Summary: Mental maps in the brain are activated when thinking about sequences of experiences even without physical movement. In an animal study, they found that the entorhinal cortex hides a cognitive map of experiences that is activated during mental simulation.
This is the first study to show the cellular basis of mental simulation in the non-spatial domain. The findings could improve our understanding of brain function and memory formation.
Key facts:
- Mental maps are created and activated without physical movement.
- The entorhinal cortex contains cognitive maps of experiences.
- This study provides insight into the cellular basis of mental simulation.
Source: HAVE
As you travel your usual route to work or the grocery store, your brain engages the cognitive maps stored in your hippocampus and entorhinal cortex. These maps store information about the paths you’ve taken and the places you’ve already been, so you can be navigated whenever you go there.
New research from MIT has found that such mental maps are also created and activated when you just think about sequences of experiences, without any physical movement or sensory input.
In an animal study, researchers found that the entorhinal cortex contains a cognitive map of what animals experience when they use a joystick to scroll through a sequence of images. These cognitive maps are then activated when thinking about these sequences, even when the images are not seen.
This is the first study to show the cellular basis of mental simulation and imagination in the non-spatial domain through the activation of a cognitive map in the entorhinal cortex.
“These cognitive maps are recruited to perform mental navigation without any sensory input or motor output.” We are able to see the signature on this map that presents itself as the animal mentally goes through these experiences,” says Mehrdad Jazayeri, associate professor of brain and cognitive sciences, member of the McGovern Institute for Brain Research at MIT, and lead author. studies.
McGovern Institute research scientist Sujaya Neupane is lead author of the paper, which appears in Nature. Ila Fiete, MIT professor of brain and cognitive sciences, member of MIT’s McGovern Institute for Brain Research, director of the K. Lisa Yang Integrative Computational Neuroscience Center, is also an author on the paper.
Mental maps
A large body of work in animal models and humans has shown that representations of physical locations are stored in the hippocampus, a small seahorse-shaped structure, and the nearby entorhinal cortex. These representations are activated whenever the animal moves through the space it was previously in, just before it passes through the space, or when it sleeps.
“Most previous studies have focused on how these areas reflect the structures and details of the environment as the animal physically moves through the space,” says Jazayeri.
“When an animal moves around a room, its sensory experiences are nicely encoded by the activity of neurons in the hippocampus and entorhinal cortex.”
In the new study, Jazayeri and his colleagues wanted to investigate whether these cognitive maps are also created and then used during purely mental flybys or imagining motion in non-spatial domains.
To investigate this possibility, the researchers trained animals to use a joystick to follow a path along sequences of images (“landmarks”) spaced at regular time intervals. During training, animals were shown only a subset of picture pairs, but not all pairs. Once the animals had learned to navigate through the training pairs, the researchers tested whether the animals could handle new pairs that they had never seen before.
One possibility is that animals do not learn a cognitive map of the sequence and instead solve the task using a memorization strategy. If so, they would be expected to fight with the new couples. Instead, if animals were to rely on a cognitive map, they should be able to generalize their knowledge to new pairs.
“The results were clear,” says Jazayeri. “The animals were able to mentally navigate between new image pairs from the first test. This finding provided strong behavioral evidence for the presence of a cognitive map. But how does the brain create such a map?”
To address this question, the researchers recorded from individual neurons in the entorhinal cortex as the animals performed this task.
The neural responses had a remarkable feature: When the animals used a joystick to navigate between two landmarks, the neurons showed significant bursts of activity associated with the mental representation of the intervening landmarks.
“The brain goes through these bursts of activity at the expected time when the intervening images would pass through the animal’s eyes, which never happened,” says Jazayeri.
“And the timing between those impacts, critically, was exactly the timing that the animal would expect to reach each one, which in this case was 0.65 seconds.”
The researchers also showed that the speed of mental simulation was related to the animals’ performance on the task: When they were a little late or early in completing the task, their brain activity showed a corresponding change in timing.
The researchers also found evidence that mental representations in the entorhinal cortex do not encode specific visual features of images, but rather an ordinal arrangement of landmarks.
Learning model
To further investigate how these cognitive maps might work, the researchers created a computational model that mimics the brain activity they found and demonstrated how it might be generated.
They used a type of model known as a continuous attractor model, which was originally developed to model how the entorhinal cortex tracks the position of an animal as it moves based on sensory input.
The researchers modified the model by adding a component that was able to learn patterns of activity generated by sensory input. This model was then able to learn to use these patterns to reconstruct these experiences later when there was no sensory input.
“The key element we needed to add is that this system has the ability to learn bi-directionally by communicating with sensory inputs. Through the associative learning that the model goes through, it actually recreates those sensory experiences,” says Jazayeri.
The researchers now plan to investigate what happens in the brain when landmarks are not evenly spaced, or when they are arranged in a ring. They also hope to record brain activity in the hippocampus and entorhinal cortex when the animals first learn to perform a navigation task.
“Seeing how the memory of a structure crystallizes in the mind and how that leads to the neural activity that emerges is a really valuable way to ask how learning happens,” says Jazayeri.
Funding: The research was funded by the Natural Sciences and Engineering Council of Canada, the Québec Research Funds, the National Institutes of Health and the Paul and Lilah Newton Brain Science Award.
About this memory research report
Author: Abby Abazorius
Source: HAVE
Contact: Abby Abazorius – MIT
Picture: Image is credited to Neuroscience News
Original Research: Closed access.
“Vector production through mental navigation in the entorhinal cortex” by Mehrdad Jazayeri et al. Nature
Abstract
Vector production through mental navigation in the entorhinal cortex
A cognitive map is a suitably structured representation that enables new computations using previous experience; for example, planning a new route in a known area. Work in mammals has found direct evidence for such representations in the presence of exogenous sensory inputs in both spatial and nonspatial domains.
Here, we tested a fundamental postulate of the original cognitive map theory: that cognitive maps support endogenous computations without external input.
We recorded from the entorhinal cortex of monkeys in a mental navigation task that required monkeys to use a joystick to create one-dimensional vectors between pairs of visual landmarks without seeing the intermediate landmarks.
The monkeys’ ability to perform the task and generalize to novel pairs indicated that they relied on a structured representation of landmarks. Task-modulated neurons exhibited periodicity and ramping that matched the temporal structure of landmarks and exhibited features of continuous attractor networks.
A continuous attractor network model of path integration augmented with a Hebbian-like learning mechanism provided an explanation for how the system can endogenously recall landmarks.
The model also made the unexpected prediction that endogenous landmarks transiently slow path integration, resetting dynamics and thereby reducing variability. This prediction was confirmed by reanalysis of firing variability and behavior.
Our findings link structured patterns of activity in the entorhinal cortex to the endogenous acquisition of a cognitive map during mental navigation.