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Learning Expands the Brain’s Capacity to Store Information

Learning Expands the Brain’s Capacity to Store Information
Kristen Harris and her team used an electron microscope to make 3D images of brain structures like this one to understand how learning alters the structures. They discovered that learning causes some synapses (red) to grow and others to shrink, leading to an increase in their capacity to store information. In this image, axons (green) carrying signals from multiple brain cells connect via synapses to the shaft-like input of a single brain cell, called a dendrite (yellow). Credit: Univ. of Texas at Austin.

The act of learning causes connections between brain cells, called synapses, to expand their capacity to store information, according to a new discovery from neuroscientists at The University of Texas at Austin, the Salk Institute for Biological Sciences and The University of Otago in New Zealand.

When you use a flash drive to store digital files, you can add or remove bits of information, but the total capacity of the drive is fixed at the factory. The brain appears to be much more flexible, adding new room for information as it's needed.

The researchers were surprised by the way in which the storage capacity of synapses changed. In a region of the brain necessary for long-term memory, called the hippocampal dentate gyrus, they discovered that learning causes some synapses to grow and others to shrink, creating a wider range of synapse sizes. The size of a synapse correlates with the strength of signals it can transmit between brain cells and the amount of information it can store.

"We expected all the synapses to get bigger," said Kristen Harris, professor of neuroscience at UT Austin and senior author of a new study appearing this week in the Proceedings of the National Academy of Sciences. "Instead, there remains a balance. Some synapses had to shrink in order for others to grow, all within the same region that 'learned.'"

Harris said that keeping the overall signal strength the same during learning, but spread over a wider range of synapse sizes, might help explain never-before-answered questions about how connections in the brain work together to support memory.

"This paper provides new insights into how new experiences can be stored in the brain while maintaining overall stability," Harris said.

Knowing the storage capacity in different brain regions and how it responds to learning and memory might help in understanding what goes wrong when memories fail to form, as can occur in many diseases, such as Alzheimer's.

The researchers implanted electrodes in the brains of living rats and stimulated them with painless electrical pulses designed to mimic the effects of learning. Learning reshapes structures in the brain through a process called long-term potentiation, or LTP. Sections of the brains were later imaged using electron microscopy and analyzed with 3D reconstruction tools developed in Harris' lab. To estimate the information storage capacity of various brain structures, the researchers analyzed the data using Information Theory—the same powerful tool that underlies digital technologies, including the Internet and cell phones, and is used to study everything from black holes to human language.

Using these same tools for a 2015 study, Harris and her colleagues estimated that the human brain has a much higher capacity for storing information than previously thought.

Comparing results of the two studies, the team discovered that two different parts of the hippocampus—the dentate gyrus and an area called CA1—have different capacities for storing information. Harris said this makes sense given that the two regions play different roles in learning and memory.

Last year, the National Science Foundation awarded a $9 million grant to Harris, Terrence Sejnowski at the Salk Institute and James Carson at UT Austin's Texas Advanced Computing Center to explore the brain in microscopic detail and understand the cell biology of the nervous system. Over the course of the next five years, they will improve technologies for synapse reconstruction to investigate information storage capacity in other brain regions in both rodent and human brains.

"We hope to explore many additional questions such as whether the increase in information storage is accompanied by a compensatory decrease in information storage capacity in adjoining layers, and how long the temporary increase in storage capacity at particular synapses lasts," said Cailey Bromer, postdoctoral fellow at the Salk Institute and first author of the study published this week.

The study's other co-authors at UT Austin are Jared Bowden, Dusten Hubbard, Dakota Hanka, Paola Gonzalez, Masaaki Kuwajima, John Mendenhall and Patrick Parker. Other co-authors at the Salk Institute are Thomas Bartol and Terrence Sejnowski. The paper's author at The University of Otago is Wickliffe Abraham.

The work was supported by the National Institutes of Health, the National Science Foundation, the Howard Hughes Medical Institute and the Texas Emerging Technologies Fund.

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Saturday, 26 May 2018

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