According to a new study published in the journal eLife, when rats (Rattus rattus) sleep or rest, their brains simulate journeys to a desired goal.
Rats dream paths to a tasty treat. Image credit: Duke University.
A team of scientists at University College London, UK, monitored brain activity in rats, first as the rodents viewed food in a location they could not reach, then as they rested in a separate chamber, and finally as they were allowed to walk to the food.
The activity of the hippocampal place cells – specialized brain cells involved in navigation – suggested that during the rest the animals simulated walking to and from food that they had been unable to reach.
“During exploration, mammals rapidly form a map of the environment in their hippocampus (part of the brain),” said Dr Hugo Spiers, who is the senior author on the study.
“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 some people with damage to the hippocampus are unable to imagine the future.
In the study, rats were individually placed on a straight track with a T-junction ahead. Access to the junction as well as the left and right hand arms beyond it was prevented by a transparent barrier. One of the arms had food at the end, the other side was empty.
After observing the food the rats were put in a sleep chamber for an hour. Finally after the barrier was removed, the rodents were returned to the track and allowed to run across the junction and on to the arms.
During the rest period, the data showed that place cells that would later provide an internal map of the food arm were active.
Cells representing the empty arm were not activated in this way. This indicates that the brain was simulating or preparing future paths leading to a desired goal.
The results suggest that the hippocampus plans routes that have not yet happened as well as recording those that have already happened, but only when there is a motivational cue such as food.
This may also imply the ability to imagine future events is not a uniquely human ability.
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H Freyja Ólafsdóttir et al. 2015. Hippocampal place cells construct reward related sequences through unexplored space. eLife 4: e06063; doi: 10.7554/eLife.06063
