Summary: Time spent in a new environment increases neural representations in surprising ways.
Source: Salk Institute
Young children sometimes believe that the moon follows them or that they can reach out and touch it. He appears to be much closer than is proportional to his actual distance. As we move around in our daily lives, we tend to think of ourselves as navigating through space in a linear fashion.
But the Salk scientists found that time spent exploring an environment causes a surprising growth in neural representations.
The findings, published in Natural neuroscience on December 29, 2022, show that hippocampal neurons essential for spatial navigation, memory, and planning represent space in a manner consistent with nonlinear hyperbolic geometry – a three-dimensional expanse that grows exponentially . (In other words, it’s shaped like the inside of an expanding hourglass.)
The researchers also found that the size of this space increases with time spent in a location. And the size increases in a logarithmic way which corresponds to the maximum possible increase in information 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 demonstrates that the brain does not always act in a linear fashion. Instead, neural networks operate along an expanding curve, which can be analyzed and understood using hyperbolic geometry and information theory,” says Professor Salk Tatyana Sharpee, Edwin K. Hunter Professor, who led the study.
“It was exciting to see that the neural responses in this area of the brain formed a map that expanded with experience as a function of time spent in a given location. The effect held even for tiny deviations over time when the animal ran slower or faster in the environment.
Sharpee’s lab uses advanced computational approaches to better understand how the brain works. They have recently pioneered the use of hyperbolic geometry to better understand biological signals such as olfactory molecules, as well as the perception of smell.
In the current study, scientists found that hyperbolic geometry also guides neural responses. Hyperbolic maps of molecules and sensory events are perceived with hyperbolic neuronal maps.
Spatial representations developed dynamically in correlation with the time the rat spent exploring each environment. And, when a rat moved more slowly through an environment, it gained more information about the space, which further increased neural representations.
“The results offer 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 various brain systems.”
“You would think that hyperbolic geometry only applies on a cosmic scale, but that’s not true,” says Sharpee.
“Our brains run much slower than the speed of light, which could be one reason why hyperbolic effects are observed over graspable spaces instead of astronomical ones.” Next, we would like to know more about how these dynamic hyperbolic representations in the brain develop, interact and communicate with each other.
Other authors include P. Dylan Rich of Princeton University and Albert K. Lee of the Janelia Research Campus of the Howard Hughes Medical Institute.
About this research on spatial perception
Author: Press office
Source: Salk Institute
Contact: Press Office – Salk Institute
Picture: Image is credited to the Salk Institute
Original research: Free access.
“Hippocampal spatial representations exhibit hyperbolic geometry that expands with experience” by Huanqiu Zhang et al. Natural neuroscience
Spatial representations of the hippocampus exhibit 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 in a nonlinear hyperbolic geometry. This geometry uses an exponential scale and produces more positional information than a linear scale.
We found that the size of the representation corresponds to the optimal predictions for the number of CA1 neurons. The representations also grew dynamically in proportion to the logarithm of the time spent by the animal exploring the environment, corresponding to the maximum mutual information that could be received. Dynamic changes followed even small variations due to changes in the running speed of the animal.
These results demonstrate how neural circuits achieve efficient representations using dynamic hyperbolic geometry.
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