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Rats dream about the places they wish to go

Do you dream of where you’d like to go tomorrow? It looks like rats do.

When the animals are shown a food treat at the end of a path they cannot access and then take a nap, the neurons representing that route in their brains fire as they sleep – as if they are dreaming about running down the corridor to grab the grub.

“It’s like looking at a holiday brochure for Greece the day before you go – that night you might dream about the pictures,” says Hugo Spiers of University College London.

Like people, rats store mental maps of the world in their hippocampi, two curved structures on either side of the brain. Putting electrodes into rats’ brains as they explore their environment has shown that different places are recorded and remembered by different combinations of hippocampal neurons firing together.

These “place cells” fire not only when a rat is in a certain location, but also when it sleeps, as if it is dreaming about where it has been in the past.

Spiers’s team wondered whether this activity during sleep might also reflect where a rat wants to go in future. They placed four rats at the bottom of a T-shaped pathway, with entry to the top bar of the T blocked by a grille. Food was placed at the end of one arm, in a position visible to the animals.

Next they encouraged the rats to sleep in a cosy nest and recorded their hippocampus activity with about 50 electrodes each as they rested. Finally they put the rats back into the maze, but now with the grille and the treat removed.

As expected, the animals scampered along the arm where they had seen the food, with their place cells firing in a pattern corresponding to the new route. Crucially, these same cells had fired while the rats were asleep – unlike those that encoded the route to the other arm. “It suggests we construct our mental maps a little bit by looking, and then pad them out more by doing,” says Spiers.

Picturing the future

David Redish of the University of Minnesota in Minneapolis says this shows that rats’ dreams can be shaped by their goals. “It’s not just reflecting some baseline activity; it depends on the animals’ desires,” he says.

The work supports the idea that the hippocampus helps us to imagine the future, as well as encoding our memories of the past. Some people with damage to their hippocampi have problems imagining future events with any richness or detail.

Spiers admits that we cannot know for sure whether the rats were dreaming, or even if the rats were just having a peaceful rest rather than sleeping. But previous work in humans has suggested that brain activity while we snooze does reflect the content of our dreams.

In one experiment, people slept inside a brain scanner and were then woken up and asked to describe their dreams. The brain patterns observed when they dreamt of specific items matched the activity recorded when participants were shown the same items while conscious.

Researchers have also been able to alter the dreams of mice, a bit like in the film Inception. When electrodes stimulated brain areas that signal reward as the animals dreamed about a certain place, they were more likely to go to that spot when awake.

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