Sleep is one of the most fascinating phenomena observed in both humans and animals. It is known to be characterized by distinct cycles and types of brain activity. Previous neuroscience studies have found that during sleep, groups of neurons in the hippocampus replay past experiences and predict possible future experiences.

Researchers at the Yale School of Medicine recently conducted a study examining how these neural ensembles in the hippocampus represent multiple sequential experiences—that is, experiences or events that occur in a specific order.

Her essay, published in Natural Neurosciencedescribes some of the processes that enable the brain to process numerous experiences during sleep and create new mental representations, often based on prior knowledge.

The main goal of this Yale team’s work was to understand how the mammalian brain organizes multiple experiences in a non-disruptive manner (that is, encodes new memories without disturbing old ones). The researchers also wanted to investigate how well the brain forms memories of multiple experiences that occur during the day and how it generates new representations during sleep.

“You can think of this experiment as analogous to the experiences of people who spend a long day exploring 15 streets in a new city, with several one- to two-hour rest breaks in between,” George Dragoi, co-author of the study, told MedicalXpress.

“In this scenario, what do people remember at the end of the day, how is what happened on each street separated from what happened on the others without major disruption, and how is the overall experience of the day organized in the day resolution, in addition to the detailed experiences on each street (e.g., the order in which the streets were visited)?”

To investigate the neural processes involved in replaying and replaying sequential experiences during sleep, Dragoi and his colleagues conducted a series of experiments with freely moving rats. These rats were placed on 15 linear tracks and allowed to explore these tracks freely.

The team recorded the activity of neuronal ensembles in the animals’ hippocampus throughout the entire trail exploration session. They also recorded the activity of these neurons during sleep and in sessions before, between, and after trail exploration.

“We recorded this activity with a time resolution in the millisecond range from 128 recording channels,” explained Dragoi.

“This temporal resolution is crucial to identify the temporally highly compressed offline representations during sleep and in particular the fine dynamics of multiplexed flickering representations of two or more experiences within the same event.

“I also think that physically exploring linear tracks (as opposed to virtual reality environments currently used in many labs) provides animals with a unique opportunity to participate in truly unique multisensory contextual experiences.”

Dragoi and his colleagues’ experiments revealed some interesting observations. For example, the team found that the rats’ earliest and most recent experiences during exploration were more frequently represented in the hippocampus.

As sleeping rats replayed past experiences or reenacted future ones, different experiences were represented together according to certain patterns, increasing the representational capacity of their brains. Moreover, pairs of explored traces that were close to each other appeared to be connected, creating continuous representations.

Finally, the entire sequence in which the rats had explored the tracks during the day seemed to be replayed in the brain in a hypercompressed ratio during sleep.

“Our results demonstrate the brain’s great capacity to process numerous experiences without major disruption, particularly through the rapid multiplexing of two or more parallel representations,” Dragoi said. “Moreover, they outline the principles by which representations of multiple experiences can be combined in a generative and multiplexed manner to produce seemingly new representations based on prior knowledge.”

The current study by Dragoi and his colleagues contributes significantly to the understanding of how the rat brain replays past sequential experiences and uses them to create representations of future experiences during sleep. The findings of these researchers may pave the way for further research in the future aimed at examining in more detail the outcomes of the processes they uncovered.

“One of our most surprising and fascinating discoveries was the hyper-compressed representation of multiple experiences of an animal spanning an entire day, in the order in which they occurred in fractions of a second of higher-order sequential reactivation, an index of that day’s experiences,” Dragoi added.

“We now want to better understand how the hippocampal calculations are evaluated by the downstream areas, or in other words, how parallelization on the one hand and the binding of several information processing processes in the hippocampus on the other hand affect functionally connected downstream areas.”

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