New Delhi: New findings suggest it is more complicated than scientists thought
Every day our brains grapple with various last-minute decisions. We adjust our gait to avoid a patch of ice; we exit to hit the rest stop; we switch to our backhand before thwacking a tennis ball.
Scientists have long accepted that our ability to abruptly stop or modify a planned behavior is controlled via a single region within the brain's prefrontal cortex, an area involved in planning and other higher mental functions. By studying other parts of the brain in both humans and monkeys, however, a team from Johns Hopkins University has now concluded that last-minute decision-making is a lot more complicated than previously known, involving complex neural coordination among multiple brain areas. The revelations may help scientists unravel certain aspects of addictive behaviors and understand why accidents like falls grow increasingly common as we age, according to the Johns Hopkins team.
The findings, published Thursday in Neuron, reveal reneging on an intended behavior involves coordinated cross talk between several brain regions. As a result, changing our minds even mere milliseconds after making a decision is often too late to alter a movement or behavior. Using functional magnetic resonance imaging—a technique that monitors brain activity in real time—the Johns Hopkins group found reversing a decision requires ultrafast communication between two specific zones within the prefrontal cortex and another nearby structure called the frontal eye field, an area involved in controlling eye movements and visual awareness.
Lead author Kitty Xu, formerly a Johns Hopkins graduate student and now a researcher at the social media site Pinterest, explains that when it comes to split-second decisions, the longer a decision has to take hold in the brain, the harder it is to reverse. "Stopping a planned behavior requires extremely fast choreography between several distinct areas of the brain, our research found," she says. "If we change our mind about pressing the gas pedal even a few milliseconds after the original "go" message has been sent to our muscles, we simply can't stop." Xu adds that if we change our minds within roughly 100 milliseconds of making a decision, we can successfully revise our plans. If we wait more than 200 milliseconds, however, we may be unable to make the desired change—in other words we may land a speeding ticket or a tumble down the stairs. As we age, our neural communication slows, and that likely contributes to more of these glitches, Xu says.
To identify the brain regions involved in canceling a decision, the new study recruited 21 subjects for a modified "stop signal task," a commonly used neuroscientific behavioral test that involves canceling a planned movement. Participants undergoing functional MRI were instructed to watch a screen, and to immediately stare at a black dot when it appeared. But just after they focused on the black dot a colored dot would appear, prompting their gaze to shift to the new stimulus—essentially causing them to abandon their initial plan to fix their eyes on the black dot. The researchers watched what areas of the brain lit up during those decision-making steps, and after the volunteers terminated their plan. To confirm their findings, the authors then ran the same experiment on a single macaque. Using an implanted electrode, they saw activation in monkey brain regions analogous to those reported on in humans when the monkey stopped looking at the black circle in favor of the colored dots.
Tracking these eye movements and neural action let the researchers resolve the very confusing question of what brain areas are involved in these split-second decisions, says Vanderbilt University neuroscientist Jeffrey Schall, who was not involved in the research. "By combining human functional brain imaging with nonhuman primate neurophysiology, weave together threads of research that have too long been separate strands," he says. "If we can understand how the brain stops or prevents an action, we may gain ability to enhance that stopping process to afford individuals more control over their choices."
Xu hopes these insights into how difficult it is for the brain to amend its plans—a task that only gets harder as we age and neural communication slows—can eventually help researchers devise ways to intervene and help us make faster, safer decisions. In the short term she hopes key targets will include helping seniors avoid falls and modifying last-minute impulses in people with addictions.
"The sooner I can turn off the plan to drink or use the drug," she says, "the less likely I am to carry out that plan."
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