Map of brain's memory hub reveals unexpected results

Australian scientists say they have made the most detailed map ever of the communication links between the hippocampus (the brain’s memory control centre) and the rest of the brain — and it may very well change how we think about human memory.

The hippocampus is a complex structure that resembles a seahorse and is tucked deep within the brain. As a vital component of the brain, it is important for memory formation and plays a key role in the transfer of memories from short-term to long-term storage. But it also plays a part in navigation, imagining fictitious or future experiences, creating mental imagery of scenes in the mind’s eye, and even in visual perception and decision making.

To generate their map, scientists from The University of Sydney relied on MRI scans from a neuroimaging database created for the Human Connectome Project (HCP), a research consortium led by the US National Institutes of Health. They processed the existing HCP data using tailored techniques that they developed, which allowed them to follow the connections from all corners of the brain to their termination points in the hippocampus — a milestone in the mapping of the human brain, which has since been published in the journal eLife.

“What we’ve done is take a much more detailed look at the white matter pathways, which are essentially the highways of communication between different areas of the brain,” said Dr Marshall Dalton, a Research Fellow at The University of Sydney. “And we developed a new approach that allowed us to map how the hippocampus connects with the cortical mantle, the outer layer of the brain, but in a very detailed way.

“What we’ve created is a highly detailed map of white matter pathways connecting the hippocampus with the rest of the brain. It’s essentially a roadmap of brain regions that directly connect with the hippocampus and support its important role in memory formation.”

Technical limitations inherent to previous MRI investigations of the human hippocampus meant it was only possible to visualise its connections in very broad terms. “But we have now developed a tailored method that allows us to confirm where within the hippocampus different cortical areas are connecting — and that hasn’t been done before in a living human brain,” Dalton said.

The researchers were delighted that their results largely aligned with data from previous studies from overseas, which had relied on post-mortem studies of primate brains. However, they found that the number of connections between the hippocampus and some brain areas was either much lower or higher than expected.

“We were surprised to find fewer connections between the hippocampus and frontal cortical areas, and more connections with early visual processing areas, than we expected to see,” Dalton said. “Although, this makes sense considering the hippocampus plays an important role not only in memory but also imagination and our ability to construct mental images in our mind’s eye.”

These differences in connectivity could indicate that although some pathways were conserved as humans evolved, human brains may also have developed unique patterns of connectivity different from other primates. They may, for example, help explain why some of our primate cousins — especially chimpanzees — are better at some memory tasks than humans, especially those relying on short-term memory. Chimpanzees have bested humans at cognitive tasks involving a form of mathematics known as game theory, which relies on short-term memory, pattern recognition and rapid visual assessment.

“Although we have achieved this high-resolution mapping of the human hippocampus, the tract tracing method conducted on non-human primates — which can see down to the cellular level — is able to see more connections than can be discerned with an MRI,” Dalton said.

“Or it could be that the human hippocampus really does have a smaller number of connections with frontal areas than we expect, and greater connectivity with visual areas of the brain. As the neocortex expanded, perhaps humans evolved different patterns of connectivity to facilitate human-specific memory and visualisation functions which, in turn, may underpin human creativity.

“It’s a bit of a puzzle — we just don’t know. But we love puzzles and will keep investigating.”

Image credit: iStock.com/solarseven

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