Scientists have discovered a brain signal in monkeys that seems to act like a cheerleader, motivating them to keep working on a task in anticipation of a reward.

Researchers found the electrical activity signal in reward experiments with monkeys. The animals pressed a bar in response to seeing certain coloured shapes on a computer screen.

A coloured bar at the top of the screen brightened each time the monkey moved closer to the reward, which was a few drops of juice.

Brain's motivation signals may be overly active in obsessive-compulsive disorder or drug addiction

The researchers found a brain signal was boosted as the monkeys worked harder, nearing the reward. They also made progressively fewer mistakes as they approached the juice finale.

But when the brightness of the bar changed randomly, the monkeys made few errors throughout. The researchers say this suggests the reward made the monkeys adjust their motivation level.

Munetaka Shidara, of the National Institute of Advanced Industrial Science and Technology in Ibaraki, Japan, and Barry Richmond of the National Institute of Mental Health Laboratory of Neuropsychology in the United States conducted the study, which appears in Friday's issue of the journal Science.

Richmond said understanding the reward is a "sure thing" may be more important than actually receiving it.

"Imagine you're in a tall building, waiting for the elevator, but it doesn't come," said Richmond. "Your anxiety increases, but then you hear a bell ding on the next floor, and you feel better, because you're sure what's going to happen next. The signal we saw is like that."

The researchers speculate that the brain's reward expectancy circuitry may be hijacked in conditions such as obsessive-compulsive disorder or drug addiction.