MCGILL (CAN) — By responding to sudden movements first, the brain allows the body to readjust quickly and stay in balance.
Scientists have 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.
For over 70 million people in North America with symptoms of vestibular dysfunction, even subtle head movements from everyday movements like getting dressed and moving around cause vertigo and dizziness.
Although researchers have a basic understanding of how the brain constructs our perceptions of ourselves in motion, McGill University researchers are the first to identify the crucial step by which the neurons in the brain select the information needed to keep us in balance.
The peripheral vestibular sensory neurons in the inner ear take in the stimuli caused by our movements, such as riding in a car that goes from a standstill to 50 km per hour.
These neurons transmit detailed information about these stimuli to the brain—information that allows a person 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 the 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.
Representations of the outside world change at each stage in this sensory pathway. For example, in the visual system neurons located closer to the periphery of the sensory system like ganglion cells in the retina tend to respond to a wide range of sensory stimuli, but central neurons like those at the back of the head tend to respond much more selectively.
As reported in PLoS Biology, Chacron and Cullen have discovered that the selective transmission of vestibular information that they were able to document occurs 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,” explains Cullen.
“I keep describing it as elegant, because that’s really how it strikes me.”
This kind of selectivity in response is important for everyday life, since it enhances the brain’s perception of sudden changes in body posture.
The brain’s nonlinear response is the reason that you can step off an unseen curb, and within milliseconds your brain has both received the essential information and performed the sophisticated computation needed to help you readjust to stay balanced.
This discovery is expected to apply to other sensory systems and eventually to the development of better treatments for patients who suffer from vertigo, dizziness, and disorientation during their daily activities.
The findings should also lead to treatments that will help alleviate the symptoms that accompany motion and/or space sickness produced in more challenging environments.
The research was conducted by Corentin Massot, a postdoctoral fellow in the department of physiology, and Adam Schneider a Ph.D. student in the department of physics.
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