The basal ganglia, structures deep in the forebrain already known to control voluntary movements, also may play a critical role in how people form habits, both bad and good, and in influencing mood and feelings.
"This system is not just a motor system," says Ann Graybiel. "We think it also strongly affects the emotional part of the brain."
Graybiel, an investigator at the McGovern Institute of the Massachusetts Institute of Technology and professor in MIT’s department of brain and cognitive sciences, believes that a core function of the basal ganglia is to help humans develop habits that eventually become automatic, including habits of thought and emotion.
"Many everyday movements become habitual through repetition, but we also develop habits of thought and emotion," she says."If cognitive and emotional habits are also controlled by the basal ganglia, this may explain why damage to these structures can lead not only to movement disorders, but also to repetitive and intrusive thoughts, emotions and desires."
Graybiel’s research focuses on the brain’s relationship to habits—how we make or break them—and the neurobiology of the habit system. She and her team have identified and traced neural loops that run from the outer layer of the brain—“the thinking cap,” as she calls it—to a region called the striatum, which is part of the basal ganglia, and back again. These loops, in fact, connect sensory signals to habitual behaviors.
Her work ultimately could have an impact not just on such classic movement disorders as Parkinson’s and Huntington’s diseases, but in other conditions where repetitive movements commonly occur, such as Tourette Syndrome, autism, or obsessive-compulsive disorder, the latter when sufferers experience unwanted and repeated thoughts, feelings, ideas, sensations or behaviors that make them feel driven to do something, for example, repeatedly washing their hands.
Moreover, the research could have an immediate value for trying to understand “what happens in the brain as addiction occurs, as bad habits form, not just good habits,” she says. “There are many psychiatric and neurologic conditions in which these same brain regions are disordered.
"These conditions may in part be influenced by the very system we are working on," Graybiel adds. "We are working with models of anxiety and depression, stress and some of these movement disorders."
It turns out that the emotional circuits of the brain have strong ties to the striatum, she says. Graybiel’s research suggests that activity in the striatum strongly affects the emotional decisions that people make: whether to accept a good outcome or a potentially bad one, for example, and that there are circuits favoring good outcomes, and, surprisingly, other circuits that favor bad ones.
"This work ties into new research suggesting that there are brain systems for ‘good’ and brain systems for ‘bad,’" she says. "What is intriguing is that we may have identified the circuits that decide between the two."