SCIENTIFICAMERICAN.COM | 57locate oneself in space and determine distance and direction.
These navigational tools are crucial for building mental maps.
(O’Keefe and the Mosers received the 2014 Nobel Prize in Medi-
cine or Physiology for their work on place and grid cells.)
A wide variety of information is useful for creating such a
map, and the hippo campus-entorhinal system encodes much of
it. Discovering the location of a physical goal is one example: as
an animal navigates toward an objective, some hippocampal neu-
rons fire depending on the direction and distance to reach it. The
cells increase their firing rate as the animal approaches the goal.
Other cells also enter the picture. A dedicated population of
“reward” cells encodes reward locations across different envi-
ronments, providing a signal to guide an animal’s navigation
(think of an “X” marking the spot of treasure on a pirate’s map).
Other cells track speed and direction and in doing so act like
internal speedometers and compasses that compute an ani-
mal’s progress as it travels through the environment. Specific
cells that signal the locations of landmarks in the surroundings
serve as references to correct errors in the animal’s trajectory. A
map must also have edges: cells that fire more as the animal
approaches the map’s perimeter.
For humans, the importance of such an abundance of cell types
seems obvious: the brain is responsible for knowing the location
of home and work, walls and dead ends, a favorite shop or the cor-
ner store. It is still a mystery as to how all of this information is
drawn together into a coherent map, but these cells appear to pro-
vide the parts list for the elements of neural mapmaking.
This hippo campal-entorhinal system is more than a map-
maker, though, and the maps are more than a way to locate one-
self in space. These maps also are used for active planning. When
a rat comes to a junction in a familiar maze, it will pause while
place cell firing sequences that relate to the different options are
activated, as if the animal is contemplating the choices.
Humans engage similar processes. Research in participants
navigating virtual environments while their brains were
scanned with functional magnetic resonance imaging shows
that the hippo campus becomes active in ways consistent with
spatial planning, such as considering and planning routes.Shaping plans also occurs during sleep. Sequences of place
cell activity can be reactivated during sleep to replay the past or
simulate the future. Without the ability to simulate new behav-
iors, we would have to explore a multitude of real-world options
before deciding on what action to take. We would be constant
empiricists, able to act only on direct observations. Instead off-
line simulations give us the ability to envision possibilities
without directly experiencing them.MENTAL TIME TRAVEL
time and space are inextricably linked. It is difficult to talk
about time without borrowing a spatial metaphor: time “passes”
as we “move” through it. We look “forward” to the future and
“back” on our past. The same hippocampal-entorhinal system
tracks move ment through time. Work done largely in the lab of
the late Howard Eichenbaum of Boston University re vealed neu-
rons in the hippocampal-entorhinal system that encode the
time course of an animal’s experience. Time cells fire at succes-
sive moments but do not track time in a simple clocklike fash-
ion. Instead they mark temporal context—stretching or shrink-
ing their firing durations if the length of a task changes, for
example. Some time cells encode space as well. In the brain, in
fact, physical and temporal space may be bound together.
The discovery of the crucial importance of these brain areas
in space and time was not totally surprising. Psychologists had
long suspected it to be the case. In 1953 Henry Molaison under-
went bilateral hippocampal resection surgery to reduce ex -
treme, life-disrupting epileptic seizures. The surgery was success-
ful at quelling the seizures. But Molaison—known for decades
only as H.M.—became one of the most renowned cases in the
history of the brain sciences.
Molaison could remember most experiences from before
his surgery—people he knew and recollections from culture
and politics. But his ability to form such explicit memories
postsurgery was practically nonexistent. Even so, certain types
of learning and memory remained untouched: he could still
learn some new skills with enough practice. But his recollec-
tions of new people, facts and events were immediately lost.Social spaceAffiliationMoreLessPowerx Close DistantSelfPhysical space Social information in physical spaceyxyIncreasing AbstractionIllustrations by Jen ChristiansenGiving Way to the Abstract
Maps simplify the world by reducing an overwhelming amount of sensory and cognitive data into a format that can be used for
navigating physical space, pointing to shortcuts and detours to reach a destination faster. The organization of such maps—built on
the activity of cells dedicated to tracking both space and time—scales in the abstraction of what they represent: from the recognition
of another individual along the way to even a complex space that denotes social power and closeness to others.Source: “Navigating Social Space,” by Matthew Schafer and Daniela Schiller, in
Neuron,
Vol. 100; October 24, 2018