Researchers at the Massachusetts Institute of Technology have revealed insights into how the human brain navigates through space, relying on specialized neurons known as 'place cells' in the hippocampus. Their findings, published in Cell Reports, stem from experiments conducted on mice.
According to Yi Gu, a professor in the Department of Brain and Cognitive Sciences at MIT, when an individual visits a new location, the brain initially struggles to recognize it. 'On the first day, the brain does not have a strong sense of the new environment because the neurons only remember specific locations. By the fifth day, however, they are ready to form a cognitive map,' he explained. 'For mapping to occur, these neurons must work together in a coordinated manner.'
The researchers placed mice in mazes without incentivizing them with rewards, monitoring the activity of neurons in the CA1 region of the hippocampus through calcium ion flashes. These flashes were recorded during exploration and during sleep when the mice reactivated their experiences.
They observed an immediate and sustained increase in neuronal activity over the following days. However, this activity alone does not clarify the complex processes involved in mapping.
Using data dimensionality algorithms, the scientists examined interactions between individual neurons, identifying 'weakly place-coupled' cells whose activity was not linked to specific memories but to patterns of activity within the network.
Gu noted, 'In the first few days after visiting a new place, a cognitive map is formed in the hippocampus, which is a conceptual map rather than a schematic one.' Matthew Wilson, the lab director who conducted the experiments, added, 'This map is enriched with memories associated with the location. These distinctive characteristics could serve as reference markers, although it remains unclear what information precisely remains on the mental map since the mice explored featureless mazes.'
The study confirmed that cognitive mapping necessitates sleep. To further investigate how the brain constructs spatial maps during sleep, researchers found that the mice 'practiced' their recent explorations in their dreams, enhancing their memories of those experiences.
In subsequent experiments, researchers modified the conditions slightly. Mice explored a new maze twice a day with a three-hour rest between trials. Some were allowed to sleep, while others were deprived of it. The first group developed a deeper understanding of the new environment, whereas the second group showed no significant improvements.
These findings suggest that sleep aids the brain's neurons in better 'tuning' themselves for learning about new spaces. Neuroscientists plan to conduct further research using more complex mazes.