Friday, July 27, 2012
If you have ever looked over the edge of a cliff and felt dizzy, you understand the challenges faced by people who suffer from symptoms of vestibular dysfunction such as vertigo and dizziness. There are over 70 million of them in North America. For people with vestibular loss, performing basic daily living activities that we take for granted (e.g. dressing, eating, getting in and out of bed, getting around inside as well as outside the home) becomes difficult since even small head movements are accompanied by dizziness and the risk of falling. We've known for a while that a sensory system in the inner ear (the vestibular system) is responsible for helping us keep our balance by giving us a stable visual field as we move around. And while researchers have already developed a basic understanding of how the brain constructs our perceptions of ourselves in motion, until now no one has understood the crucial step by which the neurons in the brain select the information needed to keep us in balance. The way that the brain takes in and decodes information sent by neurons in the inner ear is complex. The peripheral vestibular sensory neurons in the inner ear take in the time varying acceleration and velocity stimuli caused by our movement in the outside world (such as those experienced while riding in a car that moves from a stationary position to 50 km per hour). These neurons transmit detailed information about these stimuli to the brain (i.e. information that allows one to reconstruct how these stimuli vary over time) in the form of nerve impulses. Scientists had previously believed that the brain decoded this information linearly and therefore actually attempted to reconstruct the time course of velocity and acceleration stimuli. But by combining electrophysiological and computational approaches, Kathleen Cullen and Maurice Chacron, two professors in McGill University's Department of Physiology, have been able to show for the first time that the neurons in the vestibular nuclei in the brain instead decode incoming information nonlinearly as they respond preferentially to unexpected, sudden changes in stimuli.