As you travel a familiar route, your brain utilizes cognitive maps stored in the hippocampus and entorhinal cortex. These maps, containing information about previously encountered paths and locations, aid in navigation. New research from MIT reveals that such mental maps are also generated and employed when you simply contemplate sequences of experiences, even without physical movement or sensory input.
In an animal study, researchers discovered that the entorhinal cortex houses a cognitive map of what animals experience while using a joystick to navigate a sequence of images. These cognitive maps are then activated when thinking about these sequences, even when the images are not visible. This study marks the first to demonstrate the cellular basis of mental simulation and imagination in a nonspatial domain through the activation of a cognitive map in the entorhinal cortex.
“These cognitive maps are being recruited to perform mental navigation, without any sensory input or motor output. We are able to see a signature of this map presenting itself as the animal is going through these experiences mentally,” says Mehrdad Jazayeri, an associate professor of brain and cognitive sciences, a member of MIT’s McGovern Institute for Brain Research, and the senior author of the study.
McGovern Institute Research Scientist Sujaya Neupane is the lead author of the paper, which appears today in Nature. Ila Fiete, a professor of brain and cognitive sciences at MIT, a member of MIT’s McGovern Institute for Brain Research, and director of the K. Lisa Yang Integrative Computational Neuroscience Center, is also an author of the paper.
Extensive research in animal models and humans has established that representations of physical locations are stored in the hippocampus and the nearby entorhinal cortex. These representations are activated when an animal moves through a familiar space, just before it traverses the space, or even during sleep.
“Most prior studies have focused on how these areas reflect the structures and the details of the environment as an animal moves physically through space,” Jazayeri says. “When an animal moves in a room, its sensory experiences are nicely encoded by the activity of neurons in the hippocampus and entorhinal cortex.”
Jazayeri and his colleagues aimed to investigate whether these cognitive maps are also constructed and utilized during purely mental run-throughs or imagining of movement through nonspatial domains. To explore this, researchers trained animals to navigate a path through a sequence of images (“landmarks”) spaced at regular temporal intervals using a joystick. During training, the animals were only shown a subset of image pairs, not all. Once the animals learned to navigate the training pairs, researchers tested their ability to handle new, unseen pairs.
One possibility was that the animals might rely on memorization instead of learning a cognitive map of the sequence. If so, they would struggle with the new pairs. Conversely, if they relied on a cognitive map, they should be able to generalize their knowledge to the new pairs.
“The results were unequivocal,” Jazayeri says. “Animals were able to mentally navigate between the new pairs of images from the very first time they were tested. This finding provided strong behavioral evidence for the presence of a cognitive map. But how does the brain establish such a map?”
To answer this, the researchers recorded from single neurons in the entorhinal cortex as the animals performed the task. Neural responses exhibited a striking feature: As the animals navigated between two landmarks, neurons displayed distinctive bumps of activity associated with the mental representation of the intervening landmarks.
“The brain goes through these bumps of activity at the expected time when the intervening images would have passed by the animal’s eyes, which they never did,” Jazayeri says. “And the timing between these bumps, critically, was exactly the timing that the animal would have expected to reach each of those, which in this case was 0.65 seconds.”
The study also revealed a correlation between the speed of mental simulation and task performance: When animals were slightly late or early in completing the task, their brain activity showed a corresponding change in timing. Additionally, the researchers found evidence suggesting that the mental representations in the entorhinal cortex encode the ordinal arrangement of the landmarks rather than specific visual features of the images.
To further investigate the workings of these cognitive maps, the researchers developed a computational model to simulate the observed brain activity and demonstrate its potential generation. They employed a continuous attractor model, originally designed to model how the entorhinal cortex tracks an animal’s position based on sensory input as it moves.
The researchers customized the model by incorporating a component capable of learning the activity patterns generated by sensory input. This enabled the model to learn and utilize those patterns to reconstruct experiences later, even without sensory input.
“The key element that we needed to add is that this system has the capacity to learn bidirectionally by communicating with sensory inputs. Through the associational learning that the model goes through, it will actually recreate those sensory experiences,” Jazayeri says.
The researchers now plan to investigate what happens in the brain if the landmarks are not evenly spaced or arranged in a ring. They also aim to record brain activity in the hippocampus and entorhinal cortex as animals learn to perform the navigation task.
“Seeing the memory of the structure become crystallized in the mind, and how that leads to the neural activity that emerges, is a really valuable way of asking how learning happens,” Jazayeri says.
The research was 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.
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