Researchers at Princeton found that brain cells known to be involved in making maps of the external environment, collectively known as the brain’s “GPS,” are also active in representing other tasks involved in memory and cognition. The researchers found that the neurons fired in sequence in accordance with the rats’ activities as they listened to sounds and pressed levers to get rewards when the tone achieved a predetermined frequency. Above, the left diagram shows the neural activity (orange) in a group of neurons over a period of several seconds as the rats pressed and released the levers in response to the sounds they heard. The diagram on the right shows that individual cells (labeled 1 through 9) fire in sequence to represent the press and release of the lever. Credit: David Tank, Princeton University
The part of the brain that creates mental maps to help us navigate may have a larger role in memory and learning that previously believed.
“Almost 40 years of research suggested that a certain region of the brain was devoted to spatial navigation,” David Tank, Princeton’s Henry L. Hillman Professor in Molecular Biology and co-director of the Princeton Neuroscience Institute. “We found that this same region is also involved when navigating not only spatial environments but also cognitive ones.”
The study examined the hippocampus region of the brain. In previous studies with mice, this area is known to become active when the animals travel around their environment. Research and related work showing that cells in the nearby entorhinal cortex fire when animals reach specific locations, led to the finding that the brain creates an internal representation of the outside world—a sort of mental positioning system—that tells an animal where it is in its environment.
The new research found that those same brain regions are active when the brain is exploring a very different kind of environment, one involving listening to sounds. The researchers monitored neural activity as the rats listened and responded to certain sounds and found similar firing patterns to those seen when rats are exploring their environments.
The researchers theorized that the hippocampus and the nearby entorhinal cortex, which work together to make these mental maps, were not specific to mapping but were involved in more general cognitive tasks and that mapping was just one aspect of larger cognitive tasks involving learning and memory.
They gave rats a different task like exploring sounds to see evidence of cognitive activities in the hippocampal-entorhinal circuit.
The researchers selected sound as an analogy to space because both can vary along a continuum—the rats can explore ever-increasing frequencies the way they would move forward along a lengthy corridor.
To test this theory, they monitored the electrical activity of neurons in the hippocampal and entorhinal regions while the rats manipulated sounds and learned to associate certain sound frequencies with rewards.
They first taught the rats to depress a lever to increase the pitch or frequency of a tone being played over a speaker. The rats learned that if they released the lever when the tone reached a predetermined frequency range, they would receive a reward.
A pattern of neuronal firing that corresponded to the rats’ behaviors emerged during the task. Sequences of neural activity were produced as the rats advanced through the progression of places in space and there were even patterns that corresponded to particular sound frequencies.
The neurons involved in these firing patterns were identical to those involved in mapping and navigation.
The findings suggest that there are common mechanisms in the hippocampal-entorhinal system that can represent diverse sorts of tasks.
“The implication from our work is that these brain areas don’t represent location specifically, but rather they can represent other relevant features of the animal’s experience,” Tank said. “When those features vary in a continuous way, sequences of neural activation are produced.”
Dmitriy Aronov, first author on the paper who conducted the work while a postdoctoral researcher at the Princeton Neuroscience Institute and who is now an assistant professor of neuroscience at Columbia University, explained how the discovery fits with how researchers think about mapping our environment in the context of learning about new places and forming memories of experiences.
“When you visit a new location, you don’t only make a mental map, but you also form memories of your location,” Aronov said in a statement. “We feel that this study solves the mystery of the hippocampus in representing both memory and location, in that these neurons are general purpose neurons capable of representing any relevant information.”
The study was published in Nature.
