Science News - June 21, 2008 Sample

kind of autobiographical memory exists precisely for this purpose.

“It’s not entirely clear what we have memory for in the first place,” says psychologist Kathleen McDermott of Washington University in St. Louis. “The idea of sitting around reminiscing about the peanuts we ate yesterday just doesn’t seem to have a clear and compelling adaptive value.”

But if it’s the flip side of an ability to envision and so better negotiate an unknown future, then memory’s evolutionary usefulness suddenly becomes clear, McDermott says.

Harvard psychologist Daniel Schacter agrees, noting that this reverse role of memory may help explain why the human memory system is designed the way it is. Remembering personal experiences from the past and envisioning the future draw upon many of the same neural mechanisms, says Schacter, who co-organized a session on the topic in Boston at this year’s annual meeting of the American Association for the Advancement of Science.

Though current studies focus on episodic memory, or memories of events, times and places, Schacter says that other forms of memory such as semantic memory and generalized knowledge are no doubt also relevant to thinking about the future. “Episodic memory seems to be important when people think about their personal futures because it is the source of the details that allow one to build simulations of what might happen.”

Down memory lane

For more than a century, scientists studying memory have focused on its

role in preserving and recovering the past. Eventually, memory’s neurochemi-cal nuances were mapped mainly to the hippocampus and prefrontal cortex.

In the early 1980s, researchers identified additional regions used in planning and foresight. Studies of patients with brain lesions suggested that such patients struggled in these tasks as well as in remembering the past. About that time, psychologist Endel Tulving of the University of Toronto suspected that the mental powers enabling humans to remember episodes from the past, such as a disagreement with a client, also confer the ability to foresee possible futures, as in planning an upcoming meeting with that client.

Schacter, a graduate student in Tulving’s lab at the time, was struck by a patient known as K. C. A motorcycle accident had damaged K.C.’s hippocampus. Though he retained some general knowledge about the world, he couldn’t recall any past personal experiences. When asked what he would be doing next week, K.C. had just as much trouble responding. “It was kind of an informal observation, but it always struck me,” Schacter says.

Three years ago, while working on why memory can be unreliable, Schacter thought back to K.C.’s lapses and came up with a theory: Perhaps the overlap between memory and imagining could be explained by memory’s “ constructive” nature. Rather than pulling up a file, like a computer, human memory strings together all the bits and pieces — place, people, sounds — needed to re-create an episode. That flexible, piecemeal system can make memory errors, but it might pro-

Many of the same brain regions used to recall vivid memories (left) are engaged during the construction and elaboration of future events (right). These functional MRI images reveal extensive overlap in regions of the brain’s memory and planning networks.

vide a way to gather information from the past to prepare for future challenges.

At the time, only one imaging study had been done to examine the general brain regions common to both thinking ahead and remembering the past. By using a more systematic approach, Schacter reasoned, he might be able to pinpoint the components involved in both activities.

Using functional MRI to capture patterns of brain activation, Schacter tested his theory by examining the overlap between brain regions used in remembering and imagining. In the study, 14 participants described a series of events from their personal past involving common objects, such as a table. The same participants then envisioned a plausible future scenario with that object.

In the early stage of constructing an event, the left hippocampal region appeared equally active in remembering and imagining. The overlap was most apparent at the “elaboration phase,” when subjects gave details on the events. In addition, certain regions in the right hippocampus became active when subjects imagined a future event, but not when they remembered a past one. Schacter says these activations may reflect a process of recombining details from various past events into a new imaginary episode.

Letting imagination run

Schacter’s findings were confirmed last year when McDermott, with doctoral student Karl Szpunar, published another MRI study in the Proceedings of the National Academy of Sciences. In that study, participants were given a cue, such as a birthday party, and instructed to recall a personal event from their life and remember it as vividly as possible for 10 seconds. In a second condition, participants thought about a personal event and projected it into the future. They then envisioned a third event, placing a familiar individual (not themselves) in the future.

Brain activity patterns for past and future events looked a lot alike, but only when the participants imagined themselves in the future picture. When imagining a third party’s future, the same regions were busy but not to the same extent.