This tiny bit of the brain could offer clues about addiction
It's responsible for getting you to stop doing things.
Neuroscientists know a lot about what happens in the brain when someone decides to do something, like reach for a cookie on the office snack table. What they’re less certain about, though, is what happens when someone starts to do something, and then quickly decides to stop—like starting to reach for a cookie, seeing that there’s a spider sitting on it, and pulling your hand away.
New research published this week in the journal Neuron points to a small area of the brain, called the right ventrolateral prefrontal cortex (rVLPFC), as the region responsible for taking in contextual information (like the spider) and using it to update the original plan. “It’s not directly involved. It’s more monitoring the intent to stop, and not actually doing the stopping,” says Kitty Xu, who conducted the research during her doctoral studies at Johns Hopkins. Xu is now a user-experience researcher at Pinterest.
Pinning down the way the brain handles inhibition, and how it shuts down certain behavior, is important, says Joshua Brown, a professor of cognitive science at Indiana University. “It has a lot of practical implications for issues like ADHD, or addiction.”
The role of the rVLPFC in inhibition is a bit controversial among neuroscientists, who sometimes disagree about the degree of control the region has over the task of stopping a planned activity. Some think it actually carries out the decision making, while others think it’s a few steps removed from the process, Xu says. The results of this study, she says, add more evidence for the second hypothesis.
To pinpoint the exact mechanism involved, Xu and the rest of the team gave the 21 human participants a test, called a stop signal task, while they were in a fMRI machine. “It involved a cancellation of a movement that has already been made,” Xu says. In this case, the movement was eye movement: the participants were shown a signal, either to the left or the right, and they had to move their eyes to look at the target as fast as possible. After they initiated the movement, they were shown either a blue or yellow circle—one which meant stop, and one that meant go—and had to either continue or cancel their eye movement.
To make the task more complicated, the participants had to keep track of two different sets of rules. At the start of each round, they were shown either a square or a triangle, which told them which rule that round would follow. For one, blue meant to continue to make the movement, and yellow means stop. In the other, that was reversed.
Including the rule—about whether blue or yellow meant ‘go’ in that particular round—helped clarify that the rVLPFC was handling that contextual information. “In many other studies on inhibition, the task is to do something or not,” Xu says. “But when you really think about decision making, it’s more complicated than that.” To make the decision, she says, the brain has to see a signal, realize what it means, and process that information in context. “Our study was designed to address this layer of complication.”
The team took the additional step of tracking the effects of the same tasks on the brain of a monkey. Scientists are able to measure the response from individual cells of the monkey brain during a task, while in humans, they’re only able to see larger areas that come through on an MRI scan. “We have both monkey data and the human data on the task, so there’s converging evidence,” Xu says. “You’re looking at both the brain imaging and the direct neuronal response.”
After teaching the monkey to perform the eye tracking task, Xu says it ended up performing better on the task than the humans. But that’s probably just because the monkey had a lot more practice.
The results of this study are consistent with other research showing that this particular brain region handles contextual information, Brown says. “The VLPFC represents the state of the world, and that includes things like what the current rule is that determines what you can and can’t do.”
For Brown, the combined evidence from human brain scans and individual neurons back up the study’s findings. “The evidence lines up pretty well,” he says.
In clinical studies, Xu says, people struggling with addiction have more difficulty executing the type of stop signal task used in this study. Although the research didn’t address addiction directly, Xu says it raises questions about the nature of addictive behavior. When studying a drug problem, she says, it’s not just about the ability of someone to control their movement towards a drug or other substance. “It’s the early evaluation process,” Xu says, “from the moment you realize there’s this attraction to a drug, to how am I going to make or fail to make a decision.”
While it can take a long time for basic neuroscientific findings to translate to clinical treatments, Brown says understanding the way the brain functions is the critical first step. “If we understand what regions are involved in inhibiting behavior, maybe down the road we’ll figure out a way to manipulate them,” he says. “We have a better shot at hitting the target if we know what it is.”