A neural circuit connecting areas of the brain related to risk and reward can make it difficult to start a difficult task. Credit: Plume Creative/Getty
Sometimes the hardest part of an unpleasant task is just getting started: typing the first word of a long report, lifting dirty dishes over an overfilled sink, or removing clothes from an unused exercise machine. The obstacle is not necessarily a lack of interest in completing the task, but the brain’s resistance to taking the first step.
Scientists may have identified the neural circuit causing this resistance and a way to alleviate it. In a study published today in Cell ReportsResearchers describe a pathway in the brain that appears to act as a “motivation brake,” dampening the urge to begin a task.1. When the team selectively removed this circuit in macaque monkeys, goal-directed behavior rebounded.
“The change after this modulation was dramatic,” says Ken-ichi Amemori, a neuroscientist at Kyoto University and co-author of the study.
The impediment to motivation, which can be particularly stubborn in people with certain psychiatric disorders such as schizophrenia and major depressive disorder, is distinct from task avoidance motivated by risk aversion in anxiety disorders.
Pearl Chiu, a computational psychiatrist at Virginia Tech in Roanoke who was not involved in the study, says understanding this difference is key to developing new treatments and refining current ones. “Being able to find motivation again is particularly exciting,” she says.
Motivated macaques
Previous work on task initiation has involved a neural circuit connecting two parts of the brain known as the ventral striatum and ventral pallidum, both of which are involved in processing motivation and reward.2,3,4. But attempts to isolate the role of the circuit have failed. Electrical stimulation, for example, inadvertently activates downstream regions, affecting motivation, but also anxiety.
In the new study, Amemori and his team used a more precise approach. They first trained two male macaque monkeys to perform two decision-making tasks. In one, completion yielded a water reward; in the other, the reward was accompanied by an unpleasant puff of air to the face. Each trial required the monkeys to initiate the task by fixing their gaze on a central location on a screen until the reward-punishment offer appeared. This allowed the researchers to measure motivation based on how often the monkeys failed to get going.
It’s not surprising that monkeys are more hesitant when the possibility of punishment looms. But that changed when the team used a targeted genetic technique to suppress signaling from the ventral striatum to the ventral pallidum. Although suppression had little effect on the monkeys’ behavior during reward-only trials, it made them much more willing to engage in the face of a potentially unpleasant outcome. The suppression, however, did not change the way the animals weighed reward and punishment.
The team had effectively deactivated the brake on motivation. The study’s behavioral data and electrophysiological recordings suggest that the ventral striatum detects aversive conditions and suppresses activity in the ventral pallidum, making the animals less likely to act. “The ventral pallidum could be the center of motivation deficit or apathy in depression,” explains Amemori.
