The Brain’s Ability to Perceive Space Expands Like the Universe
Summary: Time spent in a new environment causes neural representations to grow in surprising ways.
Source: Salk Institute
Young children sometimes believe that the moon is following them, or that they can reach out and touch it. It appears to be much nearer than is commensurate with its true distance. As we move through our daily lives, we tend to think of ourselves as navigating through space in a linear fashion.
But Salk scientists have found that time spent exploring an environment causes neural representations to grow in surprising ways.
The findings, published in Nature Neuroscience on December 29, 2022, show that neurons in the hippocampus essential for spatial navigation, memory and planning represent space in a way that conforms to a nonlinear hyperbolic geometry—a three-dimensional space that grows outward in exponentially. . (In other words, it’s shaped like the inside of an expanding hourglass.)
The researchers also found that the size of that space increases with time spent in a place. And the size is increasing in a logarithmic fashion that matches the maximum possible increase in information being processed by the brain.
This discovery provides valuable methods for analyzing data on neurocognitive disorders involving learning and memory, such as Alzheimer’s disease.
“Our study shows that the brain does not always operate in a linear fashion. Instead, neural networks operate along an expanding curve that can be analyzed and understood using hyperbolic geometry and information theory,” says Salk Professor Tatyana Sharpee, holder of the Edwin K. Hunter Chair, who led the study.
“It is exciting to see that the neural responses in this area of the brain formed a map that expanded with experience based on the amount of time spent in a particular location. The effect even held for small deviations in time when the animal ran slower or faster through the environment.”
Sharpee’s lab uses advanced computational approaches to better understand how the brain works. They recently pioneered the use of hyperbolic geometry to better understand biological signals such as olfactory molecules, as well as olfactory perception.
In the current study, the scientists found that hyperbolic geometry also drives neural responses. Hyperbolic maps of sensory molecules and events are perceived with hyperbolic neural maps.
New experiences are absorbed into neural representations over time, symbolized here by a hyperboloid hourglass. Credit: Salk Institute
Spatial representations were dynamically expanded in correlation with the amount of time the mouse spent exploring each environment. And, when a mouse moved more slowly through an environment, it gained more information about the space, which caused the neural representations to grow even more.
“The findings provide a new perspective on how neural representations can be changed with experience,” says Huanqiu Zhang, a graduate student in Sharpee’s lab.
“The geometric principles identified in our study may also guide future efforts to understand neural activity in different brain systems.”
“You would think that hyperbolic geometry only applies on a cosmic scale, but that’s not true,” says Sharpee.
“Our brain works much slower than the speed of light, which may be one reason that hyperbolic effects are observed in tangible rather than astronomical spaces. Next, we would like to learn more about how these dynamic hyperbolic representations in the brain grow, interact and communicate with each other.”
Other authors include P. Dylan Rich of Princeton University and Albert K. Lee of the Janelia Research Campus at the Howard Hughes Medical Institute.
About this news about spatial perception research
Author: Press Office
Source: Salk Institute
Contact: Press Office – Salk Institute
Image: Image credited to the Salk Institute
Original Research: Open Access.
“Hippocampal spatial representations exhibit a hyperbolic geometry that expands with experience” by Huanqiu Zhang et al. Nature Neuroscience
Hippocampal spatial representations exhibit a hyperbolic geometry that expands with experience
Everyday experience suggests that we perceive distances near us in a linear fashion. However, the actual geometry of spatial representation in the brain is unknown.
Here we report that neurons in the CA1 region of the rat hippocampus that mediate spatial perception represent space according to a nonlinear hyperbolic geometry. This geometry uses an exponential scale and provides greater positional information than a linear scale.
We found that the size of the representation matches the optimal predictions for the number of CA1 neurons. The representations also expanded dynamically in proportion to the logarithm of the time the animal spent exploring the environment, consistent with the maximum mutual information that could be obtained. Dynamic changes also tracked small changes due to changes in the animal’s running speed.
These results show how neural circuits achieve efficient representations using dynamic hyperbolic geometry.