Remembering the Future

We rely upon our memories to envision the future. Through the trick of mental time travel, we picture yesterday and imagine tomorrow. 

By Carl Zimmer|Saturday, September 01, 2012

One day not long ago a 27-year-old woman was brought to the Tel Aviv Sourasky Medical Center, sleepy and confused. Neuropsychologist Fani Andelman, with colleagues, gave the woman a battery of psychological tests to judge her state of mind. At first the woman seemed pretty normal. She could see and speak clearly. She could understand the meaning of words and recognize the faces of famous people. She could even solve logic puzzles, including a complex test that required her to plan several steps ahead. But her memory had holes. She could remember recent events outside her own life, and she could tell Andelman details of her life up to 2004. Beyond that point, however, her autobiography was in tatters. The more doctors probed her so-called episodic memory—the sequential recollection of personal events from the past—the more upset she became. As for envisioning her personal future, that was a lost cause. Asked what she thought she might be doing anytime beyond the next day, she couldn’t tell them anything at all.

The patient, Andelman realized, hadn’t lost just her past; she had lost her future as well. It was impossible for her to imagine traveling forward in time. During her examination, the woman offered an explanation for her absence of foresight. “I barely know where I am,” she said. “I don’t picture myself in the future. I don’t know what I’ll do when I get home. You need a base to build the future.”

The past and future may seem like different worlds, yet the two are intimately intertwined in our minds. In recent studies on mental time travel, neuroscientists found that we use many of the same regions of the brain to remember the past as we do to envision our future lives. In fact, our need for foresight may explain why we can form memories in the first place. They are indeed “a base to build the future.” And together, our senses of past and future may be crucial to our species’ success.

Past Becoming Future

Endel Tulving, a neuroscientist at the University of Toronto, first proposed a link between memory and foresight in 1985. It had occurred to him as he was examining a brain-injured patient. “N.N.,” as the man was known, still could remember basic facts. He could explain how to make a long-distance call and draw the Statue of Liberty. But he could not recall a single event from his own life. In other words, he had lost his episodic memory. Tulving and his colleagues then discovered that N.N. could not imagine the future. “What will you be doing tomorrow?” Tulving asked him during one interview. After 15 seconds of silence, N.N. smiled faintly. “I don’t know,” he said.

“Do you remember the question?” Tulving asked.

“About what I’ll be doing tomorrow?” N.N. replied.

“Yes. How would you describe your state of mind when you try to think about it?”

N.N. paused for a few more seconds. “Blank, I guess,” he said. The very concept of the future seemed meaningless to N.N. “It’s like being in a room with nothing there and having a guy tell you to go find a chair,” he explained.

On the basis of his study of N.N., Tulving proposed that projecting ourselves into the future requires the same brain circuitry we use to remember ourselves in the past. During the past decade, as scientists have begun to use fmri (functional magnetic resonance imaging) scanners to probe the activity of the brain, they have found support for his hypothesis. Last year, for example, Tulving and his colleagues had volunteers in an fMRI scanner imagine themselves in the past, present, and future. The researchers saw a number of regions become active in the brains of the volunteers while thinking of both past and future, but not the present.

Studies with children lend support to this time travel hypothesis. Children start to develop strong episodic memory around age 4. Psychologist Thomas Suddendorf of the University of Queensland in Australia explored whether foresight develops at the same time. He showed 3- and 4-year-olds a box with a triangular hole on one side and demonstrated how to open it with a triangular key. He swapped the box for one equipped with a square lock and gave the children three different keys. Even the youngest of the 96 subjects correctly picked the square key.

Now, Suddendorf changed the test. Instead of asking them to choose a key right away, the kids were distracted in another room for 15 minutes before being offered a choice of keys, which they would have to carry back to the box. In other words, they had to anticipate what key they’d need. Suddendorf found a sharp break between the 3-year-olds and the 4-year-olds. The older kids did much better—probably because, with more developed episodic memories, they remembered the square lock and used that knowledge to anticipate a future in which only a square key would unlock the box.

It turns out that episodic memory isn’t the only category of memories involved in foresight. Another patient had the exact opposite problem as N.N.: He could remember personal past and imagine personal future, but could not recall or anticipate general facts. Episodic memory is important to mental time travel, but it is not the only type of memory required.

Animal Imagination

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 tested this hypothesis in a series of experiments. He probed the memory of 224 undergraduates, asking some to recall a camping trip they’d taken, and asking others to simply envision a campsite. A third group was told to imagine planning a camping trip. Students in all three groups then looked at a list of words, including camping-themed ones like knife and rope, and irrelevant ones like tv and microwave. After a few minutes, they were asked to write down as many words from the list 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: Students’ memories worked best when they actively planned for the future.

A precursor to mental time travel may have evolved in mammals more than 100 million years ago, studies with rats suggest. 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 then at MIT, observed rats traveling along a 10-yard-long track. The researchers identified place cells that fired at different spots all along the way. From time to time, the rats stopped on the track for a rest. Davidson noticed something intriguing: The place cells sometimes became active again during those breaks, firing in the same order as they did when the rats ran the track, but at 20 times the speed. It looked like the rats were rapidly replaying their journey 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 rectangle with a shortcut running through its midsection. As the rats ran up the midsection, they could choose to go left or right, but only one direction led to food. The scientists eavesdropped on their test rats’ brains using implanted electrodes.

As expected, the animals’ place cells fired as they ran through 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 seemed to be simulating alternatives, Redish concluded, projecting themselves into different futures to decide where to go next.

As with rats, our own hippocampus seems to be crucial to the power of foresight. When people think about their future, the hippocampus is part of the network that becomes active. But our ability to think about the future goes far beyond a rodent’s. We travel forward into a social future as well, in which we predict how people will react to the things we do.

Scientists do not know when this sophisticated kind of time travel emerged. Some intriguing stories suggest the transition started with our primate ancestors. For example, a recently published paper describes a chimpanzee in a zoo in Sweden that kept flinging rocks at human visitors. Before the zoo opened each day, the chimp collected a pile of rocks, as if stockpiling ammunition for his later attacks. 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 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, may have changed how we traveled through time. With language, we could 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. 

[This article originally appeared in print as "Remembering the Future."]

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