Apr. 21, 1999 Do you recall spinning in place as a child? Remember the feeling that you were still turning even after you'd flopped down on the grass? Researchers at the Neurological Sciences Institute at Oregon Health Sciences University now can recreate that sensation so that it can be studied scientifically. Results of a study incorporating the machine, published in the April 15 edition of the journal Nature, show the brain essentially creates its own version of reality when it receives conflicting information from different senses. The research is a step towards understanding how things go wrong with the extraordinarily complex systems that govern human balance and movement.
The scientists use a device that might not be out of place in an amusement park. A seat is mounted within two frames. The rectangular outer frame tilts backward and forward. The inner frame is mounted to the outer frame so that the seat can be spun around rapidly in addition to being tilted. The research method is straightforward, according to Robert Peterka, Ph.D., associate scientist at the Neurological Sciences Institute and co-author of the article.
"We spin a person for a couple of minutes in pitch darkness, then stop them suddenly. This creates a false sense of rotation. Then we quickly tilt them into a given orientation," said Peterka.
The results are, at least according to some study participants, "confusing, unpleasant and difficult to describe". Subjects who were tilted into a position with their back toward the ground reported they felt like they were lying on their side. And even though they had stopped moving, they felt like they were still rotating and their eyes continued to move as though they were still spinning.
These stomach-wrenching experiments are revealing important information about the delicate system of fluids and nerves that make up what's called the vestibulo-ocular reflex. This system is at work when you rotate your head or move from side to side while looking at someone or something. As your head moves, your eyes move in their sockets to remain fixed on their focal point. These seemingly automatic eye movements are regulated by two types of sensors in the inner ear. The semi-circular canals sense rotation and the otolith organs sense gravity and linear acceleration.
The spinning and tilting create conflicting signals between the semi-circular canals and the otolith organs. "Once the subject stops spinning, the sensation of movement continues for thirty seconds or so, just as for a kid spinning on the playground, as fluids in the inner ear regain equilibrium," said Peterka. "But the otolith organs are telling the brain the person is stationary. During this period, we use video cameras to measure the study subject's eye movements, and we found the eyes move as though the person is still both rotating and translating, even though he or she is perfectly still." The brain resolves the conflict between the senses by inventing its own reality. The researchers call it "internal modeling" essentially creating a sensation of movement that could be happening based on the sensory information.
The researchers say their work could be an important step towards understanding the causes and finding treatments for some balance disorders. "The National Institutes of Health recently reported that 50 percent of Americans will have a balance problem at some time in their lives. It's a hidden medical problem that's often misdiagnosed," said Daniel Merfeld, Ph.D., recently appointed director of the Vestibular Physiology Laboratory at the Massachusetts Eye and Ear Infirmary and researcher at Harvard Medical School. Merfeld and research associate Lionel Zupan Ph.D., also of Harvard Medical School, were members of the research team at the Neurological Sciences Institute and are co-authors of the Nature article.
Continuing research is focusing on developing clinical tests to determine if a patient's vestibular system is functioning normally. Understanding how the system works normally may be useful in treating balance disorders in the general population, but also in special situations, such as helping astronauts regain their sense of balance after a return from space.
This research also points the way towards research into other conflicts between different senses. "We're confident this kind of neural processing applies to other sensory systems, such as sight and hearing," said Merfeld.
The research was funded by the National Aeronautics and Space Administration, The National Institute on Deafness and Other Communication Disorders and the European Space Agency.
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