'Noise' Affects How Brain Directs Body To Move

Understanding where noise arises in the brain has implications foradvancing research in neuromotor control and in developing therapiesfor disorders where control is impaired, such as Parkinson's disease.

The new study was developed "to understand the brain machinerybehind such common movements as typing, walking through a doorway orjust pointing at an object," says Stephen Lisberger, PhD, senior studyinvestigator who is director of the W.M. Keck Center for IntegrativeNeuroscience at the University of California, San Francisco.

Study co-investigators are Leslie C. Osborne, PhD, apostdoctoral fellow at UCSF, and William Bialek, PhD, professor ofphysics at Princeton University.

The study findings, reported in the September 15 issue of thejournal Nature, are part of ongoing research by Lisberger andcolleagues on the neural mechanisms that allow the brain to learn andmaintain skills and behavior. These basic functions are carried outthrough the coordination of different nerve cells within the brain'sneural circuits.

"To make a movement, the brain takes the electrical activity ofmany neurons and combines them to make muscle contractions," Lisbergerexplains. "But the movements aren't always perfect. So we asked, whatgets in the way?"

The answer, he says, is "noise," which is defined as thedifference between what is actually occurring and what the brainperceives. He offers making a foul shot in basketball as an example. Ifthere were no noise in the neuromotor system, a player would be able toperform the same motion over and over and never miss a shot. But noiseprevents even the best players in the NBA from having perfectfoul-shooting percentages, he says.

"Neuroscientists are interested in what limits virtuosity. Ourfinding is significant because it demonstrates that errors in what isseen can have a bigger impact on motor performance than errors incontrolling muscles," says co-investigator Osborne, who conducted theresearch.

"By studying how the brain reduces noise, we can learn moreabout how it processes sensory inputs, makes decisions and executesthem. Understanding how noise is reduced to very precise commands helpsus understand how those commands are created," adds Lisberger, who alsois a Howard Hughes Medical Institute investigator and a UCSF professorof physiology.

In the study, the research team focused on a movement that allprimates, including humans, are very skilled at: an eye movement knownas "smooth pursuit" that allows the eyes to track a moving target.

In a series of exercises with rhesus monkeys in which theanimals would fixate on and track visual targets, the researchersmeasured neural activity and smooth pursuit eye movements. From thisdata, the team analyzed the difference between how accurately theanimals actually tracked a moving object and how accurately the brainperceived the trajectory.

Findings showed that both the smooth pursuit system and the brain's perceptual system were nearly equal.

"This teaches us that these very different neural processes arelimited to the same degree by the same noise sources," says Lisberger."And it shows that both processes are very good at reducing noise. Thedifferences that exist are likely caused by the separate parts of thebrain that are responsible for the separate processes."

He concludes, "Because the brain is noisy, our motor systemsdon't always do what it tells us to. Making precise movements in theface of this noise is a challenge. This study gives us new insightsinto how the brain works to do that."

The research was supported by grants from the National Institutes of Health and the Howard Hughes Medical Institute.