they learned the word pairs, the researchers found that the faster a per-
son’s neural activity changed during the learning task, the better they
performed on memory recall later on.^11 Electrical stimulation of partic-
ipants’ brains during the learning task didn’t have a consistent effect on
the rate of activity change, Zaghloul adds, “but when it made it faster,
people tended to do better at remembering the word pairs, and when
it made it slower... people tended to do worse,” he says. The findings,
published last year, suggest “that this representation of time does play
a role in your ability to lump or distinguish memories,” he says.
That the sense of time in episodic memory might be dependent
on neural activity rather than on a traditional clock reinforces some
researchers’ belief that the brain perceives time rather differently
from how people imagine it to. Buonomano and New York Uni-
versity neuroscientist György Buzsáki have independently argued
before and since Tsao’s work that neuroscientists should rely less
on preconceived notions of time and instead think more about
how time-related information might be used by the brain. “The
sole function of memory is to allow animals to better prepare for the
future,” says Buonomano. “Sometimes the field forgets that detail.”
Thinking ahead
Tsao is still studying the brains of rats as a postdoc at Stanford
University, although his focus has shifted to other topics in neuro-
science. But for other researchers studying the representation of
time in episodic memory, the work has only just begun.
May-Britt Moser says her group is continuing the line of
research Tsao started, exploring how the hippocampus in rats inte-
grates temporal and spatial information from the LEC and MEC
during memory formation.^12 The idea’s been around for a while.
Several years ago, Eichenbaum and colleagues reported that rats’
time cells seem sensitive to spatial as well as temporal information.
More-recent research has complicated the story further, identifying
time cells outside the hippocampus, and finding that some place
cells seem to respond to time-related signals from the LEC, leading
some neuroscientists to propose that the hippocampus possesses
different time-tracking systems for different timescales.
To Howard, one of several theoreticians who has modeled how
the brain might combine signals encoding the when and where
of episodic memories, the blurred boundary between space and
time is intuitive. Having originally trained in physics, he says, “I
was pretty sure that the brain’s representation of space and the
brain’s representation of time ought to obey the same equations,”
whatever the scale. He and many other neuroscientists are now
working under the assumption that the brain uses a unified rep-
resentation of space and time in remembered experiences. And
at least for some aspects of memory, Howard says, “I think that’s
the story that’s starting to unfold now.”
While Tsao’s work focused on how time is encoded during
memory formation, some groups are working on the other side of
the coin: what happens during the process of memory retrieval.
Researchers at the University of California, Irvine, recently
reported that people who showed higher LEC activity during
a memory retrieval task were better at recalling when specific
events in a sequence happened, supporting a role for the LEC in
a sense of time during memory retrieval as well as formation.^13
Zaghloul, Howard, and others, meanwhile, have independently
published work showing that when people successfully recall
memories, they seem to reinstate the activity patterns in the
medial temporal lobe—a region that includes the hippocampus
and the entorhinal cortex—that were present when that mem-
ory was formed. It’s an effect, notes Zaghloul, that’s thought to
allow a sort of “jump back in time” on recalling a memory.14,15
Such an ability to reinstate past activity patterns could have
applications to the brain’s representation of events that haven’t yet
happened, too, Howard says. “It occurred to us quite a while ago
that if the brain has equations of the past, you could construct an
estimate of the future with the same types of properties.” Empirical
data to test the idea are lacking for now. One of the first things to
do will be to figure out how the brain could skip back or forward
to different activity states, because “we don’t currently have algo-
rithms that can do that,” Howard notes. “We’re actively working
on... figuring out a set of equations to describe the how of jump-
ing back in time. Actually, I’m looking at my chalkboard right now,
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