Individuals found to expend the least effort had increased dopamine response in the insula, a part of the brain involved in self-awareness, social behavior and perception.
Michael Treadway and team asked volunteers to press a button rapidly in order to receive varying amounts of financial rewards. They had to determine how hard they were prepared to work, depending on what their chances of a payout were, as well as the size of the possible payout.
The "motivated" participants accepted harder tasks which offered higher prizes, even with high odds, while the less motivated ones would miss a go if they felt there would be too much effort involved.
In a separate study, the same volunteers underwent a PET scan which measured the activity of various dopamine systems in several parts of the brain.
The authors checked to see whether participants' levels of effort were linked to dopamine responsiveness.
Previous animal experiments using mice had shown that dopamine activity in motivational centers in the brain is important for decisions when the odds are long. In this study, the scientists were surprised to find that the participants with higher dopamine activity in the insula had a considerably lower likelihood of trying very hard on tasks.
"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."
Marco Leyton, PhD, an expert on dopamine at McGill University, said:
"Previous research has indicated that dopamine influences the motivation to seek out rewards. Now, this elegant new study provides the clearest evidence to date that individual differences in dopamine-related motivation might be a trait. A striking implication highlighted by the authors is that abnormal dopamine transmission could affect a wide range of decision-making processes and susceptibility to depression."
(Leyton was not involved in the study)
Written by Christian Nordqvist