If you've ever dreamed about what you'll eat when you wake up, you're not alone. Even rats seem to dream about strategies for getting food in the future, according to a new study, potentially shedding light on how our brains make plans while we sleep.
Published in the journal eLife, the study tracked rats' brain activity in three situations: first as they viewed inaccessible food, then as they rested in a separate chamber, and finally as they were allowed to reach the food. The resting rats showed activity in specialized brain cells that deal with navigation, suggesting they simulated walking to and from food they'd been unable to reach while awake.
This could help us better understand the hippocampus, the researchers say, a brain region that's key to forming, organizing and storing memories. The rats in the study were apparently using the hippocampus to not just remember the food they saw, but to map out prospective journeys for reaching it.
"During exploration, mammals rapidly form a map of the environment in their hippocampus," says study co-author Hugo Spiers, a neuroscientist at University College London, in a press release. "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."
It's still unclear if rats experience this brain activity as dreams, Spiers adds. But it at least indicates their hippocampus takes advantage of down time to strategize, which could have implications for humans. "Our new results show that during rest the hippocampus also constructs fragments of a future yet to happen," he says. "Because the rat and human hippocampus are similar, this may explain why patients with damage to their hippocampus struggle to imagine future events."
A dream come true?
Previous research has shown how rats (and humans) remember specific locations with neurons in the hippocampus known as "place cells." These neurons fire when a rat is actually in a location but also when it's later asleep, possibly because it's dreaming about where it was earlier. The new study was designed to see if this brain activity can also indicate where the rat wants to go in the future.
To test that, the researchers started by placing each rat on a straight track with a T-junction ahead. One branch of the junction was empty and one had food at the end, but both were blocked by a transparent barrier. After the rats had time to soak up this conundrum, they were removed from the track and spent an hour inside a "sleep chamber." The researchers later took down the barrier, returned the rats to the track and let them run through the junction to reach the food.
Hungry hungry hippocampus
Since the rats had been wearing electrodes throughout the experiment, the researchers could then see what their hippocampi were doing at various stages. During the rest period, the data showed activity in the rats' place cells — specifically those that would later provide a map to the food. Place cells representing the empty branch of the junction didn't show the same activity, suggesting the brain was plotting future routes toward a goal rather than just remembering scenery.
"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," says co-author Freyja Ólafsdóttir, also a neuroscientist at UCL. "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 ability to imagine future events may not be unique to humans, the researchers say, although more research is needed before we really understand the purpose of these simulations. "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," says co-author and UCL biologist Caswell Barry. "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."
Despite all the obvious differences between humans and rats, this research reminds us we're more similar than it might seem. Not only do we both have a hippocampus that helps us remember where we've been, and maybe plan where we're going next, but we also have at least one dream in common: breakfast.
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