Navigating Our Thoughts: Fundamental Principles of Thinking

It is one of the most fundamental
questions in neuroscience: How do humans think? Until recently, we
seemed far from a conclusive answer. However, scientists from the
Max-Planck-Institute for Human Cognitive and Brain Sciences (MPI CBS) in
Leipzig, Germany, have combined the available evidence, in
collaboration with Nobel Prize laureate Edvard Moser from the Kavli
Institute for Systems Neuroscience in Trondheim, Norway and Peter
Gärdenfors from Lund University in Sweden, to paint a new, comprehensive
picture in the journal Science: Humans think using
their brain’s navigation system.

When we navigate our environment, two important cell types are active
in our brain. Place cells in the hippocampus and grid cells in the
neighboring entorhinal cortex form a circuit that allows orientation and
navigation. The team of scientists suggests that our inner navigation
system does much more. They propose that this system is also key to
‘thinking’, explaining why our knowledge seems to be organized in a
spatial fashion.

“We believe that the brain stores information about our surroundings
in so-called cognitive spaces. This concerns not only geographical data,
but also relationships between objects and experience,” explains
Christian Doeller, senior author of the paper and the new director at
the MPI CBS.

The term ‘cognitive spaces’ refers to mental maps in which we arrange
our experience. Everything that we encounter has physical properties,
whether a person or an object, and can therefore be arranged along
different dimensions. “If I think about cars, I can order them based on
their engine power and weight for example. We would have racing cars
with strong engines and low weights as well as caravans with weak
engines and high weight, as well as all combinations in between,” says
Doeller. “We can think about our family and friends in a similar way;
for example, on the basis of their height, humor, or income, coding them
as tall or short, humorous or humorless, or more or less wealthy.”
Depending on the dimensions of interest individuals might be stored
mentally closer together or further away.

A Theory of Human Thinking

In their proposal, Doeller and his team combine individual threads of
evidence to form a theory of human thinking. The theory begins with the
Nobel Prize-winning discoveries of place and grid cells in rodents’
brains, which were subsequently shown to exist in humans. Both cell
types show patterns of activity representing the animal’s position in
space, for example, while it forages for food. Each position in space is
represented by a unique pattern of activity. Together, the activity of
place and grid cells allows the formation of a mental map of the
surroundings, which is stored and reactivated during later visits.

The very regular activation pattern of grid cells can also be
observed in humans—but importantly, not only during navigation through
geographical spaces. Grids cells are also active when learning new
concepts, as shown by a study from 2016. In that study, volunteers
learned to associate pictures of birds, which only varied in the length
of their necks and legs, with different symbols, such as a tree or a
bell. A bird with a long neck and short legs was associated with the
tree whereas a bird with a short neck and long legs belonged to the
bell. Thus, a specific combination of bodily features came to be
represented by a symbol.

In a subsequent memory test, performed in a brain scanner, volunteers
indicated whether various birds were associated with one of the
symbols. Interestingly, the entorhinal cortex was activated, in much the
same way as it is during navigation, providing a coordinate system for
our thoughts.

“By connecting all these previous discoveries, we came to the
assumption that the brain stores a mental map, regardless of whether we
are thinking about a real space or the space between dimensions of our
thoughts. Our train of thought can be considered a path though the
spaces of our thoughts, along different mental dimensions,” Jacob
Bellmund, the first author of the publication, explains.

Mapping New Experience

“These processes are especially useful for making inferences about
new objects or situations, even if we have never experienced them,” the
neuroscientist continues. Using existing maps of cognitive spaces humans
can anticipate how similar something new is to something they already
know by putting it in relation to existing dimensions. If they’ve
already experienced tigers, lions, or panthers, but have never seen a
leopard, we would place the leopard in a similar position as the other
big cats in our cognitive space. Based on our knowledge about the
concept ‘big cat’, already stored in a mental map, we can adequately
react to the encounter with the leopard. “We can generalize to novel
situations, which we constantly face, and infer how we should behave”,
says Bellmund.

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