Summary: The hippocampus is often described as the brain’s “GPS,” but new research from the University of Chicago shows it does much more than map space. The study reveals the hippocampus flexibly changes how it organizes activity depending on whether events match our expectations. When sequences unfold as predicted, activity flows smoothly along the structure. When expectations are violated, the hippocampus segments its processing: the anterior (front) region responds to changes in meaning or identity, the posterior (back) region responds to changes in location, and the middle zone reconciles combined changes. This dynamic arrangement helps the brain balance semantic and spatial information in real time.
Researchers used high-resolution functional MRI and eye tracking to measure how the hippocampus represented sequences of objects presented at different positions. Participants first learned sequences of five images positioned around a circular array. During scanning, the sequences were replayed with occasional mismatches: an unexpected object, an unexpected location, or both. The resulting activity patterns reveal how the hippocampus shifts between a continuous gradient and discrete, content-specific modules depending on memory demands.
Key Facts
- The expectation dial: When events match memory, hippocampal activity flows in a continuous wave from anterior to posterior along the long axis.
- The “What” region (anterior): When the identity of an object is unexpected (for example, a cat instead of a dog), activity increases in the anterior hippocampus, which is linked to conceptual and semantic networks.
- The “Where” region (posterior): When an object appears in the wrong location, the posterior hippocampus becomes more active, reflecting its ties to visual and spatial processing systems.
- The reconciliation center (middle): When both object identity and location are mismatched, activity concentrates in the middle portion of the hippocampus, suggesting this region integrates spatial and semantic discrepancies.
- Beyond the GPS: While place cells and grid cells emphasize spatial mapping, this study highlights the hippocampus’s equal role in representing semantic meaning and how meaning and place are combined in episodic memory.
Source: University of Chicago
Why this matters: The hippocampus supports episodic memory by binding together what happened and where it happened. This study clarifies a long-standing debate about whether the hippocampus is organized as a smooth gradient from coarse to fine coding or as discrete modules for different functions. The answer is both: the hippocampus operates as a flexible architecture that can act like a gradient when experience is predictable and reorganize into content-sensitive modules when expectations are violated.
The GPS of the brain
Often called the brain’s GPS, the hippocampus is best known for its role in navigation and spatial memory. Discoveries such as place cells and grid cells emphasize its spatial functions, and many studies have investigated how anterior regions support broader, coarse representations while posterior regions encode finer details. Yet real-world memories also depend on the identities and meanings of objects in those places. This research shows how spatial and semantic elements are represented simultaneously along the hippocampal long axis.
In the experiment, participants learned image-location sequences and then viewed replays while undergoing fMRI. When sequences matched expectations, hippocampal activity varied smoothly along the long axis. When sequences contained mismatches, the structure adopted a more modular pattern: anterior areas signaled semantic mismatches, posterior areas signaled spatial mismatches, and an intermediate zone became active only when both dimensions deviated from memory. Eye tracking confirmed that these neural patterns were linked to how participants attended to and processed the sequences.
Sorting and responding
Different parts of the hippocampus communicate with distinct cortical networks. The anterior hippocampus connects with regions that process abstract, conceptual information, while the posterior hippocampus connects with visual and spatial systems. By routing content-specific signals to these wider networks, the hippocampus can rapidly detect deviations from expectation and guide behavior—retrieving relevant memories or updating representations as needed.
“Real memories involve more than objects or locations—they are bound to concepts and meaning,” said James Kragel, PhD, Research Assistant Professor in the Department of Neurology at UChicago and senior author of the study. “Our results suggest flexibility, not fixed architecture, is a core principle in how the brain organizes memory across spatial and semantic dimensions.”
Funding: The study, “Spatial and semantic memory reorganize a hippocampal long-axis gradient,” was supported by the National Institutes of Health. Additional authors include Anikka G. Jordan and Joel L. Voss, both from the University of Chicago.
Key Questions Answered:
A: Specialization speeds processing. The anterior hippocampus and connected networks are tuned to conceptual, “big picture” information, while posterior regions specialize in precise spatial and visual details. By separating these tasks, the brain can quickly determine which part of a memory needs updating without reprocessing the entire scene.
A: If the hippocampus cannot shift between gradient-like and modular states, you could experience difficulty reconciling identity and location—recognizing someone but feeling disoriented about where they are. This flexibility helps us feel grounded and accurately navigate changing environments.
A: The hippocampus supports prediction by running expectations against incoming information. Smooth anterior-to-posterior activity indicates predictions are confirmed. When that flow breaks, it signals a need to encode new information and adjust future expectations.
Editorial Notes:
- This article was edited by a Neuroscience News editor.
- The journal paper was reviewed in full.
- Additional context was added by staff.
About this neuroscience research news
Author: Cassandra Belek
Source: University of Chicago
Contact: Cassandra Belek – University of Chicago
Image: The image is credited to Neuroscience News
Original Research: Closed access. “Spatial and semantic memory reorganize a hippocampal long-axis gradient” by Anikka G. Jordan, Joel L. Voss, and James E. Kragel. PNAS
DOI: 10.1073/pnas.2525724123
Abstract
Spatial and semantic memory reorganize a hippocampal long-axis gradient
The hippocampus supports episodic memory by binding spatial and semantic information, but how these content types are organized along the hippocampal long axis has been an open question. Gradient models propose a continuous shift from broad, coarse representations in anterior regions to detailed representations in posterior regions. Modular accounts propose discrete subregions specialized for distinct functions.
Using high-resolution fMRI together with eye tracking during sequence learning, the study tested these competing models. Predictable sequences produced continuous variation in hippocampal activity along the long axis, while mismatched sequences produced modular organization. Anterior and posterior subregions were selectively sensitive to semantic and spatial mismatches respectively, and an intermediate region specifically responded when both dimensions were violated simultaneously. These content-sensitive subregions were embedded within distinct cortical networks that reorganized according to memory demands.
Overall, the findings reveal a dynamic hippocampal architecture that flexibly combines gradient and modular principles to represent the spatial and semantic content defining episodic memory.