Differences in dopamine responsivity, identified using PET imaging, have been linked to variation in human choice behavior, specifically the drive to work harder for larger rewards, according to a study published May 2 in the Journal of Neuroscience.

The study, conducted by researchers at Vanderbilt University in Nashville, Tenn., suggests that brain chemistry in three specific areas of the brain may go a long way toward influencing whether a person is a "slacker" or more motivated.

"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,” Michael Treadway, post-doctoral student at Vanderbilt and co-author of the study, said in a statement.

Treadway and colleagues took 25 study participants, all healthy adults aged 19 to 29, and had them undergo two PET scans, as well as participate in a multi-trial game. The first PET scan was a baseline placebo scan, and the second, on a separate day, was performed as the participant received a dose of d-amphetamine.

The game consisted of rapidly pushing buttons, and subjects were given the choice of either an easy or a hard version of the task. Successful completion of the easy tasks earned $1, while the harder tasks had rewards up to and including $4. Once participants had made their choice, they were told they had a high, medium or low probability of getting the reward. Each trial last about 30 seconds, and subjects were asked to perform them repeatedly for about 20 minutes.

Results showed that those who were willing to work harder for the higher rewards had a higher release of the neurotransmitter dopamine in the striatum and ventromedial prefrontal cortex—areas of the brain known to play an important role in reward and motivation.

Those who were less willing to work for a reward, on the other hand, had high dopamine levels in the area of the brain that plays a role in emotion and risk perception, the anterior insula.

The latter finding came as a surprise to the researchers as it demonstrated that dopamine can have opposing effects in different parts of the brain. This means that medications affecting dopamine levels used to treat various psychiatric illnesses may not have the same effect throughout the brain.

The results could have implications for the diagnosis and treatment of disorders such as depression, schizophrenia and attention-deficit disorder. “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,” said study co-author David Zald, PhD, of the department of psychology at Vanderbilt. “With objective measures we could treat the underlying conditions instead of the symptoms."

Future research will be needed to examine how dopamine levels affect motivation in sufferers of depression and addiction.

"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,” said Zald.