A person’s willingness to work may be determined by the levels of dopamine in three different parts of their brain, new research suggests.

Researchers from the new study, published Wednesday in The Journal of Neuroscience, used an imaging technique called positron emission tomography (PET scan) and found that “go-getters,” or people who were willing to work hard to earn rewards, had higher levels release of the neurotransmitter dopamine in the striatum and ventromedial prefrontal cortex, areas of the brain that play an important role in reward and motivation.

However, researchers were surprised to find that “slackers” or those who were least likely to work hard for a reward, had higher levels of greater release of dopamine in the anterior insula, the brain region involved in emotion, perception, social behavior, and self-awareness.

The study consisted of 25 healthy participants aged 18 to 29. Researchers measured participants’ willingness to work for a monetary reward.

Participants were asked to rapidly press a button to earn different amounts of money, and they could choose whether they wanted an easy of difficult button-pushing task. Participants would earn $1 for easy tasks and up to $4 for hard tasks.

After subjects chose whether they wanted an easy or hard task, they were informed whether they had a low, medium or high probability of getting the reward.

Individual tasks took about 30 seconds to complete, and participants were asked to perform them repeatedly for 20 minutes.

Participants would be labeled as “go-getters” if they accepted harder challenges for more money, even when they were told they had a slim chance of winning, whereas less motivated participants would skip an attempt at a task if it cost them too much effort.

"Past studies in rats have shown that dopamine is crucial for reward motivation but this study provides new information about how dopamine determines individual differences in the behavior of human reward-seekers," study author Michael Treadway, a post-doctoral student, said in a university news release.

While, dopamine has generally been linked to motivation, researchers said that they were surprised to find that participants increased dopamine activity in the insula were the least likely to put in effort.

"These results show for the first time that increased dopamine in the insula is associated with decreased motivation — suggesting that the behavioral effects of dopaminergic drugs may vary depending on where they act in the brain," said Treadway.

The latest findings, which show that dopamine can have contrasting effects in different regions of the brain, make the use of psychotropic medications that affect dopamine levels for treating disorders like attention-deficit disorder, depression and schizophrenia more complex because it questions the accepted assumption that dopaminergic drugs have the same effect throughout the brain.

"At this point, we don't have any data proving that this 20-minute snippet of behavior corresponds to an individual's long-term achievement but if it does measure a trait variable such as an individual's willingness to expend effort to obtain long-term goals, it will be extremely valuable," co-author David Zald, a professor of psychology at Vanderbilt University, said in a news release.

More research is needed to determine whether difference s in dopamine levels can lower levels of motivation seen in people with certain mental disorders like ADD, depression and schizophrenia.

"Right now our diagnoses for these disorders is often fuzzy and based on subjective self-report of symptoms," said Zald. "Imagine how valuable it would be if we had an objective test that could tell whether a patient was suffering from a deficit or abnormality in an underlying neural system. With objective measures we could treat the underlying conditions instead of the symptoms."