Primitive structures deep within the brain may have a far greater role in our high-level everyday thinking processes than previously believed, report researchers at the MIT Picower Center for Learning and Memory in the Feb. 24 issue of Nature.
The results of this study led by Earl K. Miller, associate director of the Picower Center at MIT, have implications about how we learn. The new knowledge also may lead to better understanding and treatment for autism and schizophrenia, which could result from an imbalance between primitive and more advanced brain systems.
Our brains have evolved a fast, reliable way to learn rules such as "stop at red" and "go at green." Dogma has it that the "big boss" lobes of the cerebral cortex, responsible for daily and long-term decision-making, learn the rules first and then transfer the knowledge to the more primitive, large forebrain region known as the basal ganglia, buried under the cortex.
Although both regions are known to be involved in learning rules that become automatic enough for us to follow without much thought, no one had ever determined each one's specific role.
In this study, Miller, who is the Picower Professor of Neuroscience, and postdoctoral associate Anitha Pasupathy found that in monkeys, the striatum (the input structure of the basal ganglia) showed more rapid change in the learning process than the more highly evolved prefrontal cortex. Their results suggest that the basal ganglia first identify the rule, and then "train" the prefrontal cortex, which absorbs the lesson more slowly.
"These findings suggest new ways of thinking about learning," Miller said. "They suggest that new learning isn't simply the smarter bits of our brain such as the cortex 'figuring things out.' Instead, we should think of learning as interaction between our primitive brain structures and our more advanced cortex. In other words, primitive brain structures might be the engine driving even our most advanced high-level, intelligent learning abilities," he said.
The cortex--the "thinking" part of the brain--is highly developed in humans. This is especially true for the prefrontal cortex. Common wisdom suggests that when we learn new things, the prefrontal cortex figures things out first. Then, as our behaviors become familiar and habitual, the more primitive, subcortical basal ganglia take over so that the now-familiar routines can be run off automatically and occupy less of our thoughts.
"What we found was evidence for something very different," Pasupathy said. "We found that as monkeys learn new, simple rules--associations analogous to 'stop at red, go at green'--the striatum of the basal ganglia shows evidence of learning much sooner and faster than the prefrontal cortex. But, an interesting wrinkle is that the the monkeys' behavior improved at a slow rate, similar to that of the slower changes in prefrontal cortex."
This suggests that while the basal ganglia "learn" first, their output forces the prefrontal cortex to change, albeit at a slower rate.
The researchers speculate that perhaps the faster learning in the basal ganglia allows us (and our primitive ancestors who lacked a prefrontal cortex) to quickly pick up important information needed for survival. The prefrontal cortex then monitors what the basal ganglia have learned. Its slower, more deliberate learning mechanisms allow it to gather a more judicious "big picture" of what is going on by taking into account more history and thereby exert executive control over behavior, Miller said.
This work was supported by the National Institute of Neurological Disorders and Stroke and the Tourette's Syndrome Association.
A version of this article appeared in the March 2, 2005 issue of MIT Tech Talk (Volume 49, Number 19).
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