Summary: Researchers propose a new theory of human thinking, suggesting our brain’s navigation system is key to thinking. This may explain why our knowledge seems to be organized in spatial fashion.
Source: Max Planck Institute.
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, and the Kavli Institute for Systems Neuroscience in Trondheim, Norway, among them Nobel prize laureate Edvard I. Moser, offer a new proposal in the current issue of 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.
Source: Jacob Bellmund – Max Planck Institute
Publisher: Organized by NeuroscienceNews.com.
Image Source: NeuroscienceNews.com image is credited to Ella Maru Studio & MPI CBS.
Original Research: Abstract for “Navigating cognition: Spatial codes for human thinking” by Jacob L. S. Bellmund, Peter Gärdenfors, Edvard I. Moser, and Christian F. Doeller in Science. Published November 9 2018.
Navigating cognition: Spatial codes for human thinking
Ever since Edward Tolman’s proposal that comprehensive cognitive maps underlie spatial navigation and, more generally, psychological functions, the question of how past experience guides behavior has been contentious. The discovery of place cells in rodents, signaling the animal’s position in space, suggested that such cognitive maps reside in the hippocampus, a core brain region for human memory. Building on the description of place cells, several other functionally defined cell types were discovered in the hippocampal-entorhinal region. Among them are grid cells in the entorhinal cortex, whose characteristic periodic, six-fold symmetric firing patterns are thought to provide a spatial metric. These findings were complemented by insights into key coding principles of the hippocampal-entorhinal region: Spatial representations vary in scale along the hippocampal long axis, place cells remap to map different environments, and sequential hippocampal activity represents nonlocal trajectories through space. In humans, the existence of spatially tuned cells has been demonstrated in presurgical patients, and functional magnetic resonance imaging provides proxy measures for the noninvasive investigation of these processing mechanisms in human cognition. Intriguingly, recent advances indicate that place and grid cells can encode positions along dimensions of experience beyond Euclidean space for navigation, suggesting a more general role of hippocampal-entorhinal processing mechanisms in cognition.
We combine hippocampal-entorhinal processing mechanisms identified in spatial navigation research with ideas from cognitive science describing a spatial representational format for cognition. Cognitive spaces are spanned by dimensions satisfying geometric constraints such as betweenness and equidistance, enabling the representation of properties and concepts as convex regions of cognitive space. We propose that the continuous population code of place and grid cells in the hippocampal-entorhinal region maps the dimensions of cognitive spaces. In these, each stimulus is located according to its feature values along the relevant dimensions, resulting in nearby positions for similar stimuli and larger distances between dissimilar stimuli. The low-dimensional, rigid firing properties of the entorhinal grid system make it a candidate to provide a metric or distance code for cognitive spaces, whereas hippocampal place cells flexibly represent positions in a given space. This mapping of cognitive spaces is complemented by the additional coding principles outlined above: Along the hippocampal long axis, cognitive spaces are mapped with varying spatial scale, supporting memory and knowledge representations at different levels of granularity. Via hippocampal remapping, spaces spanned by different dimensions can be flexibly mapped and established maps can be reinstated via attractor dynamics. The geometric definition of cognitive spaces allows flexible generalization and inference, and sequential hippocampal activity can simulate trajectories through cognitive spaces for adaptive decision-making and behavior.
Cognitive spaces provide a domain-general format for processing in the hippocampal-entorhinal region, in line with its involvement beyond navigation and memory. Spatial navigation serves as a model system to identify key coding principles governing cognitive spaces. An important question concerns the extent to which firing properties of spatially tuned cells are preserved in cognitive spaces. Technological advances such as calcium imaging will clarify coding principles on the population level and facilitate the translation to human cognitive neuroscience. Spatial navigation is mostly investigated in two dimensions and naturally limited to three dimensions; however, the processing of complex, multidimensional concepts is vital to high-level human cognition, and the representation of such high-dimensional spaces is an intriguing question for future research. Further, the role of brain networks acting in concert with the hippocampus, in navigation specifically and cognitive function in general, will provide insight into whether and how cognitive spaces are supported beyond the hippocampal-entorhinal region. Finally, the precise way in which cognitive spaces and trajectories through them are read out for behavior remains to be elucidated.