Researchers, using a new combination of techniques, have discovered that
dopamine levels in our brains vary the most in situations where we are unsure if we are going to be rewarded, such as when we are gambling or playing the lottery.
The research results, "Dopamine Transmission in the Human Striatum during Monetary Reward Tasks," were published online April 28 in the Journal of Neuroscience.
Dopamine has long been known to play an important role in how we experience rewards from a variety of natural sources, including food and sex, as well as from drugs such as cocaine and heroin, but pinning down the precise conditions that cause its release has been difficult.
"Using a combination of techniques, we were actually able to measure release of the dopamine neurotransmitter under natural conditions using monetary reward," said David Zald, assistant professor of psychology at Vanderbilt University.
Zald believes the primary significance of the study is the possibilities it raises for future research on measuring what causes us to experience reward from a variety of sources and what happens in our brains when we are disappointed in our quest for those rewards. The research lays a foundation for a better understanding of what happens in the brain during unpredictable reward situations such as gambling and offers promise for exploring the chemical foundation of problems such as gambling addiction.
"We're moving to a point where we can measure what's happening to people's neurotransmitter systems in a way that we haven't been able to do before," he said.
Zald and his colleagues used positron emission topography (PET scanners) to view brain activity in nine human research subjects who had been injected with a chemical that binds to dopamine receptors in the brain, but is less able to bind when the brain is releasing dopamine. A decrease in binding to the receptors is associated with an increase in dopamine release, while an increase in binding indicates reduced release of dopamine. This technique allows researchers to study the strength and location of dopamine release more precisely than has previously been possible.
The team studied the subjects under three different scenarios. Under the first scenario, the subject selected one of four cards and knew a monetary reward of $1 was possible but did not know when it would occur. During the second scenario, subjects knew they would receive a reward with every fourth card they selected. Under the third scenario, subjects chose cards but did not receive or expect any rewards.