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The Brain Memories Are Crucial for Looking Into the Future
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Paul Jang
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2016-07-08 03:12:06, hit : 2,319 |
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Mind & Brain / Memory, Emotions, & Decisions
The Brain Memories Are Crucial for Looking Into the Future
Without remembering how the past unfolded, trying to plan ahead is \\"like being in a room with nothing there and having a guy tell you to go find a chair.\\"
by Carl Zimmer
From the April 2011 issue; published online April 24, 2011
Stan Klein, a psychologist at the University of California, Santa Barbara, argues that the intertwining of foresight and episodic memory may help explain how this type of memory evolved in the first place. In Klein’s view, episodic memory probably arose in part because it helped individuals make good decisions about what to do next. For instance, it could have guided our ancestors not to visit a local watering hole on moonlit nights because that was when saber-toothed tigers hung out there.
Klein has run a series of experiments to test this hypothesis. In one study published last year, he probed the memory of 224 undergraduates. Some of the students were asked to recall a camping trip they’d taken in the past. Others were asked simply to envision a campsite. A third group was told to imagine the process of planning a camping trip. Students in all three groups then looked at a list of 30 words—including food, trees, and sadness—and, after spending a few minutes on other tasks, had to write down as many of the listed words as they could. The students asked to plan a camping trip recalled more words than the others. Klein says his results illustrate the decision-making value of memory: When students were actively planning the future, their memories worked best.
The precursor to mental time travel may have evolved in mammals more than 100 million years ago. Scientists can get clues to its origins by studying lab rats. When a rat moves around a space—be it a meadow or a lab maze—it encodes a map in its hippocampus, a structure located near the brain’s core. Neurons there become active at particular spots along the route. When the rat travels that route again, the same “place cells” fire in the same order.
In 2009 a group led by Tom Davidson and Fabian Kloosterman, neuroscientists at MIT, observed rats as the animals traveled along a winding,
10-meter track. The researchers were able to identify place cells that fired at different spots all along the way. From time to time, the rats would stop on the track for a rest. Davidson noticed something intriguing: Sometimes during these breaks the place cells became active again, firing in the same order (but at 20 times the speed) as they did when the rats were navigating the track. It seemed that the rats were rapidly replaying their journey through the track in their heads.
David Redish, a neuroscientist at the University of Minnesota, is exploring this process in detail. He and his colleagues recently built a more complex rat maze, a rectangular loop with a shortcut running through its midsection. As the rats ran up the midsection, they had a choice to go left or right, with only one direction leading to food. Using implanted electrodes, the scientists eavesdropped on the hippocampi of their test rats.
As expected, the animals’ place cells fired along the way as they were running the maze. But sometimes when the rats were resting or deciding which way to turn, the firing of the place cells indicated that they were imagining running through the maze in a different direction. In fact, the signals seemed to cover every possible route, both forward and backward. The rats were pondering lots of alternatives, Redish concluded, projecting themselves into different futures to help them decide where to go next.
A number of studies suggest that the hippocampus continues to be crucial to our own power of foresight. Damage to the hippocampus can rob people of their foresight, for example, and when people with healthy brains think about their future, the hippocampus is part of the network that becomes active. But our powers of foresight go far beyond a rodent’s. We don’t just picture walking through a forest. We travel forward into a social future as well, in which we can predict how people will react to the things we do.
Scientists cannot say for sure exactly when our ancestors shifted to this more sophisticated kind of time travel. It is possible that the transition started in our primate ancestors, judging from some intriguing stories about our fellow apes. In the 1990s, for example, zookeepers in Sweden spied on a chimpanzee that kept flinging rocks at human visitors. They found that before the zoo opened each day, the chimp collected a pile of rocks, seemingly preparing ammunition for his attacks when the visitors arrived. Did the chimp see itself a few hours into the future and realize it would need a cache of artillery? The only way we could know for sure would be for the chimp to tell us.
The fact that chimpanzees can’t explain themselves may itself be a clue to the nature of time travel. Full-blown language, which evolved only within the past few hundred thousand years, is one of the traits that make us humans different from other species. It is possible that once language evolved in our ancestors, it changed how we traveled through time. We could now tell ourselves stories about our lives and use that material to compose new stories about our future. Perhaps the literary imagination that gave rise to Dickens and Twain and Nabokov is, in fact, a time machine we carry in our head.
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