How the brain replays past emotional experiences during sleep

For decades, neuroscientists have been trying to uncover the neural processes that allow humans and various other animals to recall emotional experiences of past events. Past studies have identified a network of brain regions that support the encoding and consolidation of these memories. These regions include the hippocampus and the amygdala, as well as the para-hippocampal, perirhinal, prefrontal, parietal and retrosplenial cortices.

Researchers at Neuroscience NeuroSU and the Institute of Biology Paris-Seine- IBPS carried out a study on rats aimed at better understanding how the dorsal and ventral hippocampus, two segments of the hippocampus known to have different functions, contribute to the consolidation of emotional memories of past events.

Their findings, published in Nature Neuroscience, suggest that these two regions coordinate during sleep to consolidate memories of past experiences and the emotions associated with them.

“We already knew a lot about the role of sleep-dependent reactivation in the dorsal hippocampus, but comparatively much less is known about the ventral part of the hippocampus,” Gabrielle Girardeau, senior author of the paper, told Medical Xpress.

“In addition, it was previously shown that the dorsal part of the hippocampus communicates during sleep with other structures related to emotions, like the amygdala. However, the anatomy shows that the dorsal hippocampus is not connected to these structures.”

How rats process emotional experiences while sleeping

Earlier neuroscience studies found that the dorsal hippocampus is not connected to other brain regions associated with the processing of emotions. This suggests that it communicates with these regions via an intermediate brain region when consolidating emotional memories of past events.

Girardeau and her colleagues hypothesized that this intermediate region is the ventral hippocampus, the other segment of the hippocampus. Contrarily to the dorsal hippocampus, this region is known to communicate with emotion-processing brain regions.

“We hypothesized that the dorsal and ventral parts of the hippocampus coordinate during sleep to associate contextual and emotional information, and potentially mediate the communication with the rest of the emotional network,” said Girardeau. “We wanted to test this, as it was not known how dorso-ventral communication occurred during sleep following emotional experiences.”

To test their hypothesis, the researchers carried out a series of experiments involving freely moving rats. These rats had tiny electrodes implanted in their brains, which recorded the activity of many neurons simultaneously both in the dorsal and ventral hippocampus.

“We collected electrophysiological recordings while the rats were undergoing an emotional experience, and then sleeping,” explained Girardeau. “We used computational methods to analyze the activity of these neurons and examine how they coordinate/dialogue during sleep, and how accurately this coordination reflects the preceding emotional experience.”

While they were awake, some rats experienced a small electric shock, while others received a reward. The researchers looked at the activity of neurons in the dorsal and ventral hippocampus both during these experiences and after them, while the rats were sleeping.

“We observed neural reactivation (a phenomenon known to support sleep-dependent memory-processing) during sleep that spans the entire axis of the hippocampus following an emotional positive or negative experience,” said Girardeau.

Towards a better understanding of emotional memory consolidation

The recordings collected by this research team confirmed that while rats are sleeping, their brain consolidates memories of emotional experiences they had while awake. The consolidation of both aversive and pleasurable experiences appears to be supported by coordinated activity between the dorsal and ventral hippocampus.

“We also found that reactivation is more faithful to the original experience when the experience was negative,” said Girardeau. “This might explain the bias towards better memories of negative compared to positive events. More broadly, it identifies a mechanism that allows us to form memories combining context and emotions, positive or negative.”

If validated in humans, the results of this recent study could help to shed more light on the intricate neural processes that support the consolidation of traumatic memories and could play a role in trauma-related mental health conditions. For instance, they might improve the understanding of post-traumatic stress disorder (PTSD) and other mental health disorders that are linked to intrusive and sometimes debilitating memories of traumatic events.

“We are now planning to move beyond the hippocampus and extend our understanding of cross-area communication involved in emotional-memory consolidation to other structures like the amygdala or prefrontal cortex,” added Girardeau. “We also hope to understand how stress might additionally influence these sleep-dependent processes.”

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