jects reported more vivid simulations
when imagining events that might occur
in the near future, or in a familiar setting.
Again, the brain regions used in thinking
about the future and remembering the
past were almost indistinguishable.
“We sat around scratching our heads
about this for a while,” McDermott says.
“Whatever we’re doing when we remember the past, the same things happen when
we envision the future.”
These results support the idea that
autobiographical information is used not
just to remember, but also to construct
simulations of future events. “Except now
you’re kind of recombining information
from the past and relating it in a new way
to think about what might happen in the
future,” Schacter says.
He and his group recently pinpointed
a region in the posterior hippocampus
used to construct and elaborate on past
and future events. That finding suggests
that region may be where memories are
“grabbed” during the construction process, Schacter and colleagues reported in
the journal Hippocampus in February.
Making a scene
Schacter and postdoctoral researcher
Donna Addis spelled out their “
constructive episodic simulation” hypothesis last
year in an essay in Nature . About that time,
another study emerged linking memory to
the future from Eleanor Maguire and colleagues at the Wellcome Trust Centre for
Neuroimaging at the University College
London. Maguire asked five amnesiacs to
imagine and then describe in-detail situations in commonplace settings, such as a
beach, pub and market. The patients were
also asked to describe plausible future
events, such as a Christmas party.
Despite being given cues to help probe
their memories, the patients were unable
to put together the elements of an imagined event. Instead of visualizing a single
scene in their mind, such as a crowded
beach filled with sunbathers, the patients
reported seeing just a collection of disjointed images, such as sand, water, people
and beach towels.
The patients seemed to lack a “
spatial context” in which to place the events,
Maguire says. “The events in your life hap-
we’re doing when
we remember the past,
the same things happen
when we envision
pen in a specific location — a store, at work
or in a certain room in your house,” she
explains. “These patients simply didn’t
have that kind of context to draw from.”
Maguire says those results show that
the hippocampus, in particular, plays a role
in helping humans bind together snippets
of events that allow construction of both
past and future scenes.
Working to tease out more detail on the
mechanism behind spatial context, Maguire and her colleagues recently went back
to the scanner. This time, the scientists
compared real and imagined experiences
to see what, if any, other brain areas were
activated under each type of scenario.
“Our argument was, if we compare
real memories with imagined, fictitious
experiences, it might allow us to identify
brain areas concerned with the self, with
mental time travel and with that feeling
that something actually, really happened,”
Maguire says. “Because real memories
have all of these qualities, while imagined
events don’t have any of these qualities.”
That study revealed that a core network of brain regions—including the
hippocampus, parahippocampal gyrus
and retrosplenial cortex — provides a basis
for mentally generating and maintaining
a complex and coherent scene in both real
and imagined events. This core network
of brain regions appears to underpin the
critical scene construction process in such
scenarios, Maguire says.
She proposes that this scene construction is a key part of retrieving past experiences in any memory process, including
navigation, planning for the future, daydreaming and mind wandering.
“We think scene construction underpins not just autobiographical and spatial
memory and imagination, but a whole
host of other critical cognitive functions,”
Maguire says. This includes semantic
memory, multisensory information,
short-term and long-term memory.
Her findings, published in December in
the Journal of Neuroscience , also revealed
brain areas that help distinguish real from
imagined experiences. Maguire says those
regions, including the cortex and the posterior cingulate cortex, appear to be co-opted into the scene construction network
when the subject recalls experiences that
have actually happened.
“Memories that happened to you, very
vivid memories, have something about
them that help you distinguish them from
imagined experiences or things that may
or may not happen in the future,” she says.
“If you take scene construction as the core,
you may need other processes on top of
that to give you a sense that this is a real
experience that happened to you.”
Imagining future applications
Maguire and her colleagues are continuing studies to track the neural basis
for scene construction. Other scientists
are looking at how the autobiographical
memory system contributes to memory
impairment in aging, depression, Alzheim-er’s disease and head injuries.
By understanding how this memory
system works — what each part does and
what each brain area contributes — scientists may be able to develop ways to treat
those with memory problems.
The ongoing studies may also provide new insights into other functions of
memory as it relates to future-oriented
thinking, such as planning, prediction and
remembering intentions, scientists say.
Despite the recent progress, Maguire says scientists are a long way off from
understanding how memory’s various
brain components talk to each other and
interact to simulate future events.
“We’re still at that stage of trying to
understand how this amazing thing happens.” s
Susan Gaidos is a freelance science writer
based in Maine.
s Daniel Schacter. The Seven Sins of
Memory: How the Mind Forgets and
Remembers. Houghton Mifflin, 2002.