Specialized cells allow brain's navigation systems to keep us on our feet
(NC&T/WUSTL) To successfully orient yourself and move about the environment, you have to look at the world both from the viewpoint of your own sensory organs, which are fixed in your head and body, and from the viewpoint of your relationship to the space around you and to the force of gravity.
Now researchers at Washington University School of Medicine in St. Louis have identified the brain cells that mathematically massage sensory data to allow us to perform this tricky perspective shift. Using an animal model, they directly recorded Purkinje cells in the cerebellar cortex changing sensory data to transform it to a point of view that encompasses the broader world around us.
"If we couldn't consider this non-egocentric point of view, we'd be clueless as to how to approach or avoid objects in our environment," says study coauthor J. David Dickman, Ph.D., professor of neurobiology. "We would, for example, be unable to separate whether we are approaching a train, or a train is approaching us. Obviously it is to our advantage to be able to tell the difference."
Loss of orientation is associated with several disorders, including brain damage from stroke, cancer and Alzheimer's disease. By studying how the healthy brain handles the challenges of orientation and navigation, scientists hope to lay the groundwork that will one day permit better understanding of how those systems go awry and what medical science can do about it.
The paper appears in Neuron.
Senior author Dora Angelaki, Ph.D., Alumni Endowed Professor of Neurobiology, notes that the brain unconsciously performs many complex mathematical calculations. Confronting some of those same math problems with our conscious brain would leave many of us stumped, but the subconscious brain easily handles them on a daily basis to enable such basic tasks as walking across a room or lying down in bed.
"We don't appreciate that the brain's navigation system does all of this math, but we know right away when the system stops working," Angelaki says. "We get disoriented, we get motion sickness and we know very quickly that something is wrong."
In a primate model, Angelaki's lab studies how the brain uses sensory inputs from the vestibular system in the inner ear to establish balance and orientation. Using implanted electrodes in the brain, they have tracked these signals through the brainstem at the base of the brain and into the cerebellum, a region just above the brain stem that handles sensory inputs and motor controls.
As researchers tracked the signals' paths, they looked for signs that cells were transforming the data from a self-centered point of view to the broader perspective. In various regions, they saw signs that some brain cells were doing the calculations. But they had never previously identified a region where the data was uniformly transformed.
For the new study, Angelaki and her colleagues looked at Purkinje cells, a special group responsible for conveying all output signals from the cerebellum to the rest of the brain. They found the cells uniformly and "elegantly" transformed the sensory data to put it in the context of the broader point of view.
For follow-up, scientists plan to study the Purkinje cells and their connections in the cerebellum in closer detail. They hope to learn more about how the brain circuits that handle these transformations are engineered.
"Another way we'd like to continue our studies is to look at how visual stimuli affect the processing of these signals," Angelaki says. "We've been doing our tests in the dark to isolate the signal from the vestibular organs, but a more natural way to do it will be to include visual stimuli as well."
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