A new brain imaging study has shown that an individual’s willingness to work hard to earn money is strongly influenced by the chemistry in three specific areas of the brain. Researchers claim that this could have important implications for the treatment of attention-deficit disorder, depression, schizophrenia and other forms of mental illness characterized by decreased motivation.
The study was published May 2 in the Journal of Neuroscience and was performed by a team of Vanderbilt scientists including postdoctoral student Michael Treadway and Professor of Psychology David Zald.
The study was conducted with 25 healthy volunteers (52 percent female) ranging in age from 18 to 29. To determine their willingness to work for a monetary reward, the participants were asked to perform a button-pushing task. First, they were asked to select either an easy or a hard button-pushing task. Easy tasks earned $1 while the reward for hard tasks ranged up to $4. Once they made their selection, they were told they had a high, medium or low probability of getting the reward. Individual tasks lasted for about 30 seconds and participants were asked to perform them repeatedly for about 20 minutes.
The team of researchers used a brain mapping technique called positron emission tomography (PET scan). They found that the “go-getters” who are willing to work hard for rewards had higher release of the neurotransmitter dopamine in areas of the brain known to play an important role in reward and motivation, the striatum and ventromedial prefrontal cortex. But the “slackers” who are less willing to work hard for a reward had high dopamine levels in another brain area that plays a role in emotion and risk perception, the anterior insula.
“If you look around at the people you know, yourself included, and think of the people always driven to work hard vs. the people who prefer to take it easy, what this study shows is that the range in motivation is in part due to how the dopamine system functions,” said researcher Michael Treadway, a clinical fellow at McLean Hospital and Harvard Medical School. “Past studies in rats have shown that dopamine is crucial for reward motivation,” said Treadway, “but this study provides new information about how dopamine determines individual differences in the behavior of human reward-seekers.”
Researchers were surprised by the role of dopamine in the anterior insula. The finding was unexpected because it suggests that more dopamine in the insula is associated with a reduced desire to work, even when it means earning less money. The fact that dopamine can have opposing effects in different parts of the brain complicates the picture regarding the use of psychotropic medications that affect dopamine levels for the treatment of attention-deficit disorder, depression and schizophrenia because it calls into question the general assumption that these 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,” said Zald, “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.”
The research is part of a larger project designed to search for objective measures for depression and other psychological disorders where motivation is reduced. “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.”
“Previous research has indicated that dopamine influences the motivation to seek out rewards,” Dr. Marco Leyton, a dopamine researcher at McGill University in Montreal who was not involved with the study, said in a written statement. “Now, this elegant new study provides the clearest evidence to date that individual differences in dopamine-related motivation might be a trait.”
The new research suggests scientists should look at a greater number of brain networks when studying dopamine, said Brian Knutson, an associate professor of psychology and neuroscience at Stanford University. He reviewed the findings. “This may have implications for how drugs that influence dopamine may affect effort,” he said of the new research. The study shows that the amount of effort a person decides to expend seems to depend on where the dopamine is in the brain, says John Salamone, a professor of psychology and neuroscience at the University of Connecticut in Storrs. The study, he said, points out that differences in dopamine location from person to person were linked with various degrees of effort.
Additional research is under way to examine how medications specifically impact these motivational systems. The National Institute of Drug Abuse funded the research.