How the brain maps memories without movement

summary: Mental maps are activated in the brain when thinking about sequences of experiences, even without physical movement. In an animal study, they found that the entorhinal cortex contains 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 the non-spatial domain. The findings could enhance our understanding of brain function and memory formation.

Key facts:

  1. Mental maps are created and activated without the need for physical movement.
  2. The entorhinal cortex contains cognitive maps of experiences.
  3. This study provides insight into the cellular basis of mental simulation.

source: Massachusetts Institute of Technology

As you travel your usual route to work or the grocery store, your brain interacts with cognitive maps stored in the hippocampus and entorhinal cortex. These maps store information about the routes you’ve taken and locations you’ve visited before, so you can navigate when you go there.

New research from MIT has found that such mental maps are also created and activated when you think only about sequences of experiences, in the absence of any physical movement or sensory input.

In an animal study, researchers found that the entorhinal cortex contains a cognitive map of what animals experience while using a joystick to scroll through a series 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 demonstrate the cellular basis of mental simulation and imagination in a non-spatial domain through cognitive map activation in the entorhinal cortex.

“These cognitive maps are employed to perform mental navigation, without any sensory input or motor output. We are able to see the signature of this map,” says Mehrdad Jazayeri, associate professor of brain and cognitive sciences, a member of MIT’s McGovern Institute for Brain Research, and lead author. It manifests itself as the animal mentally goes through these experiences.” The study.

Sujaya Neupane, a research scientist at the McGovern Institute, is lead author of the paper, which will appear in nature. Ella Vitti, professor of brain and cognitive science at MIT, a member of MIT’s McGovern Institute for Brain Research, and director of the K. Lisa Yang of Integrative Computational Neuroscience is also an author on the paper.

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Mind maps

A great deal of work in animal and human models 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 when the animal moves through a space it has been in before, immediately before traversing it, or when it is asleep.

“Most previous studies have focused on how these regions reflect the structures and details of the environment when the animal physically moves through space,” Jazayeri says.

“When an animal moves through 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 his colleagues wanted to explore whether these cognitive maps are also built and then used during purely mental processes or imagining movement across non-spatial domains.

To explore this possibility, the researchers trained the animals to use a joystick to trace a path through a series of images (“landmarks”) spaced at regular intervals. During training, animals were shown only a subset of the picture pairs but not all pairs. Once the animals learned how to navigate through the training pairs, the researchers tested whether the animals were able to handle new pairs that they had never seen before.

One possibility is that the animals do not learn the cognitive map of the sequence, and instead solve the task using a memorization strategy. If so, they are expected to have difficulty with new pairs. Instead, if animals rely on a cognitive map, they should be able to generalize their knowledge to novel pairs.

“The results were clear and unambiguous,” Jazayeri says. “The animals were able to mentally navigate between novel pairs of images from the first time they were tested. This finding provided strong behavioral evidence for the existence of a cognitive map. But how does the brain create such a map?”

To answer this question, the researchers recorded from single neurons in the entorhinal cortex while the animals performed this task.

The neural responses had a striking feature: When the animals used a joystick to move between two landmarks, the neurons showed distinct spikes of activity associated with the mental representation of the overlapping landmarks.

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“The brain experiences these bumps of activity at the expected time when the overlapping images pass in front of the animal’s eyes, which never happens,” Jazayeri says.

“And the timing between these bumps, crucially, was exactly the timing the animal was expecting to arrive at, 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 slightly late or early in completing the task, their brain activity showed a similar 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 the ordinal arrangement of features.

Model for learning

To further explore how these cognitive maps work, the researchers built a computational model to mimic the brain activity they found and show how it was generated.

They used a type of model known as the persistent attraction model, which was originally developed to model how the entorhinal cortex tracks an animal’s position as it moves, based on sensory input.

The researchers customized the model by adding a component capable of learning activity patterns generated by sensory input. This model was then able to learn to use those patterns to reconstruct those 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 those sensory experiences,” Jazayeri says.

The researchers now plan to study what happens in the brain if landmarks are not evenly spaced, or if 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 the memory of a structure crystallize in the mind, and how that leads to the neural activity that arises, is a really valuable way of asking how learning happens,” Jazayeri says.

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Financing: The research was funded by the Natural Sciences and Engineering Research Council of Canada, the Quebec Research Funds, the National Institutes of Health, and the Paul and Lily Newton Brain Science Award.

About this memory research news

author: Abby Appazorius
source: Massachusetts Institute of Technology
communication: Abby Apazourius – Massachusetts Institute of Technology
picture: Image credited to Neuroscience News

Original search: Closed access.
Vector production via mental navigation in the entorhinal cortex“By Mehrdad Jazayeri and others. nature


a summary

Vector production via mental navigation in the entorhinal cortex

A cognitive map is an appropriately organized representation that allows new calculations to be made using prior experience; For example, planning a new route in a familiar place. Work on mammals has found direct evidence for such representations in the presence of external sensory inputs in both spatial and non-spatial domains.

Here we tested a basic postulate of the original cognitive map theory: that cognitive maps support internal 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 produce 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 suggests that they relied on an organized representation of the landmarks. Task-modulated neurons showed periodicity and steepness consistent with the temporal structure of landmarks and showed signatures of persistent attractor networks.

The continuous attractor network model of path integration augmented with a Hippie-like learning mechanism provided an explanation for how the system recalls landmarks internally.

The model also made an unexpected prediction that internal landmarks transiently slow path integration, resetting dynamics, and thus reducing variability. This prediction was confirmed in a reanalysis of firing rate variability and behavior.

Our findings link organized activity patterns in the entorhinal cortex to the endogenous recruitment of the cognitive map during mental navigation.

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