Sleeping Rats Dream Of A Better Place

Rats’ brains simulate journeys to a desired future, such as a yummy meal, when they rest, new University College London research says.

Brain activity in rats was monitored by the researchers, first as the animals looked at food in a location they couldn’t reach, next as they rested in a separate chamber, and finally as they were allowed to get to the food.

Activity of brain cells specialised in navigation suggested that when they were resting, the rats simulated walking to and from food that they had been unable to reach.

Senior author Dr Hugo Spiers said:

“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."

The study may help explain why some people with damage to a region of the brain called the hippocampus are unable to imagine the future.

“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,” explains co-lead author Dr Freyja Ólafsdóttir. “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."

The results imply that the hippocampus plans routes that have not yet happened in addition to recording those that have already happened, but only when there is a motivational cue such as food.

It may also suggest the ability to imagine future events is not only a human ability.

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

Hippocampal place cells construct reward related sequences through unexplored space
In anticipation of a reward, hippocampal place cells simulate desired journeys through areas that have been seen, but not explored.
H Freyja Ólafsdóttir, Caswell Barry, Aman B Saleem, Demis Hassabis, Hugo J Spiers
eLife 2015;4:e06063 10.7554/eLife.06063