Rodents, one might guess, live in the present—seeking out the best rewards they can scurry to. Indeed, the Scottish poet, Robert Burns, encapsulated this in his poem, “To a Mouse,” with the lines: “Still, thou art blest compar‘d wi’ me! The present only toucheth thee: But Och! I backward cast my e‘e, On prospects drear! An’ forward, tho‘ I canna see, I guess an’ fear!”
Legend has it that Burns wrote the poem after turning a mouse out of its home when ploughing his fields. He felt pity for it, but also envied the mouse for its inability to worry about what the future might bring. However, it seems Burns may have been wrong. New research published by our research team in eLife indicates that rodents do in fact appear to simulate the future, and they do so during sleep/rest periods.
We have known since the 1970s than neurons, called “place cells,” in a brain area called the hippocampus form an organized map of space through their spatially localized patterns of activity. Because each cell is active in a different part of a space, the population of activity from these cells provides a sort of “you are here on the map” signal to the rest of the brain connected to the hippocampus. Place cells are typically recorded in rats, but similar patterns have been observed in humans.
One dogma is that place cells can only form a map during active physical travel through a space. However, we wondered whether this assumption might be wrong. This was because a recent study found that humans with hippocampal damage struggled to imagine future scenarios. When asked to imagine lying on a beach in a tropical bay, for example, the patients described having great difficulty creating a coherent scene in their mind’s eye. We speculated that if place cells not only map space during physical exploration, but also during mental exploration of a future space, this might underlie why the patients were unable to imagine fictitious places. The patients’ place cells were damaged making them unable to mentally construct imagined places.
