Writing in the open-access journal eLife, senior author Dr. Hugo Spiers of the UCL Department of Experimental Psychology and his colleagues explained that when rats are at rest, their brains simulate journeys to a desired future outcome, such as a tasty piece of aged cheddar.
The study authors monitored the brain activity of the rodents as they observed food in a location that they were unable to reach, then again as they rested in a separate area, and a third time when they were allowed to reach the food. They found increased activity in areas of the brain that handle navigation, suggesting that the animals “dreamed” of finding the cheese while they slept.
Hippocampus may actually help plan future journeys
Dr. Spiers said that the findings may help explain why some people who have suffered damage to the hippocampus are unable to imagine future events, as they use this part of the brain to make maps of the environment and replay journeys through this area while sleeping or resting.
“During exploration, mammals rapidly form a map of the environment in their hippocampus,” he explained. “During sleep or rest, the hippocampus replays journeys through this map which may help strengthen the memory. It has been speculated that such replay might form the content of dreams. Whether or not rats experience this brain activity as dreams is still unclear.”
He added that the results “show that during rest the hippocampus also constructs fragments of a future yet to happen. Because the rat and human hippocampus are similar, this may explain why patients with damage to their hippocampus struggle to imagine future events.”
The experiments involved placing rats on a straight track with a T-junction ahead. The scientists blocked access to the junction and the two branches with a transparent barrier. One branch had a piece of food at the end, while the other was empty. After the rats saw the food, they were put in a sleep chamber for an hour, after which time they were granted access to the food.
Route-planning only occurs in response to motivational cues
During that one-hour rest period, the data collected by Dr. Spiers’ team showed that place cells which were later used to create an internal map of the branch containing food were active, while cells representing the empty arm were not activated in the same way. This finding indicates that the brain was simulating or preparing future paths to the desired goal, the authors said.
“What’s really interesting is that the hippocampus is normally thought of as being important for memory, with place cells storing details about locations you’ve visited,” said co-lead author Dr. Freyja Ólafsdóttir. “What’s surprising here is that we see the hippocampus planning for the future, actually rehearsing… journeys that the animals need to take in order to reach the food.”
Furthermore, the researchers said that the results appear to indicate that the hippocampus is used to plan routes that the subject has not yet taken as well as recording those that have already been traveled, but only in instances where there is a clear motivational cue (such as the food).
“What we don’t know at the moment is what these neural simulations are actually for,” said co-lead author Dr. Caswell Barry of UCL. “It seems possible this process is a way of evaluating the available options to determine which is the most likely to end in reward, thinking it through if you like. We don’t know that for sure though and something we’d like to do in the future is try to establish a link between this apparent planning and what the animals do next.”