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.