Encouraging words from a cheerleader can convince someone to keep at an arduous task in order to reap benefits once the chore is completed. Now the results of a study published today in the journal Science suggest that a part of the brain might work in a similar way. According to the report, scientists have discovered a brain signal in monkeys that increases as an expected reward nears. The findings could help unravel how the compulsion characteristic of addiction takes shape in the brain.

Munetaka Shidara of the National Institute of Advanced Industrial Science and Technology in Japan and Barry J. Richmond of the National Institute of Mental Health monitored monkeys while the animals performed a series of tasks in order to receive a reward. The animals learned to release a lever when a spot on a computer screen turned from red to green. After a number of successful attempts, they received their prize. In some of the trials, a bar on the screen tracked their progress and got brighter as the animals approached the reward. As the monkeys performed the tasks, the scientists investigated the activity of more than 100 neurons located in the anterior cingulate cortex, a region of the brain involved in performance and conflict monitoring, error detection, and response selection.

The activity of about one third of the brain cells, the team found, varied with the level of expectation of a reward--the closer the reward seemed, the more they fired. The animals also made the fewest errors in the task just prior to receiving their reward. When the possibility of receiving a reward was randomized, however, these neurons did not display increasing activity. Of particular note is the fact that the signal actually died down just before the prize was reached. "Understanding the reward is a 'sure thing' may be more important than actually receiving it," Richmond says.

This cheerleading brain signal could form a biological basis for goal-driven behavior. "Understanding how this signal works normally, as well as when its activity is abnormally high or low, may shed light on why individuals seem to have different levels of motivation for performing similar tasks," Shidara notes. Moreover, the researchers propose that misfiring of this reward expectancy circuit could be involved in some pathological behaviors. "Let's assume you had a signal like this, and it was turned up too high," Richmond explains. "It might make you feel like you had to be trying harder, telling you, 'Just go a little further and you'll get satisfied.' Our speculation is that this signal never resolves for conditions like [obsessive-compulsive disorder]."