A neural circuit connecting brain areas related to risk and reward can makes it hard to start a difficult task. Credit: Plume Creative/Getty
Sometimes the hardest part of doing an unpleasant task is simply getting started – typing the first word of a long report, lifting the dirty dish atop an overfilled sink, or removing the clothes from an unused exercise machine. The obstacle isn’t necessarily a lack of interest in completing the task, but the brain’s resistance to taking the first step.
Now, scientists may have identified the neural circuit behind this resistance, and a way to ease it. In a study published today in Cell Reports, researchers describe a pathway in the brain that seems to act as a ‘motivation brake’, dampening the drive to begin a task1. When the team selectively suppressed this circuit in macaque monkeys, goal-directed behaviour rebounded.
“The change after this modulation was dramatic,” says Ken-ichi Amemori, a neuroscientist at Kyoto University and a coauthor on the study.
The motivation brake, which can be particularly stubborn for people with certain psychiatric conditions such as schizophrenia and major depressive disorder, is distinct from the avoidance of tasks driven by risk aversion in anxiety disorders.
Pearl Chiu, a computational psychiatrist at Virginia Tech, in Roanoke, who was not involved in the study, says that understanding this difference is essential for developing new treatments and refining current ones. “Being able to restore motivation, that’s especially exciting,” she says.
Motivated macaques
Previous work on task initiation has implicated 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 reward2,3,4. But attempts to isolate the circuit’s role have fallen short. 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 earned a water reward; in the other, the reward was paired with an unpleasant puff of air to the face. Each trial required the monkeys to initiate the task by fixing their gaze on a central spot on a screen until the reward-punishment offer appeared. This allowed the researchers to measure motivation by how often the monkeys failed to begin.
Not surprisingly, monkeys were more hesitant when the possibility of punishment loomed. But that changed when the team used a targeted genetic technique to suppress signalling from the ventral striatum to the ventral pallidum. Although the suppression had little effect on the monkeys’ behaviour during the reward-only trials, it made them significantly more willing to start in the face of a potentially unpleasant outcome. The suppression did not, however, alter how the animals weighed reward against punishment.
The team had effectively disabled the motivation brake. The study’s behavioural data and electrophysiological recordings suggest that the ventral striatum detects aversive conditions and suppresses ventral pallidum activity, which made the animals less likely to act. “The ventral pallidum could be the centre for motivation deficit or apathy in depression,” says Amemori.
