One day not long ago a 27-year-old woman was brought to the
Tel Aviv Sourasky Medical Center, sleepy and confused. Fani Andelman, a neuropsychologist at the center, and colleagues gave the woman a battery of psychological tests to judge her state of mind. At first the woman seemed fine. She could see and speak clearly. She could understand the meaning of words and recall 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 still 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 just lost 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.
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 had memories of 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. Over the past decade, as scientists have begun to use fMRI 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 lie in an fMRI scanner and 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 the past and future, but not the present.
Studies on children also lend support to Tulving’s time travel hypothesis. Previous work had shown that around the age of 4, children start to develop a strong episodic memory. Thomas Sud dendorf, a psychologist at the University of Queensland in Australia, designed a series of experiments to see if foresight develops with the same timing. In one experiment, published earlier this year, 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 then swapped the box for one equipped with a square lock and gave the children three different keys. Most of the 96 subjects correctly picked the square key, regardless of their age.
Then Suddendorf ran the experiment again, but with a twist to test the children’s foresight. Instead of choosing a key for the square lock right away, the kids were first taken to another room to play for 15 minutes; only after that were they offered a choice of keys, which they had to take back to the room with the box. The children had to anticipate what would happen when they tried to unlock it. This time Suddendorf found a sharp break between the 3-year-olds and the 4-year-olds. The younger kids were just as likely to pick one of the wrong keys as the right one. The older kids did much better--probably because, with more developed episodic memories, they remembered the square lock and used that knowledge to project into a future in which only a square key would unlock the box.
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.
Copyright 2011 Carl Zimmer