking with the device.)
In the newest incarnation of the technique, scientists at the University of New South Wales in Australia found a relatively simple solution to the steering problem. They administered the vestibular stimulations while the volunteers turned their faces toward either the ground or sky. For reasons not entirely understood, this made them pivot cleanly to the left or right when stimulated, without the characteristic dizziness. As evidence of this adept level of control, researchers steered blindfolded volunteers through the Sydney Public Gardens.
But don’t expect remote-controlled humans any time soon. “This is not very practical, due to the lack of specificity of [electrical] input and the fact that stimulation is overridden by visual input,” says Moore. Instead, scientists are developing devices to try to make virtual reality environments seems more realistic or to help people with vestibular disorders.
Moore is developing a device to create better flight simulators for astronauts. A large percentage of space shuttle landings come in faster than the target speed, he says, possibly because pilots’ sensory systems have adapted to the gravity-free environment of space. “Current training simulations are too easy for these pilots, so we use [electrical stimulation] to disrupt normal vestibular function and produce illusory sensations of motion,” he says. That stimulation mimics the sensation of landing the shuttle, when pilots undergo the radical shift from zero gravity to hypergravity.
Devices applying vestibular stimulation could also be used to help people with disorders of the vestibular system, which can be caused by damage during surgery to remove brain tumors or certain types of drugs, brain injury, or other causes. “People with vestibular deficits often have one ear that functions better than the other,” says Timothy E. Hullar, an otolaryngologist at the Washington University School of Medicine in St. Louis. “It would be helpful to have a way to balance them – like trying to balance speakers on a stereo.”
Other scientists are developing implantable devices that can stimulate the vestibular nerves more specifically than non-invasive galvanic stimulation. “When you put pads on the skin, you can create the perception that the head is tilting, but that’s about as specific as you can get,” says Charley Della Santina, a vestibular expert at the John’s Hopkins School of Medicine. But by implanting electrodes behind the ear, researchers can stimulate individual nerves and therefore send more accurate signals to the brain. “We want to create a vestibular implant that works like a cochlear implant to replace the missing vestibular information,” says Daniel Merfeld, a scientist at the Massachusetts Eye and Ear Infirmary in Boston, who is also developing such a device.
So far, both Santina and Merfeld have tested their implants only in animals, but they say a human-testable version could be developed within the next five to ten years.