Like a string of Christmas lights, the brain’s decision-making areas are wired in series, with damage early in the process shutting down anything further up the line, scientists have discovered.
The finding is the latest in a long line of research trying to unravel the structure of the prefrontal cortex, which houses the brain’s most complex and intricate thought processes. According to the new research, as you move toward the eyes from the tail end of the prefrontal cortex, which abuts the motor cortex in the middle of the brain, each successive decision-making region analyzes and integrates concepts at an increasingly abstract level.
Because of this sequential progression, any brain damage in this region disrupts only the ability to make decisions at higher levels of abstraction, and not vice versa, the researchers write in a March 1 article in Nature Neuroscience.
“This is a unidirectional flow,” says lead researcher David Badre, an assistant professor of cognitive and linguistic sciences at Brown University.
Scientists have long known that the prefrontal cortex appears to activate in this back-to-front sequence, Badre says, but until now had disagreed about what such data implied about the decision-making structure of the brain.
Badre and colleagues from Brown University and the University of California, Berkeley, studied 12 people who had brain lesions as a result of stroke, as well as a group of 24 similar but healthy volunteers.
The scientists administered tests that required the participants to successfully select objects from a group. Each task required analyzing an additional level, or “order” of information—progressing, for example, from color alone to color and texture to color, texture and orientation.
“The task becomes more complex,” Badre says. “It requires larger and larger generalizations over more-abstract information.”
The results were consistent: People who failed an early task continued to underperform in all the tasks of greater complexity.
The scientists also scanned their subjects using functional magnetic resonance imaging to look at how the position of the lesions affected results. The scans were surprisingly precise. Badre and his team were able to match up two neighboring regions of the brain with the results from two steps of their testing tree, clear evidence, he says, of a hierarchical organization.
Badre adds that his results echo other studies that look at look at higher, more complex reasoning processes. With that in mind, he is now looking to perform similar analyses on people with learning and memorization difficulties, suspecting that those tasks also may fall into sequential structures.
Such research is unlikely to have many practical applications, though, he warns. While these results may give us “a more refined sense of how to treat people with brain damage clinically,” he says, “this is a basic science finding, offering insights into how you think and reason.”
This asymmetry in how the brain processes abstraction makes intuitive sense, says Matthew Botvinick, an assistant professor of psychology at Princeton University not involved in the research. By tying such thought patterns to specific regions of the brain, the new study is quite convincing, he says.
In some ways, the findings echo the visual system, Botvinick says, in which the brain pulls out progressively more complex patterns from the raw sight information relayed by the eyes. Though the functional implications are different, taken together the two systems offer a computational model that may also underlie many other brain functions.
Still, he says, the work is only one piece of an enormously complex puzzle about how the brain conducts its most sophisticated operations.
“This group is doing very important work as part of a larger number of groups looking at the way the prefrontal cortex is organized,” Botvinick