A recent study has revealed new information about how individual neurons in the hippocampus of rats stabilize and tune spatial representations during periods of rest following the animals’ first time running a maze. This study provides the first evidence of neuroplasticity during sleep. Additionally, the research found that during sleep, some neurons not only replay the recent past but also anticipate future experiences. This discovery is part of a series of insights from a study on sleep and learning published in Nature by a team of researchers from Rice University.The University of Michigan conducted research that provides a unique perspective on how individual neurons in the hippocampus of rats stabilize and adjust spatial representations after the animals complete their first run through a maze.
Kamran Diba, an associate professor of anesthesiology at Michigan and the study’s corresponding author, explained, “Certain neurons respond to specific stimuli. For example, neurons in the visual cortex respond to visual stimuli. The neurons we’re examining exhibit place preferences.”
Working with collaborators in Michigan’s Neural Circuits and Memory Lab led by Diba, , has been researching how specialized neurons create a representation of the world after a new experience. The researchers focused on sharp wave ripples, a pattern of neuronal activation that helps consolidate new memories and indicates which parts of a new experience are to be stored as memories. “For the first time, we observed how these individual neurons stabilize spatial representations during rest periods,” explained Kemere, an associate professor at Rice University.Sleep is essential for memory and learning, as proven by scientific studies that measure memory test performance after a nap compared to after waking or sleep deprivation. Years ago, researchers found that the neurons in the brains of sleeping animals who had previously explored a new environment were firing in a way that replayed their movements during exploration. This discovery supports the idea that sleep helps new experiences become lasting memories, indicating that specialized neurons play a role in spatial representations during sleep.The scientists were curious to see if there were changes in the hippocampus during sleep. They believed that some neurons might alter their activity, much like the experience of waking up with a new understanding. To investigate this, they needed to monitor how individual neurons develop spatial tuning, which is the brain’s process of learning to navigate a new route or environment. The researchers conducted their study by training rats to run on a track and observing how individual neurons responded.Neurons in the animals’ hippocampus would exhibit an increase in activity. By determining an average rate of activity over numerous journeys, the scientists were able to gauge the neurons’ preferred area in the environment. “The important thing to note is that preferred areas are determined using the animal’s behavior,” Kemere explained, emphasizing the difficulty of determining what happens to preferred areas during rest periods when the animal is not actively navigating the maze. “I’ve been contemplating for a while how we can assess the preferences of neurons outside of the maze.
According to Diba, the challenge of understanding the brain’s spatial tuning process is that neurons are not always active, such as during sleep. To tackle this challenge, the researchers linked the activity of each neuron to the activity of all the other neurons.
The key innovation of the study was the development of a statistical machine learning approach that utilized the activity of other neurons to estimate where the animal was dreaming of being. The dreamed positions were then used to estimate the spatial tuning process for each neuron in the data sets.
Diba stated that being able to track the preferences of neurons even without a stimulus was a significant breakthrough for the research team.
Both Diba and Kemere praised Kourosh Maboudi, a postdoctoral researcher at Michigan and the main author of the study, for his contribution to the development of the learned tuning approach.
The method confirmed that the spatial representations created during the exposure to a new environment remain stable for most neurons during several hours of post-experience sleep. However, as the researchers had expected, there was more to the story.
“What I found most exciting about this research is the discovery that it’s not necessarily the case that during sleep the only thing these neurons are doing is simply replaying past experiences,” Maboudi said.
Kemere stated that the primary function of sleep is to consolidate memories from experiences. However, he noted that some neurons may be engaged in other activities during this time. The research showed that changes in neuron activity occur during sleep, and these changes reflect the learning process that took place while the animals were asleep. This suggests that the brain is still active and processing information even when the body is at rest. This observation is important because it provides direct evidence of neuroplasticity occurring during sleep, which is a significant finding in the field of plasticity research.The study explores the mechanisms through which neurons can rewire and create new representations, focusing on the process that occurs when stimuli are presented during waking periods, as opposed to when the relevant stimuli are absent during sleep. According to Diba, the brain’s plasticity or rewiring capability operates on extremely fast timescales, with a fascinating connection between the duration of actual experiences – spanning from seconds to days – and the formation of memories, which are heavily condensed. Diba also emphasizes the instantaneous nature of memories, referring to a well-known literary passage by a French modernist.The author Marcel Proust’s writing describes a childhood memory that quickly reveals a whole world of past experiences. The study demonstrates how advancements in neuroscience have been made possible by technological progress in the design of stable, high-resolution neural probes and machine learning-backed computation power. Kemere, the researcher, believes that brain science is on the brink of significant progress in the future but is also concerned about the impact of recent budget cuts on ongoing research. He fears that the current budget cuts may hinder the potential advancements in this field.”Without the opportunity, we wouldn’t have been able to conduct these experiments and obtain these findings,” Kemere mentioned. “We appreciate the chance that was presented to us.”
