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Sleep plays a critical role in the ability of individuals, as well as mice, to adapt to new vacation destinations. It aids in the development of coherent mental maps of unfamiliar places, connecting weakly spatial neurons to place cells for improved navigation. Research conducted by MIT neuroscientists at The Picower Institute for Learning and Memory sheds light on the importance of sleep in this process.
When embarking on a vacation in a new city, individuals often find themselves exploring unique locations that leave a lasting impression, such as a stunning garden on a quiet side street. However, it may take days before one gains enough familiarity with the city to confidently navigate through it and direct others to similar locations. The creation of a cognitive map of the entire area plays a crucial role in this process, allowing the brain to understand how different locations fit together within the geographical landscape.
The hippocampus, a region in the brain responsible for recalling specific places, uses neurons known as “place cells” to activate when an individual is in a familiar location. These place cells play a vital role in the creation of cognitive maps, which provide an overall image of the entire space. While the concept of cognitive maps was first proposed in 1948, the exact mechanisms by which the brain creates these maps remain largely unknown.
A recent study published in Cell Reports highlighted the significance of subtle changes in the activity of weakly spatial cells in the hippocampus in the development of cognitive maps. These cells, which are not strongly attuned to individual locations, contribute to the refinement and robustness of the hippocampus’s encoding of the entire space over time. During sleep, these weakly spatial cells enhance neural network activity in the hippocampus, linking various locations and forming a cognitive map.
Lead author Wei Guo and his colleagues conducted experiments on mice to investigate how these weakly spatial cells contribute to the creation of cognitive maps. The mice were exposed to basic mazes of various designs and were allowed to explore them freely over several days. By monitoring the activity of neurons in the hippocampus, the researchers observed that the mice displayed “latent learning” of the areas after several days of exploration.
Through a technique called “manifold learning,” Guo discovered that weakly spatial cells progressively connected their activity with patterns of activity among other neurons in the network, rather than specific locations. This interconnected network of cells formed a cognitive map of the maze that closely resembled the actual layout of the space. While strongly spatial cells respond to individual locations, weakly spatial cells specialize in responding to ensemble firing patterns of other cells, acting as bridges between various locations.
The study also revealed the significance of sleep in the refinement of cognitive maps. Animals that were allowed to sleep after exploring the mazes showed a considerable improvement in their mental maps compared to those that were not permitted to sleep. Sleep helped cells become more attuned to both locations and patterns of network activity, enhancing the encoding of the cognitive map over time.
Cognitive maps, as stored by mice, are not exact replicas of the physical environment but rather schematics that provide a cognitive understanding of the space. These maps offer a topology that can be mentally examined and used for navigation and planning. The brain can overlay non-spatial information onto these maps, adding meaning and context to the represented locations.
While the study focused on the natural behavior of animals and the implicit and unsupervised learning that occurs during exploration and sleep, future research may delve into what type of information weakly spatial cells are absorbing to enhance the animals’ understanding of their surroundings. These findings have significant implications for human learning and intelligence, highlighting the importance of sleep in the process of adapting to new and unfamiliar environments.
