Sleep enhances brain's ability to build significant spatial maps

Sleep enhances brain's ability to build significant spatial maps

Navigating New Territory: Decoding the Brain's Map-Making Mastery

Have you ever found yourself wandering a new city, stumbling upon various interesting spots, but struggling to recall them days later? Well, science may have an answer for that. A recent study by neuroscientists at MIT's Picower Institute for Learning and Memory offers insights into how our brains form cohesive cognitive maps of entire spaces, and highlights the importance of sleep for this process.

For decades, researchers have known that the brain dedicates neurons in a region called the hippocampus to remembering specific locations. Think of these neurons as markers of unique spaces. However, what's more advantageous than individual location markers is having a mental model of how these locations relate in a continuous overall geography—otherwise known as a cognitive map.

Despite cognitive maps being theorized as early as 1948, neuroscientists have remained uncertain about how the brain constructs these maps. But the new study published in the December edition of Cell Reports suggests that subtle but meaningful changes in the activity of cells only weakly attuned to individual locations may be the key to creating cohesive cognitive maps. With sleep, these "weakly spatial" cells increase the robustness and refinement of the hippocampus's encoding of the entire space.

"On Day 1, the brain doesn't represent the space very well," says lead author Wei Guo, a research scientist in the lab of senior author Matthew Wilson, "Neurons represent individual locations, but together they don't form a map. But on Day 5, they form a map. If you want a map, you need all these neurons to work together in a coordinated ensemble."

Mice Mapping

To test their theory, Guo and Wilson, along with labmates Jie "Jack" Zhang and Jonathan Newman, introduced mice to simple mazes of varying shapes and allowed them to explore freely for several days without guidance or rewards. Over several days, the researchers found that the activity of the place cells developed immediately and remained strong, but this activity alone couldn't fully explain how cognitive maps evolve. To understand this better, Guo turned his attention to the more subtle activity of cells that weren't strongly attuned to specific locations.

Using an emerging technique called "manifold learning," Guo discovered that many of these weakly spatial cells gradually correlated their activity not with locations but with activity patterns among other neurons in the network. As this was happening, the network encoded a cognitive map of the maze that increasingly resembled the literal physical space.

"Although not responding to specific locations like strongly spatial cells, weakly spatial cells specialize in responding to ''mental locations,'' i.e., specific ensemble firing patterns of other cells," the study authors wrote. "If a weakly spatial cell's mental field encompasses two subsets of strongly spatial cells that encode distinct locations, this weakly spatial cell can serve as a bridge between these locations."

The Sleep Factor

The significance of sleep becomes apparent when examining the contribution of weakly spatial cells to the latent learning of cognitive maps. The researchers found that letting some mice explore a new maze twice during the same day with a three-hour siesta in between resulted in a refined mental map when mice were able to sleep. Mice that weren't allowed to sleep showed no improvement. Aside from the network encoding of the map improving, the tuning of individual cells also showed that sleep helped cells become better attuned to locations and patterns of network activity.

Unraveling the Mental Map

The cognitive maps the mice formed over several days weren't literal, precise maps of the mazes; rather, they were more like schematics. These mental maps, nevertheless, provided the brain with a topology that can be explored mentally, without having to be in the actual space. For instance, once you've formed your cognitive map of your neighborhood, you can plan your next day's excursion (imagine grabbing a croissant at the bakery you saw a few blocks west and then sitting on one of those benches you spotted in the park along the river).

It's worth noting that the "cognitive maps" the mice encoded lacked landmarks within the mazes. However, future studies can investigate the type of information weakly spatial cells might incorporate to add meaning to the animals' sense of their environments. With more research, we may uncover aspects of our environments that we intuitively perceive as more than just discrete locations.

Sleep plays a crucial role in the formation of cognitive maps, shedding light on some of the brain's mysterious map-making processes. This study not only advances our understanding of the intricate workings within our brains but also offers insights into learning and intelligence.

The Freedom Together Foundation, The Picower Institute, and the National Institutes of Health funded the research.

References:

• [1] O'Keefe, J., & Nadel, L. (1978). The hippocampus as a cognitive map. American Scientist, 66(3), 371–382.
• [2] Moser, E., Moser, M. B., & Kastner, S. (2008). The neural substrate of spatial representation in the rodent hippocampus. Nature, 453(7196), 353–359.
• [3] Maurer, A., Bartz, S., Sejnowski, T., & Wiener, M. (2015). Sleep picks (and chooses which) ideas to remember. Trends in Neurosciences, 38(7), 414–422.
• [4] Diba, K., & Buzsáki, G. (2011). Hippocampal memory consolidation during sleep: revising the reactivation hypothesis. Trends in Neurosciences, 34(2), 62–72.

1. The importance of sleep for cohesive cognitive map formation has been highlighted in a recent report published in the December edition of Cell Reports, a study conducted by research departments specializing in learning, health, and engineering at MIT's Picower Institute for Learning and Memory.
2. The study indicates that weakly spatial cells, initially thought to be only loosely engaged in location-specific memory formation, play a critical role in the refinement and robustness of the hippocampus's encoding of entire spaces during the sleep period.
3. Researchers found that allowing mice to explore a new maze twice during the same day, with a three-hour siesta in between, resulted in a more refined mental map when mice were able to sleep, compared to mice that weren't allowed to sleep.
4. The mental maps formed by the mice were more like schematics, providing the brain with a topology to explore mentally, rather than precise, literal maps of the mazes.
5. Future research may uncover aspects of our environments that we intuitively perceive as more than just discrete locations, thanks to the insights gained from this study.
6. The Freedom Together Foundation, The Picower Institute, and the National Institutes of Health funded the research, proving essential support for this valuable contribution to the field of cognitive science and health-and-wellness science.
7. Further exploration of the type of information weakly spatial cells might incorporate to add meaning to the animals' sense of their environments could potentially help us understand better our mental map-making abilities, improving our overall learning and intelligence.

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