The brain’s ability to perceive space expands like the universe
Abstract: Time spent in a new environment causes neural representations to grow in surprising ways.
Source: Salk Institute
Young children sometimes believe that the moon follows them or that they can reach out and touch it. It appears to be much closer than is proportional to its actual distance. As we move in our daily lives, we tend to think that we are moving through space in a linear fashion.
But the Salk scientists found that time spent exploring the environment causes neural representations to grow in surprising ways.
The findings, published in The neuroscience of nature Dec. 29, 2022 show that neurons in the hippocampus essential for spatial navigation, memory, and planning represent space in a way that conforms to nonlinear hyperbolic geometry—a three-dimensional expanse that grows exponentially outward. (In other words, it has the shape of the inside of an expanding hourglass.)
The researchers also found that the size of this space increases with time spent in a place. And the size increases in a logarithmic manner corresponding 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 shows that the brain does not always work linearly. Instead, neural networks function along a growing curve, which can be analyzed and understood using hyperbolic geometry and information theory,” says Salk Professor Tatyana Sharpee, the Edwin K. Hunter Chair, who led the study.
“It’s 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 was there even for minute deviations in the 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 have recently pioneered the use of hyperbolic geometry to better understand biological signals such as odorant molecules as well as odor perception.
In the current study, the researchers found that hyperbolic geometry also guides neural responses. Hyperbolic maps of sensory molecules and events are perceived by hyperbolic neural maps.
The spatial representations dynamically expanded in correlation with the amount of time the rat spent exploring each environment. And when the rat moved more slowly through the environment, it received more information about the space, which caused an even greater increase in neural representations.
“These findings provide a new perspective on how neural representations can change 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 the cosmic level, but that’s not true,” says Sharpee.
“Our brains work much slower than the speed of light, which could be the reason why hyperbolic effects are observed in tangible spaces instead of astronomical ones. 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 are P. Dylan Rich of Princeton University and Albert K. Lee of the Janelia Research Campus of the Howard Hughes Medical Institute.
About this news about spatial perception research
Original research: Open access.
“Hippocampal spatial representations exhibit a hyperbolic geometry that expands with experience” Huanqiu Zhang et al. The neuroscience of nature
Hippocampal spatial representations exhibit a hyperbolic geometry that expands with experience
Everyday experience shows that we perceive distances in our vicinity linearly. 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 nonlinear hyperbolic geometry. This geometry uses an exponential scale and provides more position information than a linear scale.
We found that the display size corresponded to the optimal predictions for the number of CA1 neurons. The displays 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 received. Dynamic changes followed even small variations due to changes in the animal’s running speed.
These results demonstrate how neural circuits achieve efficient representations using dynamic hyperbolic geometry.