The neural circuit connecting brain areas associated with risk and reward can make it difficult to start a difficult task. Credit: Bloom Creative/Getty
Sometimes the hardest part of tackling an unpleasant task is simply getting started—writing the first word of a long report, lifting a dirty dish over a full sink, or removing clothes from an unused exercise machine. The obstacle is not necessarily a lack of interest in completing the task, but rather the brain’s resistance to taking the first step.
Now, scientists may have identified the neural circuit behind this resistance, and a way to mitigate it. In a study published today in Cell reportsResearchers describe a pathway in the brain that appears to act as a “motivational brake,” dampening the motivation to start a task1. When the team selectively suppressed this circuit in macaques, goal-directed behavior rebounded.
“The change after this modification was dramatic,” says Ken-Ichi Amemori, a neuroscientist at Kyoto University and co-author of the study.
Motivational suppression, which can be particularly stubborn for people with certain psychiatric conditions such as schizophrenia and major depressive disorder, is different from risk-aversion-driven task avoidance in anxiety disorders.
Understanding this difference is essential to developing new treatments and improving existing ones, says Pearl Chiu, a computational psychologist at Virginia Tech in Roanoke, who was not involved in the study. “Being able to regain motivation is especially exciting,” she says.
Stimulated macaques
Previous research on task initiation has suggested the existence of a neural circuit linking two parts of the brain known as the ventral striatum and the ventral pallidum, both of which are involved in stimulus and reward processing.2,3,4. But attempts to isolate the department’s role failed. For example, electrical stimulation inadvertently activates the lower regions, affecting motivation, but also affecting anxiety.
In the new study, Amemori and his team used a more precise approach. They first trained two male macaques to perform two decision-making tasks. In one, completion earned a water reward; In the other condition, the reward was paired with an unpleasant puff of air to the face. Each trial required the monkeys to begin the task by fixing their gaze on a central point on the screen until the reward and punishment displays appeared. This allowed the researchers to measure motivation by the number of times the monkeys failed to start.
Not surprisingly, the monkeys were more hesitant when the possibility of punishment loomed. But this changed when the team used a targeted genetic technique to suppress signals from the ventral striatum to the ventral pallidum. While suppression had little effect on the monkeys’ behavior during reward-only trials, it did make them more willing to begin experiencing unpleasant outcomes. However, repression did not change how the animals weighed reward versus punishment.
The team has effectively disabled the incentive brake. The study’s behavioral data and electrophysiological recordings suggest that the ventral striatum detects aversive situations and suppresses ventral pallidum activity, making the animals less likely to act. “The ventral pallidum can be a focus of motivation deficits or apathy in depression,” Amemori says.