Christmas morning: The kids throw on their clothes and wolf down breakfast so they can rip open their presents, while their grandparents loiter over their toast and coffee, seemingly more intent on finishing their meals than examining the holiday loot. It's not that Grandma and Grandpa hate Santa. Rather, new research shows, the same neurochemical that triggers excitement and rapid decision-making in the young just doesn't have the same effect for older folks.
Presents and other rewards cause a release of dopamine in the brain. Animal research has shown too little or too much dopamine can throw off activity in the prefrontal cortex (PFC), where most of our high-level decision-making takes place. Research in young adults suggests a similar relationship, but no one had examined dopamine's effect on PFC activity in the elderly.
To fill this gap, Karen Berman, clinical neuroscientist at the National Institute of Mental Health in Bethesda, Maryland, and colleagues turned to the two workhorses of human neuroscience research: positron emission tomography (PET) and functional magnetic resonance imaging (fMRI). PET tracks how much of a particular chemical the brain produces; fMRI tracks moment-to-moment blood flow, pointing researchers toward areas of increased neural firing.
To examine the relationship between dopamine and the brain's response to rewards, the researchers recruited 20 individuals averaging 25 years of age and 13 individuals averaging 66 years of age. While inside an fMRI scanner, the subjects viewed a series of 16 slot machine games on a computer screen. The slot machine first showed the chance of winning a set amount of money. After a 15-second delay, the screen showed how much money the subjects actually received. The researchers repeated the test, this time using PET to measure overall dopamine production during the testing period.
Increased dopamine production in young people was linked to more brain activity in the prefrontal cortex, as measured by the fMRI. But the elderly showed the opposite relationship, meaning older subjects who made more dopamine actually had less activity in their prefrontal cortex, the researchers report online today in the Proceedings of the National Academy of Sciences. "So you're getting less bang for your [dopamine] buck," Berman says. Across the board, the elderly subjects also showed less activity in the brain areas that control arousal, suggesting they were less excited at the thought of the reward.
Amy Arnsten, a neuroscientist at Yale University who first described the relationship between dopamine and PFC activity in animals, says the work offers good and bad news. Although Grandpa may not need a top-notch PFC to coordinate his gift-unwrapping sessions, the elderly do need a PFC in good order to make major decisions such as deciding on health insurance or investing life savings. In those situations, an impaired dopamine reward system could cause major problems. "To have those functions impaired [in the elderly] when you may be needing them most is really rotten." But research like this could eventually lead to effective treatments to stabilize impaired dopamine systems in the elderly, Arnsten says.