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.
A follow-up study showed that sub-