New research has found that during sleep, some neurons not only replay the recent past but also anticipate future experiences. In a study on sleep and learning published in Nature by a team of researchers from Rice University and the University of Michigan, it was discovered that individual neurons in the hippocampus of rats stabilize and tune spatial representations during periods of rest following the animals’ first time running a maze. These specialized neurons show place preferences and play a role in consolidating new memories.
The researchers tracked sharp wave ripples, a pattern of neuronal activation known to consolidate new memories and tag which parts of a new experience are to be stored as memories. They observed how individual neurons stabilize spatial representations during rest periods, shedding light on the process by which the brain produces a representation of the world after a new experience. Sleep is critical for memory and learning, as demonstrated by performance on memory tests after a nap compared to after a period of waking or sleep deprivation.
Scientists previously discovered that neurons in the brains of sleeping animals that had explored a new setting before rest were firing in ways that replayed the animals’ trajectories during exploration. This finding suggests that spatial representations of many specialized neurons in the hippocampus are stable during sleep. The researchers wanted to see if there were more to the story by tracking how individual neurons achieve spatial tuning and navigate a new environment.
The researchers trained rats to run on a track with liquid rewards at either end and observed how individual neurons in the hippocampus spiked during the process. They estimated the neurons’ place field, the area in the environment that a neuron “cared” about the most, by calculating the average spiking rate over many laps. By developing a statistical machine learning approach that used other neurons surveyed to map out an estimate of where the animal was dreaming of being, the researchers could track the preferences of neurons during sleep.
The study confirmed that most neurons’ spatial representations were stable across several hours of postexperience sleep. However, the researchers found that some neurons underwent changes during sleep that reflected something learned while the animals were asleep. This observation constitutes direct observation of neuroplasticity during sleep, a significant finding in neuroscience. Neuroplasticity research typically focuses on rewiring and forming new representations during waking periods when stimuli are present.
Advancements in neuroscience enabled by technological progress in neural probes and machine learning have allowed for significant progress in brain science. While the study demonstrates exciting developments, recent budget cuts pose a challenge to continued research. The researchers express gratitude for the opportunity to conduct these experiments and obtain valuable results, emphasizing the importance of ongoing support for neuroscience research initiatives.