Imagine navigating your way to work or the grocery store without ever moving a muscle. Your brain effortlessly taps into cognitive maps stored deep within your hippocampus and entorhinal cortex, recalling the routes and landmarks you’ve encountered before. But what if simply thinking about these locations could activate those same mental maps? New research from MIT reveals that our brains don’t just rely on these maps when physically moving through space they also kick in during mental journeys. The study, conducted on animals, shows that these cognitive maps are activated even when the subject is merely thinking about a sequence of experiences, with no physical movement or sensory input involved.
Mental maps: More than just navigation
MIT neuroscientists have discovered that the brain creates and activates cognitive maps in the entorhinal cortex when an animal thinks about navigating through a sequence of images. This groundbreaking study is the first to demonstrate the cellular basis of mental simulation and imagination in a nonspatial domain, revealing how these cognitive maps function during mental navigation. “These cognitive maps are being recruited to perform mental navigation, without any sensory input or motor output,” says Mehrdad Jazayeri, an associate professor of brain and cognitive sciences at MIT and senior author of the study. “We can actually see a signature of this map presenting itself as the animal mentally goes through these experiences.”
How the study worked
In the study, animals were trained to use a joystick to navigate through a series of images shown at regular intervals. These images served as "landmarks" within the mental map. The researchers initially only showed the animals certain pairs of images during training. Later, they tested whether the animals could navigate through new pairs they hadn’t seen before.
If the animals were using simple memorization, they would likely struggle with these new pairs. Instead, the animals navigated them successfully on the first try, providing strong evidence that they were relying on a cognitive map, not just memory.
To understand how this cognitive map was formed and activated, researchers recorded the activity of individual neurons in the entorhinal cortex. They found that as the animals used the joystick to navigate between landmarks, specific neurons displayed distinctive bursts of activity that corresponded to the expected locations of the intervening images, images that the animals never physically saw during the task.
“The brain goes through these bursts of activity at the expected time when the intervening images would have passed by the animal’s eyes,” Jazayeri explains. “And the timing between these bursts matched exactly when the animal would have expected to reach each landmark, in this case, 0.65 seconds.” Interestingly, the speed of these mental simulations was tied to the animals’ performance. When they were late or early in completing the task, their brain activity showed corresponding shifts in timing.
Building a model of learning
To delve deeper into how these cognitive maps work, the researchers developed a computational model to replicate the brain activity they observed. They used a continuous attractor model, which was originally designed to track an animal’s position as it moves. The model was enhanced with a component that could learn from sensory input, allowing it to recreate those sensory experiences later, even without actual sensory input. “The key was adding the ability for the system to learn bidirectionally by communicating with sensory inputs,” Jazayeri says. “Through this associative learning, the model can recreate those sensory experiences.”
What’s next?
The research team plans to explore how the brain processes sequences when landmarks are unevenly spaced or arranged in a ring. They also aim to record brain activity in the hippocampus and entorhinal cortex as animals learn new navigation tasks.
“Watching how memory becomes crystallized in the mind and how this leads to emerging neural activity is a valuable way of understanding how learning happens,” Jazayeri notes. This research, funded by the Natural Sciences and Engineering Research Council of Canada, the Québec Research Funds, the National Institutes of Health, and the Paul and Lilah Newton Brain Science Award, opens up new avenues for understanding the brain’s remarkable ability to navigate both physical and mental landscapes.