April 3, 2007
By CARL ZIMMER
Humans are born time travelers. We may not be able to send our bodies into the past or the future, at least not yet, but we can send our minds. We can relive events that happened long ago or envision ourselves in the future.
New studies suggest that the two directions of temporal travel are intimately entwined in the human brain. A number of psychologists argue that re-experiencing the past evolved in our ancestors as a way to plan for the future and that the rise of mental time travel was crucial to our species' success. But some experts on animal behavior do not think we are unique in this respect. They point to several recent experiments suggesting that animals can visit the past and future as well.
The first clues about the twists and turns of mental time travel came from people with certain brain injuries that caused them to forget autobiographical details without forgetting the information they had picked up along the way. A man known in the scientific literature as K.C., for instance, could play chess with no memory of having ever played it. K.C. could remember sentences psychologists taught him without any memory of the lessons.
K.C. had lost what psychologists now call episodic memory. Endel Tulving, a Canadian psychologist, defined episodic memory as the ability to recall the details of personal experiences: what happened, where it happened, when it happened and so on.
Dr. Tulving argued that episodic memory was distinct from other kinds of memory that did not involve personal experience. People can remember how to get to a subway stop, for example, without recalling the first time they were there.
Episodic memory was also unique to our species, Dr. Tulving maintained. For one thing, he argued that episodic memory required self-awareness. You can't remember yourself if you don't know you exist. He also argued that there was no evidence animals could recollect experiences, even if those experiences left an impression on them.
Many animal behavior experts agreed with Dr. Tulving, even though they had not actually run experiments testing the idea. But when Nicola Clayton, a comparative psychologist, first heard about the claim, she had a different reaction. "I could feel my feathers ruffling," said Dr. Clayton, who is now at the University of Cambridge. "I thought, hang on, that doesn't make sense."
Dr. Clayton began to test western scrub jays to see if they met any of the criteria for episodic memory. The jays can hide several thousand pieces of food each year and remember the location of each one. Dr. Clayton wondered if scrub jays simply remembered locations, or if they remembered the experience of hiding the food.
She ran an experiment using two kinds of food: moth larvae and peanuts. Scrub jays prefer larvae to peanuts while the larvae are still fresh. When the larvae are dead for a few hours, the jays prefer peanuts. Dr. Clayton gave the birds a chance to hide both kinds of food and then put them in another cage. She later returned the birds to their caches, in some cases after four hours and in other cases after five days.
The time the scrub jays spent away from their caches had a big effect on the type of food they looked for. The birds that waited four hours tended to dig up larvae, and the birds that had to wait for five days passed the larvae by and dug up peanuts instead. (To make sure they were not just picking up the smell of rotten larvae and avoiding those spots, Dr. Clayton dumped out the caches as soon as the birds had made them, and filled all of them with fresh sand.)
In 1998, Dr. Clayton and her colleagues published the results of their experiment, declaring that scrub jays met the standards for "episodic-like" memory. Ever since, Dr. Clayton has been investigating the memories of scrub jays more deeply. Last year, for example, her team discovered that scrub jays not only remember when and where they hide food, but also whether they are being watched at the time. If one scrub jay notices another one watching it hide food, it tends to dig up the cache later and hide it somewhere else. Other scientists have followed Dr. Clayton's lead and have searched for signs of episodic-like memory in other animals. When rats are exploring a maze, for example, they seem to be able to recall which kinds of food they encountered along the way. Hummingbirds seem to remember where and when they visited individual flowers for nectar. Rhesus monkeys can remember where they put food, but not how long ago they put it there.