Brain signal boosts as monkey nears reward

May 30, 2002

Delaying gratification while working toward a goal appears to have roots in a specific brain circuit. NIMH scientists have discovered a signal in a brain area involved in motivation that strengthens as a monkey performs a task for which it has been trained to expect a reward.

Munetaka Shidara, Ph.D, and Barry Richmond, M.D., NIMH Laboratory of Neuropsychology, trained monkeys to release a lever when a spot on a computer screen turned from red to green. The animals knew they had performed the task correctly when the spot turned blue. A visual cue -- a gray bar on the screen -- got brighter as they progressed through a succession of trials required to get a juice treat. Though never punished, the monkeys couldn't graduate to the next level until they had successfully completed the current trial.

The brain signal boost occurred as the monkey worked harder and more accurately as the reward neared. Emanating from a reward-anticipating circuit in the front top center of the brain, the signal is thought to sustain the goal-driven behavior and then shuts off when the reward is assured. Signal alterations may underlie abnormal activity detected in the brain area, the anterior cingulate cortex, in disorders of motivation and reward expectation, such as obsessive compulsive disorder (OCD), propose the researchers. They report on their findings in the May 30 Science.

"If you're working toward a distant goal, you must often keep working even if you don't like what you are doing very much," explained Richmond. "It makes sense that there is such a signal that varies with degree of reward expectancy that keeps you on-task performing a long sequence of behaviors. What we're studying in a rather cartoon-like way with this task are the dynamics of this situation: the ongoing tension between the desirability of reaching the goal and the hard work needed to achieve it."

As the monkeys approached their reward, they made progressively fewer errors, with the fewest occurring during the rewarded trial. Electrical activity of about a third of more than 100 neurons, brain cells, monitored in the anterior cingulate progressively increased with reward expectancy. The progressive activity abated only when the reward was imminent and the expectancy resolved.

The researchers saw no such progressive activity when they switched the animals to a random condition in which the cues were no longer related to the rewards. In this condition, the monkeys performed the task well regardless of cue brightness, but showed little activation of the anticipatory anterior cingulate circuitry. "There is a substantial behavioral difference between knowing for certain what will happen in each successfully completed trial (cued condition) versus knowing the overall reward rate without knowing the outcome of each trial for certain (random condition)," they note. The monkeys performed poorly when they had no expectation of reward.

Feelings of increasing anticipation experienced as we work in stages toward a predicted outcome may be traceable to the reward expectancy signal, propose Richmond and Shidara, who now works at the Neuroscience Research Institute of Japan's National Institute of Advanced Industrial Science and Technology.

In a disturbance of motivation, such as OCD, Richmond speculates that the brain may be hijacked by runaway signals in this reward expectancy circuit. The individual performs the behavior that would normally alleviate the sense of expectancy, but the signal somehow fails to turn off. There is no feeling of completion, the tension remains unresolved, and a compulsion to keep repeating the behavior takes over. Brain imaging studies have detected abnormal activation in the anterior cingulate in patients with OCD.
The National Institute of Mental Health (NIMH) is part of the National Institutes of Health (NIH), the Federal Government's primary agency for biomedical and behavioral research. NIH is a component of the U.S. Department of Health and Human Services.

NIH/National Institute of Mental Health

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