The hippocampus is a crucial part of the brain that plays a role in memory and learning, especially in remembering directions and locations. New research from the University of Chicago shows how this small, curved structure reorganizes its activity depending on whether a situation matches people’s memories and expectations.
In a new study published in the Proceedings of the National Academy of Sciences1, researchers used functional MRI scanning to track the brain activity of study participants as they watched images of a sequence of objects at different locations.
When the images matched what the participants expected, activity in the hippocampus shifted smoothly from the front to the back, like a continuous dial. When the images differed from the patterns the subjects had learned, however, the researchers saw that hippocampal activity split into specialized regions:
If the “what” of the image changed—for example, the participant expected to see a picture of a dog but instead saw a cat—the activity took place in the anterior, or front, part of the hippocampus.
If the “where” of the image changed—the picture of the dog was on the left side instead of the right—activity centered in the posterior, or back, part of the hippocampus.
“Real memories involve more than just objects or locations. They are bound to concepts and meaning. How the hippocampus handles both space and meaning at the same time has been one of the central unsolved questions in memory neuroscience.”
James Kragel, PhD, a Research Assistant Professor in the Department of Neurology at UChicago
“This resolves a long-standing debate about hippocampal organization and suggests that flexibility, not fixed architecture, is a core principle of how the brain organizes memory, spanning both spatial and semantic information,” said James Kragel, PhD, a Research Assistant Professor in the Department of Neurology at UChicago and senior author of the study.
Sometimes called “the GPS of the brain,” the hippocampus is most well-known for its role in remembering places and locations. In 2014, the Nobel Prize in Medicine was awarded for the discovery of so-called place cells and grid cells in the brain that track location and map out space. A lot of research on the hippocampus to date has focused on these spatial aspects; other MRI studies in rodents and humans suggest that the hippocampus processes big-picture information in the anterior region, and more precise details in the posterior.
But just as important as spatial information is what’s in those locations. Our experience of a room is distinctly different if all the furniture is in the same place, but the couch in the corner is now bright red instead of blue, as we remembered. Kragel and his teammates wanted to go further to understand the overlap of how spatial and conceptual information are represented at the same time.
They recruited 28 participants who learned sequences of five images placed in different locations on a circular array. After they memorized the sequences, they got into an MRI machine and watched a replay of those same images—with some differences. The researchers varied the sequences, sometimes swapping an image in its expected location, moving an image to a different location, or both.
Depending on how big the difference was from the expected images, the researchers saw different activity in the hippocampus. Seeing an image of a different object triggered more activity in the anterior region, while differences in location sparked more activity in the posterior region. Changes in both the object and its location generated activity in the central region, suggesting that it plays a role in reconciling both types of information.
Different regions of the hippocampus are connected to different networks of the brain for higher-level processing. The anterior region is connected to systems involved in abstract or conceptual processing, and the posterior region is connected to systems for visual and spatial processing. This suggests that the patterns of activity the researchers saw in this study are a way for the hippocampus to sort out discrepancies and pass them along to more specialized parts of the brain for further processing.
“We need to encode and retrieve memories pretty quickly all the time, and we need to be able to switch between processing different types of information,” Kragel said. “So, this type of organization where the hippocampus receives different kinds of inputs allows it to rapidly detect when information differs from our expectations and retrieve relevant memories to guide behavior.”
Reference:
1) https://www.pnas.org/doi/10.1073/pnas.2525724123
(Newswise/HG)
