New research shows that the hippocampus plays a crucial role in allowing us to delay gratification for imagined rewards.
Would you rather have a bag of chips now, or tickets to a major league football game later?
In order to choose the game tickets, you don’t just have to resist the short-term gratification that the chips would give you. You also have to be able to imagine the football game—the excitement, the roar of the crowds, the smell of beer and hot dogs—in order to decide that the game is more fun than eating the bag of chips you can see right in front of you.
The prefrontal cortex of the brain, the foremost area that lies just behind the eyes and forehead, plays a major role in controlling impulses, long-term planning, and delaying gratification for larger gains later. People who receive damage to this area of the brain, whether from a head injury or as the result of dementia, often have difficulty controlling their decision-making. Now, research is showing that the hippocampus, the brain’s memory formation center, also plays a role.
In order to test this, the research team had to tease apart how decision-making really works. “The demonstration was made in several steps,” Maël Lebreton, a Ph.D. candidate on the MBB team and first author on the paper, told Healthline.
To simulate the decision-making we experience in our daily lives, participants in the study had to choose between an immediate reward they could observe (in a picture) and a delayed reward that was described in text, which they had to imagine. When asked to describe what they had imagined, the more vivid and detailed a participant’s description, the more likely he or she was to have chosen the delayed reward.
Imagined fantasies are constructed from memories, which is the hippocampus’ domain. It governs what the paper describes as “mental time-travel”—the ability to not just remember the past, but also to project ourselves forward into the future.
So, the team homed in on the hippocampus, examining their subjects in structural and functional magnetic resonance imaging (MRI) scans. They found that the more active a person’s left hippocampus was, the better he or she was at choosing the greater imagined reward over the smaller observed reward. In turn, the thicker and denser the grey matter in the left hippocampus, the more strongly it was able to respond.
“We thus suggest that the hippocampus, through its role in imagination, might help the valuation of imagined reward, which leads to a greater tendency to choose these rewards,” explained Lebreton.
Finally, Lebreton and his team wanted to isolate the effects of the hippocampus on decision-making from those of the prefrontal cortex. They examined patients with Alzheimer’s disease (AD), which damages the hippocampus but leaves the frontal lobe intact, and frontotemporal dementia (FTD), which damages the frontal lobe and some of the areas near the hippocampus, but leaves the hippocampus intact.
The FTD patients, who often experience disinhibition and poor decision-making, showed a general inability to delay gratification. In tests, regardless of whether they had to choose between text or pictures, they were more likely to choose the immediate-gratification option, even if it was a much smaller reward. Without the frontal lobe intact, the entire decision-making system broke down.
But the AD patients experienced no such difficulties. When asked to choose between a small reward in text and a large reward in text, or a small reward in pictures and a large reward in pictures, they performed just as well as healthy subjects in choosing a larger, delayed reward.
For them, the difficulty arose when they had to compare one reward in text with another pictured reward. With their hippocampus damaged, they were unable to access the memories they would need to construct an imaginary picture of what the reward in the text might be like. Unable to imagine properly, they preferred the reward they could see: the lesser, immediate reward in the picture.
This might provide a new diagnostic tool for dementia patients, as the impaired decision-making characteristic of AD and FTD can often look superficially similar in the early stages of the disease. By being able to diagnose dementia correctly early on, doctors would be able to treat patients with targeted medications sooner.
The team’s findings might also prove useful for user interface and customer experience designers. It offers a plausible model for how to influence people to make decisions one way or the other. With good interface design, consumers can be prompted to make healthier choices, whether it’s which groceries to buy at the store or how to fund our retirement accounts.
“This study highlights the importance of the representation of the options when we make decisions,” Lebreton said. “Especially, it suggests that people make short-sighted decisions, not only because of a deficit in control, but maybe also because of a deficit of imagination and representation of delayed, imagined, and intangible options.”