When you explore different places, memories of them may seem vivid immediately. Still, it can take days to develop a clear sense of the area—enough to guide someone to a spot and nearby locations like a café you discovered.
The brain uses neurons in the hippocampus, called “place cells,” to remember specific locations. These cells activate when you’re in an area they are tuned to. However, beyond marking particular spots, the brain creates “cognitive maps” that link places into a connected mental layout.
While the idea of cognitive maps has been known for a long time, how the brain builds them has been unclear.
A new MIT study on mice reveals that forming these maps relies on gradual changes in the activity of specific “weakly spatial” cells. These cells, which aren’t strongly tied to particular locations, help refine the hippocampus’s ability to represent the entire space. The study highlights the critical role of sleep, during which these cells enhance hippocampal activity to connect places into a unified map.
To study how mice form cognitive maps, researchers let them explore mazes of different shapes for 30 minutes daily over several days without training or rewards. Mice naturally learn spatial layouts over time, which is called “latent learning.”
The team monitored neuron activity in the CA1 region of the hippocampus using a method that made active neurons flash. They recorded this activity both during exploration and sleep, as sleep helps animals replay and refine memories of their experiences.
While “place cells” (neurons linked to specific locations) stayed active from the start, the researchers didn’t fully explain how cognitive maps formed. Instead, they studied “weakly spatial” cells, which aren’t tied to specific spots.
Using a technique called “manifold learning,” they showed that these weakly spatial cells gradually synchronized with other neurons in the network. Over time, this activity helped the hippocampus create a mental map of the maze that resembled its physical layout.
Scientists noted, “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.”
“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.”
Researchers use a novel approach to track neural signals in brain
Studies show that sleep and rest help consolidate and refine memories through neural activity like replay. A scientific team ran an experiment to test whether sleep is essential for weakly spatial cells in building cognitive maps.
They let mice explore a new maze twice in one day with a three-hour break in between. Some mice were allowed to sleep during the break, while others were kept awake. Mice that slept showed improved mental maps and better-tuned cells, while sleepless mice showed no improvement.
Sleep enhanced the network’s map encoding and helped individual cells become more attuned to specific locations and network activity patterns, creating “mental places” or “fields.”
Lead author Wei Guo, a research scientist in the lab of senior author Matthew Wilson, said, “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 that they provide the brain with a topology that can be explored mentally without being in the physical space. For instance, once you’ve formed your cognitive map of the neighborhood around your hotel, you can plan the next morning’s excursion.”
Senior author Matthew Wilson, the Sherman Fairchild Professor in The Picower Institute and MIT’s departments of Biology and Brain and Cognitive Science suggested that weakly spatial cells might add non-spatial information to cognitive maps, giving them extra meaning—for example, associating the idea of a bakery with its location. While this study didn’t include landmarks or examine specific behaviors, it confirmed that weakly spatial cells play an important role in mapping.
Future research could explore how these cells incorporate additional information into an animal’s understanding of its environment, reflecting how we naturally perceive spaces as more than individual locations.
The authors concluded that their work demonstrated significant neural changes during free exploration and sleep, even without reinforcement. They emphasized that this implicit, unsupervised learning is a key aspect of human intelligence and deserves further study.
Journal Reference:
- Wei Guo, Jie Zhang, et al. Latent learning drives sleep-dependent plasticity in distinct CA1 subpopulations. Cell Reports. DOI: 10.1016/j.celrep.2024.115028