A new study by MIT researchers explores the fascinating science behind how our brains form cognitive maps, the mental representations of our spatial surroundings, particularly the significant role sleep plays in this process. Using mice as subjects, the study sheds light on how weakly spatial neurons contribute to building cohesive mental maps of environments over time.
When we visit new places, like on vacation, the memories we create may appear vivid from the start. Yet, it often takes days before we can confidently guide someone else back to a favorite café or scenic spot. This study uncovers key insights about the cognitive maps our brains construct to navigate these spaces more effectively.
Researchers led by Wei Guo and Matthew Wilson from MIT's Picower Institute for Learning and Memory monitored the activity of neurons within the hippocampus of mice. This part of the brain is known for housing various neurons, primarily the so-called 'place cells' which activate when the animal is located at spots it has learned to recognize.
While 'place cells' are known for their strong connection to specific locations, the new findings highlight the importance of 'weakly spatial' neurons, which are not tied to exact spots but play a key role over time. The study demonstrates how these neurons gradually synchronize their activity with other neurons, enhancing the hippocampus's ability to form cognitive maps of whole spaces.
To conduct the research, mice explored simple mazes of differing shapes freely for about 30 minutes each day over the course of several days. What’s remarkable is the study did not employ any training or rewards; the mice were simply allowed to wander. Previous research has shown mice display what is termed 'latent learning' through such exploratory experiences, accumulating spatial knowledge over time.
During the exploration phase, the activity of place cells rose immediately, indicating their responsiveness as the mice navigated. Yet, this alone couldn't account for the formation of cognitive maps—what researchers noticed was how weakly spatial cells began to change their activity patterns. Over the days, these cells started to correlate not with locations but with the activity patterns of other neurons, which created broader spatial representations.
Lead author Wei Guo noted the significance of their work, saying, "On Day 1, the brain doesn’t represent the space very well. Neurons represent individual locations, but together they don’t form a map. But on Day 5 they form a map." This assertion highlights the transformative capacity of the brain when integrating experiences over time.
The researchers underscored the importance of sleep for this process. They tested the necessity of sleep by allowing some mice to rest between explorations of mazes. The results were telling: mice allowed to sleep demonstrated marked improvement in refining their mental maps, whereas those kept awake showed no such enhancements.
This aspect of the study was significant; it confirmed previous findings about sleep's role in memory consolidation and neural network refinement. The lead author added, "Sleep helped cells become more attuned both to places and to patterns of network activity, creating 'mental places' or 'fields.'" Sleep promotes connections between the weakly spatial and place cells, enabling the construction of more cohesive cognitive maps.
Interestingly, these cognitive maps were not perfect replicas of the mazes. Instead, they were described by Guo as more like schematics. He explained, "The ‘cognitive maps’ the mice encoded over several days were not literal, precise maps of the mazes. Instead, they were more like schematics. Their value is they give the brain with the topology to explore mentally."
Such cognitive maps allow the brain to mentally navigate environments, such as planning how to get to the bakery they passed earlier. While the study did not incorporate specific landmarks or investigate observed behaviors, it opens avenues for future research focused on how weakly spatial cells might contribute additional meaning to the environment.
Matthew Wilson, senior author of the study, pointed out the potential of weakly spatial cells to weave together non-spatial information with specific locations. This integration supports the idea we naturally perceive our environments as enriched by more than mere physical locations.
Concluding their findings, the authors stated: "This form of implicit and unsupervised learning constitutes a key aspect of human intelligence and deserves future investigation." The study shines light on the intricacies of cognition and serves as a stepping stone for more research to untangle the complex processes involved in how both mice and humans learn about and navigate their worlds.