A new study, conducted by Corentin Massot, a Postdoctoral in the Department of Physiology, and Adam Schneider a Ph.D. student in the Department of Physics, has developed a new understanding of how the brain processes information from the inner ear that offers hope for those suffering from vertigo. People who suffer from symptoms of vestibular dysfunction, such as vertigo and dizziness, encounter many challenges. If you have ever gazed over the edge of a cliff and felt dizzy, you understand their difficulties.
Over 70 million people in North America suffer from this condition. People with vestibular loss have a difficult time performing necessary daily living activities (like eating, dressing, getting in and out of bed, moving around the house and moving outside) because even turning their head slightly can make them dizzy and give them a risk of falling.
It has been known that a sensory system in the inner ear, known as the vestibular system, helps us keep our balance by keeping the visual field stable as we move around. Scientists have already developed basic knowledge of how the brain forms our perceptions of ourselves in motion. But until now no one has understood the most important step by which the neurons in the brain select the information needed to keep us balanced.
The information taken in and decoded by the brain, sent by neurons in the inner ear, is done so in a complex way. The peripheral vestibular sensory neurons in the inner ear take in the time varying velocity and acceleration stimuli caused by our movement in the outside world (for example, riding in a car that changes from a stationary position to 50 km per hour). Detailed information about these stimuli (information that helps reconstruct how stimuli change over time), in the form of nerve impulses, is transmitted by these neurons.
It was previously believed that the brain decoded this information linearly, attempting to reconstruct the time sequence of acceleration and velocity stimuli. However, two professors in McGill University's Department of Physiology, Kathleen Cullen and Maurice Chacron, combined electrophysiological and computational approaches and were able to show that neurons, in the vestibular nuclei in the brain, decode incoming information nonlinearly as they respond to sudden and unpredicted changes in stimuli.
At each stage in this sensory pathway, our representations in the outside world change. For example, neurons found in the visual system closer to the periphery of the sensory system (such as ganglion cells in the retina) usually respond to a wide variety of sensory stimuli (a "dense" code), unlike central neurons (primary visual cortex at the back of the head) that usually respond much more selectively (a "sparse" code). The selective transmission of vestibular information, which Chacron and Cullen documented for the first time, happens as early as the first synapse in the brain.
"We were able to show that the brain has developed this very sophisticated computational strategy to represent sudden changes in movement in order to generate quick accurate responses and maintain balance. I keep describing it as elegant, because that's really how it strikes me."
Since this kind of selectivity in response enhances the brain's perception of unexpected changes in body posture, it is important for everyday life. For example, if you step off a curb you didn't see, within milliseconds, your brain is able to receive the necessary information and perform the sophisticated computation essential to helping you readjust your position.
The researchers hope this discovery will apply to other sensory systems and eventually to the development of better treatments for patients suffering from dizziness, vertigo, and disorientation during their everyday activities. This finding also has the potential to lead to treatments that will help reduce the symptoms that come with motion and/or space sickness that take place in more challenging environments.
Written by Sarah Glynn