Summary: The brain’s mental maps are activated when we think about sequences of experiences, even without physical movement. In an animal study, they found that the entorhinal cortex houses a cognitive map of experiences, which is activated during mental simulation.
This is the first study to show the cellular basis of mental simulation in a non-spatial domain. The findings could improve our understanding of brain function and memory formation.
Highlights:
- 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: WITH
When 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 the paths you’ve taken and the places you’ve visited before, so you can find your way back every time you go.
New research from MIT has shown that such mental maps are also created and activated when you simply think about sequences of experiences, in the absence of any physical movement or sensory input.
In an animal study, researchers found that the entorhinal cortex houses a cognitive map of what animals experience when they use 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 is the first study to show the cellular basis of mental simulation and imagination in a 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 can see a signature of this map presenting itself when the animal mentally goes through these experiences,” says Mehrdad Jazayeri, associate professor of brain and cognitive sciences, member of MIT’s McGovern Institute for Brain Research and lead author. of the study.
Sujaya Neupane, a researcher at the McGovern Institute, is the lead author of the paper, which will appear in Nature. Ila Fiete, professor of brain and cognitive sciences at MIT, member of the McGovern Institute for Brain Research at MIT, director of the K. Lisa Yang Integrative Computational Neuroscience Center, is also an author of the article.
Mind maps
Extensive work in animal and human models has shown that representations of physical locations are stored in the hippocampus, a small seahorse-shaped structure, and in the neighboring entorhinal cortex. These representations are activated each time an animal moves into a space in which it has already been, just before crossing the space or when it sleeps.
“Most previous studies have focused on how these areas reflect the structures and details of the environment when an animal physically moves through space,” says Jazayeri.
“When an animal moves around a room, its sensory experiences are well encoded by the activity of neurons in the hippocampus and entorhinal cortex.”
In the new study, Jazayeri and colleagues wanted to determine whether these cognitive maps are also constructed and then used during purely mental journeys or when imagining movements across non-spatial domains.
To explore this possibility, researchers trained animals to use a joystick to trace a path through a sequence of images (“landmarks”) spaced at regular temporal intervals. During training, animals saw only a subset of image pairs, but not all pairs. Once the animals learned to navigate between the training pairs, the researchers tested whether the animals could handle the new pairs that they had never seen before.
One possibility is that the animals do not learn a cognitive map of the sequence, but solve the task using a memorization strategy. If so, they should struggle with the new pairs. Instead, if animals rely on a cognitive map, they should be able to generalize their knowledge to new pairs.
“The results were unequivocal,” says Jazayeri. “The animals were able to mentally navigate between the new pairs of images from the first test. This finding provides strong behavioral evidence for the presence of a cognitive map. But how does the brain establish such a map?
To answer this question, the researchers recorded single neurons in the entorhinal cortex while the animals performed this task.
The neuronal responses exhibited a striking feature: When the animals used the joystick to navigate between two landmarks, the neurons showed distinct bumps of activity associated with the mental representation of the intermediate landmarks.
“The brain experiences these peaks of activity at the expected time, while the intermediate images would have passed through the animal’s eyes, which is never the case,” explains Jazayeri.
“And the crucial moment between these bumps was exactly the time the animal would have expected to reach each of them, which in this case was 0.65 seconds.”
The researchers also showed that the speed of mental simulation was linked to the animals’ performance on the task: when they were a little late or early in performing the task, their brain activity showed a corresponding change in the timing.
Researchers also found that mental representations in the entorhinal cortex do not encode specific visual features of images, but rather the ordinal arrangement of landmarks.
A learning model
To further explore how these cognitive maps work, the researchers built a computer model to mimic the brain activity they found and demonstrate how it could be generated.
They used a type of model known as the continuous attractor model, originally developed to model how the entorhinal cortex tracks an animal’s position as it moves, based on inputs sensory.
The researchers customized the model by adding a component capable of learning activity patterns generated by sensory inputs. 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 bidirectionally by communicating with sensory inputs. Through the associative learning that the model goes through, it will actually recreate these sensory experiences,” says Jazayeri.
The researchers now plan to study what happens in the brain if the landmarks are not evenly spaced or if they are arranged in a ring. They also hope to record brain activity from the hippocampus and entorhinal cortex as the animals learn to perform the navigation task.
“Seeing the memory of the structure crystallize in the mind and how that leads to the neural activity that emerges is a very valuable way of asking how learning occurs,” says Jazayeri.
Funding: The research was funded by the Natural Sciences and Engineering Research Council of Canada, Fonds de recherche du Québec, the National Institutes of Health and the Paul and Lilah Newton Brain Science Award.
About this memory search news
Author: Abby Abazorius
Source: WITH
Contact: Abby Abazorius – MIT
Picture: Image is credited to Neuroscience News
Original research: Closed access.
“Vector production via mental navigation in the entorhinal cortex” by Mehrdad Jazayeri et al. Nature
Abstract
Vector production via mental navigation in the entorhinal cortex
A cognitive map is a suitably structured representation that allows new calculations using previous experience; for example, planning a new route in a familiar space. 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: cognitive maps support endogenous computations without external input.
We recorded from the entorhinal cortex of monkeys a mental navigation task that required monkeys to use a joystick to produce one-dimensional vectors between pairs of visual cues without seeing the intermediate cues.
The monkeys’ ability to perform the task and generalize it to new pairs indicates that they rely on a structured representation of the landmarks. Task-modulated neurons exhibited periodicity and ramping that matched the temporal structure of the landmarks and exhibited signatures of continuous attractor networks.
A continuous attractor network model of path integration, augmented with a Hebbian-type learning mechanism, provided an explanation of how the system could recall landmarks endogenously.
The model also made an unexpected prediction that endogenous cues transiently slow path integration, reset dynamics, and thereby reduce variability. This prediction was confirmed by a new analysis of firing rate variability and behavior.
Our results link structured patterns of activity in the entorhinal cortex to endogenous recruitment of a cognitive map during mental navigation.