Brain’s “GPS System” Switches Between Actual and Imagined

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When you are walking through a maze and meet a fork in the road, your brain will choose between the left and right options. You picture a future, or, a possibility for yourself and try to find out the solution that can lead you to the destination successfully. The process happens to every single one of us, but how the process goes varies greatly.


Neuroscientists at UC San Francisco have discovered how the brain may generate such imagined future scenarios. The work provides a new grounding for understanding not only how the brain makes decisions but also how imagination works more broadly, the researchers say.



“Place cells”


In the study, researchers put rats in a simple M-shaped maze, while recording the firing of neurons in the hippocampus called “place cells,” which are traditionally thought to keep track of an animal’s location like a neural GPS system.


When the rats met a fork in the maze, their place cell activity began to switch between representing the animal’s current position and its two alternative future paths at a rate of 8 times per second. It’s like the rat was thinking: “Should I go left? Or right?”



Place cells have also been known to keep track of an animal’s travel direction. Researchers found that place cells representing opposite travel directions could also switch at a high speed, like the rat thought “I can go back if the current way isn’t right.”



Suggestion of new conception of hippocampus as source of imagination


The hippocampus, a seahorse-shaped structure found on each side of the brain deep in the temporal lobes, is among the most intensively studied parts of the brain.


In the past, people thought hippocampus is where we store memories, and describe it as the brain’s memory center. In the 1970s, scientists found place cells in the hippocampus.



Previous work has shown that place cell activity can replay an animal's recent movements or even anticipate where an animal may be headed next, but such activity had only been seen intermittently—typically when animals were resting or pausing during ongoing movement—as actively considering their next move.


The new study is the first to show how hippocampal cells can represent different hypothetical scenarios consistently and systematically over time. Such a system could allow animals on the move to make extremely rapid decisions in the moment based on these imagined alternatives while also keeping track of the animal's present reality, the researchers say.


"The regular switching between present and possible—or actual and imagined—looks like be a robust system for generating lots of ideas, not just for mechanically remembering or predicting," Kenneth Kay, leader of the study, said. "The hippocampus could be at the root of our ability to imagine."



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