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DO YOU dream of where you’d like to go tomorrow? Rats appear to.
When the animals take a nap after seeing a food treat at the end of an inaccessible path, the neurons in their brains representing that route fire as they snooze – 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 maps of the world in their hippocampi, two curved structures on either side of the brain. Putting electrodes into rats’ brains as they go exploring 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.
Spiers’s team wondered whether this activity during sleep might also reflect where a rat wants to go in the 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 in a visible position at the end of one arm.
Next, they encouraged the rats to sleep in a cosy nest, where they recorded activity in their hippocampi with about 50 electrodes. They then put the rats back into the T, 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 route. Crucially, these same cells had fired while the rats were resting – 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.
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 hippocampi help 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 resting 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 before describing their dreams. The brain patterns when they dreamed of specific items matched patterns when they saw the same items while conscious.