Summary: The size and shape of neural assemblies, and not the strength of signals processed by neurons or the order in which they fire, is the most critical element of recording episodic memory.
Of all forms of memory, episodic memory is the most intimate. We recall the sequences of events that happen to us — a marriage, a visit to a foreign country, a personal achievement — in great autobiographical detail. But scientists have disagreed about the most important elements the brain uses to encode these episodes and consolidate them during sleep.
A group of Yale scientists, however, reports that it is the size and shape of neuronal assemblies — not the strength of signals processed by neurons or the order in which neurons fire — that are the most crucial elements in our ability to record past events.
“It is like a sketch that contains a lot of dots but has no ultimate form, but once you enlarge specific dots with a crayon then the pattern becomes clearer,” said George Dragoi, assistant professor of psychiatry and neuroscience and senior author of the research published June 25 in the journal Neuron.
While living through an experience and during sleep, the neuronal activity of the brains of rats maps the experience onto connections between cell assemblies arranged in sequences. However, the sequence of those connections is less important in determining whether an episodic memory forms than are the changes in neuronal assemblies themselves, the researchers report.
Doctoral candidate Usman Farooq is first author of the paper. Other authors are postdoctoral associates Jeremie Sibille and Kefei Liu.
About this neuroscience research article
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Highlights • Preconfigured patterns robustly preplay future place cell sequences and trajectories • Plasticity in trajectory replay is supported by increased cell assembly coordination • Rate and temporal order coding do not fully account for plasticity in replay • Plasticity in replay is mainly expressed in epochs with large neuronal participation
Summary A central goal in learning and memory research is to reveal the neural substrates underlying episodic memory formation. The hallmark of sequential spatial trajectory learning, a model of episodic memory, has remained equivocal, with proposals ranging from de novo creation of compressed sequential replay from blank slate networks to selection of pre-existing compressed preplay sequences. Here, we show that increased millisecond-timescale activation of cell assemblies expressed during de novo sequential experience and increased neuronal firing rate correlations can explain the difference between post-experience trajectory replay and robust preplay. This increased activation results from an improved neuronal tuning to specific cell assemblies, higher recruitment of experience-tuned neurons into pre-existing cell assemblies, and increased recruitment of cell assemblies in replay. In contrast, changes in overall neuronal and cell assembly temporal order within extended sequences do not account for sequential trajectory learning. We propose the coordinated strengthening of cell assemblies played sequentially on robust pre-existing temporal frameworks could support rapid formation of episodic-like memory.