Summary: When neural assemblies in the hippocampus and prefrontal cortex fail to synchronize at the proper times, short-term memories break down.
Source: University of Bristol
Researchers from the universities of Bristol and Heidelberg report that short-term memory depends on precisely timed cooperation among multiple neural assemblies located within and between the hippocampus and prefrontal cortex. When these assemblies do not coordinate correctly, the ability to encode, maintain, or recall recent information is compromised.
Short-term memory — the mental capacity that lets you hold information briefly to guide immediate decisions — relies on the coordinated activity of different brain regions. Two regions that play essential roles are the hippocampus and the prefrontal cortex. Understanding how these regions interact at the level of distinct groups of neurons is key to explaining why memory sometimes fails and how it might be restored.
To map these interactions, the team recorded brain activity in rats performing a memory task and analyzed activity patterns of neuronal populations in the dorsal hippocampal CA1 area and medial prefrontal cortex (mPFC). Their findings, published in Current Biology, show that memory encoding, maintenance during delay periods, and recall are supported by dynamically reconfiguring assemblies of neurons that form both within each region and across the two regions.
The concept of “neural assemblies”—groups of neurons that transiently cooperate to process specific information—was proposed over 70 years ago, but isolating those assemblies and linking them to particular cognitive operations has been challenging. This study provides direct evidence that distinct CA1–mPFC assemblies arise during different stages of a short-term memory task and that the timing and rhythmic coordination of these assemblies are critical for success.
Lead author Dr. Michał Kucewicz, formerly a PhD student at the University of Bristol and now Assistant Professor of Neurology at Gdansk University of Technology, explained that the study reveals both the complexity and the vulnerability of short-term memory circuits. “Our results make potential therapeutic interventions for memory restoration more challenging to target in space and time,” he said. “At the same time, we identified specific assembly-level processes that determine whether remembering succeeds or fails, and these processes could become targets for interventions that modulate assembly interactions.”
Senior author Professor Matt Jones, of Bristol’s School of Physiology, Pharmacology and Neuroscience, emphasized that these findings shift how we think about the neural substrates of memory. Instead of single, static circuits, memory appears to be distributed across heterogeneous neuronal subpopulations whose contributions change dynamically across task stages. This distributed and time-varying organization helps explain why memories can be robust under some conditions and fragile under others.
The experimental data show that during the sample or encoding phase, distinct mPFC subpopulations join with distributed CA1–mPFC assemblies that exhibit rhythmic modulation around 4–5 Hz. These rhythmic assemblies reappear during the choice phase, but without the same 4–5 Hz modulation. When rhythmic assembly activity was weakened during the delay, sustained mPFC encoding collapsed and animals made errors in the task. In other words, loss of rhythmic coordination across regions predicted unstable memory maintenance and behavioral mistakes.
Future research will test whether directly modulating assembly interactions can alter memory performance. Dr. Kucewicz is already applying stimulation approaches in human patients to investigate whether disrupting or strengthening similar assemblies affects remembering. The team expects that analogous mechanisms operate in humans and that targeted interventions—pharmacological or via brain stimulation—may one day help restore memory functions impaired by neurological disorders.
About this memory and neuroscience research news
Author: Press Office
Source: University of Bristol
Contact: Press Office – University of Bristol
Image: The image is in the public domain
Original Research: Open access. “Distinct hippocampal-prefrontal neural assemblies coordinate memory encoding, maintenance, and recall” by Aleksander P.F. Domanski et al., Current Biology.
Abstract
Distinct hippocampal-prefrontal neural assemblies coordinate memory encoding, maintenance, and recall
Highlights
- Activity between hippocampus and prefrontal cortex (CA1–PFC) reconfigures across different memory stages.
- Distributed CA1–PFC assemblies co-fire with a 4–5 Hz rhythmic modulation during memory encoding.
- Tiled or sequential activation in prefrontal cortex supports maintenance of information across delay periods.
- Breakdown of rhythmic CA1–PFC assembly coordination precedes unstable delay coding and behavioral errors.
Summary
Short-term memory allows recent experience to guide immediate choices. Both the hippocampus and prefrontal cortex participate in encoding task cues, rules, and outcomes, but exactly which neurons carry specific information at each stage has been unclear. By decoding population activity in rat medial prefrontal cortex and dorsal hippocampal CA1, the authors show that mPFC populations are crucial for maintaining sample information across delays even though most individual neurons fire only transiently.
During sample encoding, distinct mPFC subpopulations join distributed CA1–mPFC assemblies marked by 4–5 Hz rhythmic modulation. These assemblies re-emerge during choice phases but without the same rhythmic signature. Delay-dependent errors occurred when rhythmic assembly activity was attenuated, resulting in a collapse of sustained mPFC encoding. Together, these results map the component processes underlying memory-guided decisions onto diverse CA1–mPFC subpopulations and on the dynamics of physiologically distinct, distributed cell assemblies.