Ever wondered why some people are full of “get up and go” and why others drag their heels? Why some kids at school charge enthusiastically around the running track, while others prefer to go for a smoke behind the bike sheds? If work published in Science this week fulfills its promise, there might soon be an answer.

Much of what we know about emotions and feelings — where they come from and how they are processed — comes from observations of people with damaged brains. In the front part of the brain is an area that University of Glasgow scientist Graham Cairns-Smith calls the “let’s just do it” area. People who suffer damage to this area, called the anterior cingulate, lose initiative and the desire to say or do anything.

Motivation also goes down. People may feel unmotivated after being unemployed for months or years or if stuck in a dead-end McJob — but it’s far worse in stroke patients who have suffered damage to the anterior cingulate. Such people are sometimes able to answer questions but do not talk unless spoken to. On partial recovery, they report that they had lost the will to act — the “let’s just do it” area of the brain had apparently shut down. Even flipping burgers all day leaves you with more motivation that that.

Now two scientists, Munetaka Shidara of the Neuroscience Research Institute in Tsukuba, Ibaraki Prefecture, and Barry Richmond of the Laboratory of Neuropsychology in Bethesda, Md., have discovered a brain signal that might provide the biological basis for motivation. Further research in this area may also help explain what’s behind brain disorders and addiction.

For now, however, given that the number of neurons in a human brain is about the same as the number of stars in our galaxy — tens of billions — understanding how motivation and reward are signaled in the brain is a good start.

The scientists were not looking at signals in a human brain, but in one quite similar. Shidara and Richmond monitored the neural activity in monkeys performing a task for which they have been trained to expect a reward.

“Understanding how this signal works normally, as well as when its activity is abnormally high or low,” said Shidara, “may shed light on why individuals seem to have different levels of motivation for performing similar tasks.”

The scientists found that the brain signal seems to wind down just before a reward is reached. In other words, understanding that the reward is a sure thing may be more important, for the brain at least, than actually receiving it.

“Imagine you’re in a tall building, waiting for the elevator, but it doesn’t come,” Richmond elaborated. “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 monkeys in the experimental trials watched a spot on a computer screen change from red to green. When the spot was green, the monkeys were supposed to release a lever — if they did so correctly, the spot turned blue. For every correct response given, a gray bar on the screen became brighter. Thus, the monkeys knew that the treat, a juice drink, was on its way.

During the trials, Shidara and Richmond monitored the electrical activity in more than 100 neurons in the anterior cingulate. They found that there was a boost in neural activity as the timing of the reward drew nearer, and that the activity dropped off when the reward was imminent and the expectancy was resolved. Moreover, the monkeys made fewer errors as the reward drew closer. But when the brightness of the gray bar changed randomly, the monkeys did not progressively improve. This means that the monkeys didn’t work as hard to get the reward when they knew it was fairly far off in the future.

“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,” said Richmond. “It makes sense that there is such a signal that varies with the degree of reward expectancy, that keeps you on-task performing a long sequence of behaviors.”

The scenario can be compared to what a marathon runner experiences toward the end of a race. There are more and more bystanders along the road, cheering the runner on. But in the case of the brain, as soon as the winning line can be seen up ahead, the cheering bystanders disappear and the runner sprints to the end.

The scientists believe that the brain signal they’ve discovered may be at least partly responsible for conditions like obsessive-compulsive disorder or drug addiction, both of which are known to involve greater than normal activity in the anterior cingulate.

“Let’s assume you had a signal like this, and it was turned up too high. It might make you feel like you had to be trying harder, telling you ‘Just go on a little further and you’ll get satisfied,’ ” said Richmond. “Our speculation is that this signal never resolves for conditions like obsessive-compulsive disorder. Or in the case of addiction, a drug has the effect of satisfying the signal.”

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