58 | SCIENTIFIC AMERICAN | SPECIAL EDITION | WINTER 2022
From observing Molaison, neuroscientists discerned that
the hippo campus was essential in forming the episodic memo-
ries that record facts and events. Research on the role of the
hippo campus in episodic memory exploded, largely in parallel
to studies on its maplike functions.
The discoveries about the roles of the hippo campus and ento-
rhinal cortex in spatial navigation and episodic memory were
significant for at least a couple of reasons. The work in spatial
navigation in rodents marked the first time that a higher-order
cognitive function—something beyond basic sensory process-
es—mapped onto clear neural correlates. H.M. showed us that
there were multiple types of memory supported by at least par-
tially different neural systems, with the hippo campus playing
a central role in the formation and storage of new episodic mem-
ories. These discoveries hinted that mechanisms of spatial and
temporal navigation might underlie episodic memory. This syn-
thesis is perhaps best ex plained by the theoretical construct pro-
posed decades earlier by Tolman; both episodic memory and
spatial navigation might reflect the brain’s formation and use of
cognitive maps.
Maps are not accurate portraits of the world in all of its com-
plexity. Rather they are representations of relations—distances
and directions between locations and what exists where. Maps
reduce a dizzying amount of real-world information into a sim-
ple, easily readable format that is useful for effective, flexible
navigation. The cell types mentioned earlier (place cells, grid
cells and border cells, among others) may piece together such
related elements into a mental map, which other brain regions
can then read out to guide “navigation,” amounting to adaptive
decision-making. Mapping allows relations to be inferred, even
when they have not been experienced. It also allows for mental
shortcuts that go beyond the purview of the spatial and tempo-
ral domains. In fact, reasoning using abstract concepts may
depend on some of these same neural foundations.
In one example of this new line of work, researchers Alexan-
dra Constantinescu, Jill O’Reilly and Timothy Behrens, all then
at the University of Oxford, asked participants to learn associa-
tions of different symbols with images of “stick” birds with var-
ious neck and leg lengths. A bird with a long neck but short
legs, for example, might be linked with the image of a bell,
whereas a bird with a short neck and long legs might be con-
nected to a teddy bear. These linkages created a two-dimension-
al association space. Despite neuroimaging being too crude to
detect actual grid cells in the human brain, imaging conducted
during the learned-association testing nonetheless revealed a
gridlike pattern of activation within the entorhinal cortex.
This finding builds on earlier work by Christian Doeller of
the Max Planck Institute for Human Cognitive and Brain Sci-
ence in Leipzig, Germany, and Neil Burgess of University Col-
lege London that first showed an entorhinal gridlike represen-
tation in humans navigating a virtual maze. For both physical
and abstract relations, the gridlike organization is highly effi-
cient. It makes the linkages of places or concepts more predict-
able, enhancing how quickly inferences can be made about
these relations. As in physical space, this organization of infor-
mation allows for inferring shortcuts—relations between ideas
or perhaps analogies, stereotypes and even some aspects of cre-
ativity itself could depend on such inferences.Hippocampus
(home of
place cells)Entorhinal cortex
(home of grid cells)Locations that
prompt place
cell firingLocations that
prompt grid
cell firingPhysical Navigation Social NavigationAffiliationMoreLessPowerClose DistantyxSelfCognitive Cartography
Is Physical and Social
The brain forms the idea of friend or foe by stitching together
diverse social characteristics from memories that track one’s
whereabouts. The recollections, research suggests, can then
be used to place an individual within a social hierarchy that
elucidates, say, where one stands in relation to others.PLACE AND GRID CELLS
Place cells pinpoint the animal’s whereabouts, each cell firing when
a particular spot on a mental map is reached. A grid cell activates
when an animal passes over the vertices of triangles superimposed
on a mental map. The triangles’ pattern of activation helps the animal
compute the direction and distance traveled along a route.MAKING THE LEAP TO SOCIAL MAPS
Go right at the corner and continue to your destination. Building
a map of physical surroundings is the work of place and grid
cells. But the brain may also use these cells for constructing maps
for social milieus: locating an acquaintance who grows closer but
loses power in a relationship.Sources: “Scientific Background: The Brain’s Navigational Place and Grid Cell System,” by Ole Kiehn and Hans Forssberg, with illustrations by Mattias Karlen.
Nobelprize.org; “Navigating Social Space,” by Matthew Schafer and Daniela Schiller, inNeuron,Vol. 100; October 24, 2018