As you travel your usual route to work or the grocery store, your brain engages cognitive maps stored in your hippocampus and entorhinal cortex. These maps store information about paths you have taken and locations you have been to before, so you can navigate whenever you go there. New research from MIT has found that such mental maps also are created and activated when you mereThe researchers discovered that the entorhinal cortex holds a cognitive map of an animal’s experiences while they use a joystick to navigate through a series of images. This means that when animals think about these sequences, even when the images are not visible, the cognitive maps in the entorhinal cortex are activated.
This study is the first to demonstrate the cellular foundation of mental simulation and imagination in a nonspatial domain by activating a cognitive map in the entorhinal cortex.
the entorhinal cortex plays a crucial role in mental representation and imagination, even in the absence of physical movement or sensory input.Navigation occurs without sensory input or motor output, and this mental experience leaves a signature on the brain map, according to Mehrdad Jazayeri, an associate professor of brain and cognitive sciences and a member of MIT’s McGovern Institute for Brain Research. Sujaya Neupane, a Research Scientist at the McGovern Institute, is the lead author of the study, which will be published in Nature. Ila Fiete, a professor of brain and cognitive sciences at MIT and a member of the McGovern Institute, will also be part of the publication.The paper’s author, from the Neural Neuroscience Center, is also an author of the paper.
Mental maps
Many studies with animals and humans have demonstrated that the hippocampus, a seahorse-shaped structure, and the nearby entorhinal cortex are where representations of physical locations are stored. These representations are activated when an animal moves through a space it has been in before, just before it traverses the space, or when it is asleep.
“Previous studies have mostly focused on how these areas reflect the structures and the details of the environment as an animal moves physically through it.”Space is a big deal,” Jazayeri explains. “When an animal moves in a room, its sensory experiences are effectively recorded by the neurons in the hippocampus and entorhinal cortex.”
The new study aimed to investigate whether these cognitive maps are also constructed and utilized during purely mental simulations or imagining of movement through nonspatial domains.
To test this possibility, the researchers taught animals to navigate a path through a series of images (“landmarks”) using a joystick at regular intervals. During the training, the animals were only shown some pairs of images.
Not all pairs were tested, only the ones that the animals had learned to navigate through during training. The researchers then tested if the animals could handle new pairs that they had never seen before.
One possibility is that the animals do not learn a cognitive map of the sequence and instead use a memorization strategy to solve the task. If this is the case, they would struggle with the new pairs. On the other hand, if the animals rely on a cognitive map, they should be able to apply their knowledge to the new pairs.
“The results were clear,” Jazayeri explains. “Animals were able to mentally navigate between the new pairs of images from the very beginning.”At the first time they were tested, it was found that there was strong behavioral evidence for the existence of a cognitive map. But how does the brain create such a map? To address this question, the researchers observed the activity of individual neurons in the entorhinal cortex while the animals completed this task. They found that the neural responses had a notable feature: as the animals used the joystick to move between two landmarks, the neurons showed distinct bursts of activity that were linked to the mental representation of the landmarks in between. “The brain experiences these bursts of activity at the anticipated time when the animal is in a particular location.””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 researchers also found that the speed of the mental simulation was linked to the animals’ performance on the task. When they were slightly off in completing the task, their brain activity showed a corresponding change in timing. The researchers also discovered evidence suggesting that the mental representations in the entorhinal cortex do not encode specific visual features.
The emphasis is not on the images, but rather on the sequential arrangement of the landmarks.
An educational model
In order to further investigate the potential functioning of these cognitive maps, the scientists created a computational model to replicate the brain activity they observed and demonstrate how it could be produced. They utilized a type of model called a continuous attractor model, which was initially designed to simulate how the entorhinal cortex monitors an animal’s position as it moves, based on sensory input.
The scientists tailored the model by including a component that was capable of learning the activity patterns generated by sensory input. This model was
Researchers have discovered that the brain has the ability to learn from sensory input and use that information to recreate experiences later, even when there is no sensory input. According to Jazayeri, the key element needed for this system to learn bidirectionally is the ability to communicate with sensory inputs. The model is able to go through associational learning and recreate sensory experiences. The researchers plan to further investigate the effects of unevenly spaced landmarks or arranged landmarks in a ring on the brain. Additionally, they hope to record brain activity in the hippocampus and entorhinal cortex as animals learn.
To carry out the task of navigation.
“Observing the memory of the arrangement solidify in the brain, and how that results in the neural activity that arises, is a very useful method of questioning how learning occurs,” Jazayeri explains.
The study was supported 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.
