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The hippocampus and surrounding regions in the medial temporal lobe of the brain are critical for memory, but it has remained unclear to scientists how activity across these regions is dynamically organised to link different stages of memory processing.
A team of researchers in the Brain Network Dynamics Unit, at the Nuffield Department of Clinical Neuroscience (NDCN), in collaboration with the Brain and Cognition Research Center, CNRS, Toulouse, France set out to understand how the human brain coordinates memory across time, from learning an experience to storing it and later recalling it.
Existing knowledge on this subject has come largely from studies using animal models, where faster brain rhythms are thought to structure memory-related activity. However, human brain activity appears slower and more variable and it has remained unclear whether there is a unifying mechanism in humans that organises brain activity across these memory stages.
By studying human patients undergoing clinical monitoring for epilepsy, who had electrodes temporarily implanted in memory-related brain regions, researchers recorded both brain rhythms and the activity of individual neurons as patients learned associations, rested, and were later tested on what they remembered.
During the periods of learning and recalling information, the team discovered that the hippocampus generated brief bursts of very slow brain rhythms (2 beats per second). These bursts acted like timing signals, coordinating activity across different parts of the memory system and organising patterns of neural activity.
DPhil student Adrien Causse, first author on the paper, explains: “We found that the brain uses short bursts of slow activity to coordinate how memories are formed and stabilised. The same patterns seen during learning are later reactivated during rest, with the strength of this ‘reactivation’ predicting how well participants later remembered the information.”
Professor David Dupret, co-senior author on the paper, adds: “This study shows that human memory processing is not continuous, but organised into brief moments where brain activity becomes synchronised across regions on demand. These moments appear to link learning with the later strengthening of memories.”
The results of the study provide a clearer mechanistic understanding of how human memory is organised in the brain, and helps explain a fundamental aspect of how we form and retain experiences.
In the longer term this could inform new approaches to treating memory disorders. Conditions such as Alzheimer’s disease, epilepsy, or brain injury often involve disruptions to these same brain networks. Understanding the timing mechanisms that support memory may help guide strategies to restore or enhance memory function, for example through targeted brain stimulation or by improving how memory consolidation is supported during rest and sleep.
More broadly, it provides a framework for understanding why memory sometimes succeeds or fails, by linking behaviour directly to the underlying coordination of brain activity.
The full paper ‘A learning-evoked slow-oscillatory architecture paces population activity for offline reactivation across the human medial temporal lobe’ is published in Neuron.