When rats sleep, the part of their brain responsible for memory and future planning prepares them for success after they wake. Researchers at University College London monitored the animals before, during and after exposing them to the location of a desired treat. Based on similarities in activity in the Hippocampus, they determined that the path to the food dominated the rats thinking even during a sleep cycle.

Humans and mammals have two hippocampi which serve a variety of complex functions. They play a role in the transfer of information from short-term to long-term memory an are important in spatial navigation and future planning.

Scientists already know that neurons called “place cells” in the hippocampus help rats to remember territory they’ve visited before.

For their research, the rats were allowed to view an area that they could not reach which contained a desired food, they were then placed into a sleep chamber before being returned to the prior location and allowed to walk to the food.

“During exploration, mammals rapidly form a map of the environment in their hippocampus. 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, as we would need to ask them to be sure! Our new 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,” said Dr Hugo Spiers in a statement.

Spiers is senior author of the paper published in the open access journal eLife.

The rats were placed on a track with two branching arms, one arm was accessible but contained nothing, the other contained food which the rats were prevented from accessing by clear plastic. The rats were then allowed to rest before being returned to the track and allowed to access the food.

The brain activity in the rats showed that during the rest period, place cells related to the branch containing food were active. Place cells related to the branch which did not contain food remained dormant.

“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. What’s surprising here is that we see the hippocampus planning for the future, actually rehearsing totally novel journeys that the animals need to take in order to reach the food,” said Dr Freyja Ólafsdóttir, co-lead author of the paper.

According to the researchers, the findings indicate that the hippocampus maps routes which have not yet been taken if there is something of interest to the animal on that path. It is an indication that rats, and possibly other animals, possess the ability to imagine future events.

“What we don’t know at the moment is what these neural simulations are actually for. 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,” said co-lead author Dr Caswell Barry.

In humans, damage to the hippocampus is known to cause problems with memory and is thought to be related to Alzheimer’s Disease. It has also been associated with difficulty in planning for the future. The new research, as well as follow up studies, may help to shed light on why that is.