The ability to conjure up possible futures or alternative realities is
the flip side of memory. Both faculties cohabit in the brain region
called the hippocampus
June 8, 2023
Henry Molaison, known for
years as “H.M.,” was famously unable to form new memories. If someone he had
met left the room only to return several minutes later, he would greet that
person again as if for the first time. Because of surgery to treat intractable
epilepsy, H M. lacked a sea-horse-shaped brain structure called the hippocampus
and had amnesia. His case helped establish the hippocampus as an engine of
memory.
In recent years scientists
have discovered another essential deficit that burdens people with hippocampal
amnesia: they can’t envision the range of possibilities that must be considered
to make future plans. When researchers asked a group of people with hippocampal
damage to describe themselves in a fictitious scene—say, lying on a white sandy
beach—they came up largely blank, producing only fragmented images. Brain scans
of healthy people, by contrast, showed that their hippocampus was engaged even
more when they imagined the future than when they summoned the past.
Studies of neural activity
in rats have since come along to support the idea that the hippocampus plays a
central role in imagination. “It’s still responsible for creating memories of
what is happening right now,” says Loren Frank, a systems neuroscientist at the
Howard Hughes Medical Institute and the University of California, San
Francisco. “And now it seems it is also responsible for rolling out
possibilities.” Frank and his colleagues make their case in a paper entitled
“Imagination as a Fundamental Function of the Hippocampus,” which was published
in the Philosophical Transactions of the Royal Society B.
That dual role makes sense,
experts say, in part because imagination depends largely, if not exclusively,
on memory. “Why do we talk about imagination separately from memory? From the
public point of view [talking about them together] is a crazy idea. But you can
put it in a simple way: there is absolutely no way you can imagine anything
without the past,” says György Buzsáki, a systems neuroscientist at New York
University, who was not involved with the paper.
In addition, both skills
involve essentially the same process: combining bits and pieces of experience
with emotions, inner commentary and things people have read or heard about,
says Donna Rose Addis, a cognitive neuroscientist at the Rotman Research
Institute in Toronto and the University of Toronto, who was also not involved
with the recent review. This process can even distort memories by mixing them
with imaginary material. “Memory is a form of imagination,” Addis says.
From Frank’s point of view,
imagination gives memory a purpose: helping us make decisions based on what
we’ve learned—for instance, deciding to avoid a food that once made us sick.
“From an evolutionary perspective, we are reasonably sure that the purpose of
memories is actually in the future,” Frank says. “Memories allow you to take
experiences that you have and retrieve them to make predictions about what will
happen next.” This chain of neural events even loops back on itself. We also
need to form memories of our simulations of the future so that when we have an
experience, we have something to draw on. “We have found that the encoding of
an imagined simulation also involves the hippocampus,” Addis says.
Much of the recent evidence
for imagination’s roots in the brain draws on a Nobel Prize–winning discovery
in the 1970s of “place cells” in the hippocampus. When a rat runs a maze, the
activity of these cells changes in a predictable way based on the animal’s
position in the maze. These hippocampal cells tell an animal where it is in the
world. This function seemed distinct from imagination until Frank and his
colleagues showed that the activity of these cells does not always represent an
animal’s actual location.
The firing patterns of
place cells repeat about eight times per second in rats, forming what is called
the theta rhythm. And within each cycle, the researchers found, the patterns
progressively change to represent three different locations for the animal that
are separated in time: the place it just was, its current position and, late in
the cycle, a possible upcoming location. “The neural activity has this
unmistakable structure where, at certain time points, it looks like what the
animal is experiencing in the present. At these other time points, it looks
like an imaginary experience,” says Kenneth Kay, a postdoctoral researcher at the Mortimer B. Zuckerman Mind Brain
Behavior Institute at Columbia University and a co-author of the paper. (The
paper’s first author is Frank’s graduate student, Alison Comrie.)
What a rat seems to be
imagining in any given cycle varies. When the rat is approaching a T-junction
in a maze, the late theta activity alternates between two possible immediate
futures: a turn to the left in one cycle and a turn to the right in the next.
It’s as if the animal is planning its next move, akin to a soccer player who is
running toward a ball and flipping through various scenarios before deciding on
a play.
In other instances, that
late theta activity denotes a more distant place in the maze, as if the
animals’ mind wandered to some other scene or scenario, perhaps some place it
would rather be. The researchers also found instances in which this imagination
portion of the cycle reflected a hypothetical direction of travel that differed
from the animal’s actual directional heading. “They are representing things
that can roughly be thought of as possibilities or hypotheticals, things that
could be but aren’t necessarily the case in terms of a possible future or just
an alternative reality,” Frank says.
The mere existence of
spontaneous activity within the hippocampus that is not necessarily tied to a
specific place, some experts say, hints at an internal thought process that is
separated from reality. “That rhythmicity [of the theta wave] is not coming
from the environment,” Kay says. “That’s highly reminiscent of the notion that
our imaginings are coming from ourselves, and they are not from this external
reality.”
Another form of imagination
seems to occur when an animal isn’t traveling through space but is eating,
grooming or zoning out. At these times, scientists have detected bursts of
activity in the hippocampus called “sharp wave ripples,” which also occur
during sleep, that seem to represent mental replays of past events. The replays
occur about 10 times faster than the original event, a reenactment that is
reminiscent of human experience. “One huge advantage of using our minds to
think about things sometimes is: we can quickly play through things, we can
quickly simulate them,” Kay says.
While these mental replays
are a form of recollection, they can also represent events the animal has not
experienced, Frank says. Some sharp wave ripples appear to connect two
trajectories that an animal had experienced separately but not together, he
says. The ripple activity may, in essence, build a mental map so that the
animal can mentally traverse new paths, such as shortcuts and detours. In this
context, the hippocampus seems to be acting to combine past events in new ways,
something that “is more like imagination than it is just replaying the past or predicting
the future,” says Lynn Nadel, an emeritus professor of cognitive science and
psychology at the University of Arizona, who did not contribute to the recent
paper.
The experiments of neuronal
activity in rodents are important, experts say, because they place the idea of
imagination into a physical reality: that of the brain itself. “This gives us
an opportunity to take a fuzzy cognitive concept like imagination” and link it
to brain activity, says Daphna Shohamy, a cognitive neuroscientist at Columbia
University, and director and CEO of Columbia's Zuckerman Institute, who was not
involved in those studies or the review paper.
Humans’ internal worlds are
rich, however, and the studies of place cells in rats may not represent all
types of human imagination. The animal results connect most directly with
imagination that is based in experience and action, as in planning out a
strategy for moving through the world, Nadel says. But other experts believe
the hippocampus has a much broader repertoire: it may also forge ties between
ideas and information. “I don’t think the hippocampus cares, really, about what
you’re connecting,” Addis says.
Some of Shohamy’s work
supports the idea that the hippocampus might be important for mental
simulations that are not rooted in time or place. She has found that people
with damage to the hippocampus are much slower than those without brain damage
to choose between food items—say, a Kit Kat versus M&Ms—that they like
about equally well. The problem seems to be that they have trouble imagining
what the options are like. “It looks as if they are spending more time trying
to conjure up the evidence,” Shohamy says. In the end, they make a choice at
random.
Although the hippocampus
may play a central role in imagination, it is by no means performing a solo
act. It needs the cooperation of other brain areas. Frank likens the
hippocampus to an orchestra conductor that cues up neurons in other regions
that represent the sights, sounds and smells that either are part of a
recollection or “fit together in some imagined thing.”
One mystery is how people
separate a real symphony from music playing in their head. “It’s amazing that
we’re not all psychotic all the time, that we’re not all delusional, because
our brains are clearly making stuff up a lot of the time about things that
could be,” Frank says. New data from Frank’s group suggest the brain may use
sensory input—say, the feeling of a foot hitting the ground while walking—to
flag what is real versus what is just in the mind’s eye and so ground this hive
of neural activity in the physical world. The brain, he says, separates fact
from fiction by reconciling the information it receives from the outside world
with its own internal models.
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