Discovery of brain circuit associated with delayed gratification may boost research into obsessive compulsive disorder


The discovery in monkeys of a specific brain circuit associated with delayed gratification may have implications for work with conditions involving motivation and reward expectation such as obsessive compulsive disorder, according to an article in the May 30th issue of Science. The American researchers at the National Institute of Mental Health identified a signal in the anterior cingulate region that became more intense as monkeys neared completion of a task rewarded with a juice treat.

Munetaka Shidara, Ph.D, and Barry Richmond, M.D., trained monkeys to release a lever when they saw a spot on a computer screen turn 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. The brain signal boost occurred when each monkey worked harder and more accurately as the reward neared. Originating in the anterior cingulate region of the brain, the signal is thought to sustain the goal-driven behavior and to shut off when the reward is assured.

The researchers propose that signal alterations may underlie abnormal activity detected in the same brain area, the anterior cingulate cortex, in patients with disorders of motivation and reward expectation such as obsessive compulsive disorder.

"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 one third of more than 100 neurons 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.

In a disturbance of motivation such as obsessive compulsive disorder, Richmond speculates that the brain may be misled 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 repeat the behavior takes over.





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