A team of scientists from the Center for Neuroscience Imaging Research (CNIR) at the Institute for Basic Science (IBS) has unveiled how the hippocampus orchestrates multiple memory processes, including encoding new information, forming memories, and retrieving them. This study was led by SHIM Won Mok, associate professor of Biomedical Engineering at Sungkyunkwan University and YOO Michael Seng Bum, assistant professor of Biomedical Engineering at the same university. By applying advanced dimensionality reduction techniques to fMRI data, the researchers demonstrated the hippocampus’s critical role in coordinating these processes.

The human brain processes and integrates diverse information simultaneously to form memories. For example, while watching a movie, the brain integrates multiple pieces of information, such as identifying the characters and understanding the evolution of their relationships, to later recall the storyline. Previous studies have mainly explored how the hippocampus supports individual memory processes - like recognizing new information, forming memories, and recalling them - but how the hippocampus coordinates these processes has remained unclear.

To address this, the research team designed an experiment in which participants watched a movie and later recalled its content while undergoing fMRI scans. They analyzed two types of novelty in each movie scene. First, co-occurrence novelty measures how many new combinations of characters appear together in a scene. Meanwhile, valence novelty measures how the relationships between characters change in a scene.

Additionally, they measured how well participants remembered the scenes (“memorability”). Interestingly, the study revealed that participants remembered scenes with lower novelty more accurately, shedding light on how novelty encoding interacts with memory formation. Furthermore, by analyzing how the hippocampus processed novelty and memorability during movie viewing, the team explored its role in coordinating these processes.

The team hypothesized that the hippocampus coordinates these processes by aligning low-dimensional subspaces of neural activity, which represent different memory functions. Their results showed:

• - Aligned subspaces for the two types of novelty, suggesting that the hippocampus integrates diverse forms of novel information.
• - Alignment between novelty and memory formation subspaces, with participants showing better memory performance when these alignments were stronger.
• - Distinct alignment patterns for retrieval, where the subspace for memory retrieval aligned with memory formation but not with novelty, suggesting process-specific coordination by the hippocampus.

This study is significant because it combined fMRI and electrophysiological methods to reveal how the hippocampus coordinates memory processes. Unlike previous research that focused on the hippocampus’s role in individual memory processes, this study highlights how the hippocampus integrates multiple processes by analyzing the geometric characteristics of neural activity patterns. These findings not only deepen our understanding of memory encoding and retrieval but also provide insights into the hippocampus’s role as a coordinator and its interaction with the neocortex.

KWON Dasom, the first author of the study, stated, “This work expands our understanding of memory by uncovering the hippocampus's coordinating role and the neural dynamics supporting it.”

YOO Michael Seng Bum, co-lead author, added, “This research demonstrates the hippocampus’s role in coordinating multiple cognitive processes, beyond its role in individual processes.”

SHIM Won Mok, another co-lead author, described the research as “an innovative exploration of how the hippocampus coordinates memory formation and retrieval through brain activation patterns during naturalistic experiences.”

Figure 1. Alignment between hippocampal subspaces for memory processes
A. Overview of hippocampal dynamics during movie watching. FMRI data from the hippocampus were measured at the voxel level, and low-dimensional subspaces for two types of novelty and memorability were extracted. From the same hippocampal dynamics, three subspaces were derived: one for co-occurrence novelty encoding, one for valence novelty encoding, and one for memory formation.
B. Degree of alignment between the subspaces. The analysis demonstrated that the novelty subspaces were aligned with the memory formation subspace but not with the memory retrieval subspace. Statistical testing was performed by generating 1,000 random subspaces through data shuffling (gray dots) and comparing their alignment scores with those of the observed subspaces (red and purple dots).
C. Relationship between subspace alignment and memory performance. Each point represents data from an individual participant, showing that greater alignment between the novelty encoding subspaces and the memory formation subspace is associated with higher memory performance scores for the movie content.

Figure 2. Comparison between canonical components and functional subspaces of the hippocampus
This figure illustrates the relationship between the hippocampus’s intrinsic components and its functional subspaces. Three components were obtained through principal component analysis (PCA) of hippocampal neural data collected while participants watched a movie and freely recalled its content (Center). Consistent with previous findings, the components exhibited spatial gradients along the hippocampus’s longitudinal axis. The first principal component was associated with the memory formation subspace (Left), while the third principal component was linked to the novelty encoding subspaces (Right). The novelty encoding subspaces were aligned with the memory formation subspace but not with the memory retrieval subspace. In contrast, the memory formation and retrieval subspaces were aligned. These findings demonstrate that the neural subspaces identified in this study reflect the hippocampus's intrinsic functional structure.