A rat navigating a maze keeps track of where it’s been and where it’s going using the area of the brain called the hippocampus and updates its path eight times a second, say researchers from Carnegie Mellon University's Robotics Institute and the University of Minnesota in a study published online June 17 by the journal Nature Neuroscience.
Anoopum Gupta, who earned his PhD in robotics at CMU in 2011 and is now pursuing his MD at the University of Pittsburgh School of Medicine, said the rat’s neural activity patterns represent a short path that begins just behind and extends just ahead of the rat. The length of the path, and the relative length in front and behind the animal, changes depending on the rat’s behavior, including its speed, the presence of landmarks and whether the rat is deliberating which way to go.
The hippocampus plays a central role in episodic memory, navigation and episodic future thinking. A number of researchers have explored possible mechanisms for how the hippocampus encodes and recalls experiences and the findings from this study of six trained rats adds insight and evidence regarding a possible mechanism, Gupta said.
Moreover, the study provides the first evidence that the hippocampus may play a role in “cognitive chunking,” the idea that the brain collects information into discrete units that can be efficiently stored and recalled, he added.
When a rat would pass a landmark, neural activity patterns showed that the hippocampus represented more space in front of the animal; when a rat approached a landmark, more space was represented behind the animal. This suggests that the hippocampal firing patterns represented segments of space that were between important landmarks, potentially allowing those segments to be discretely stored in the brain.
In addition to Gupta, the researchers include Carnegie Mellon Computer Science Professor David S. Touretzky and A. David Redish, professor of neuroscience at the University of Minnesota and Matthijs van der Meer, assistant professor of biology at the University of Waterloo.
Previous work by this group of researchers, published two years ago in the journal Neuron, showed that rats at rest often would replay their experiences in a maze in a way suggesting that they weren’t building memories so much as creating maps that would help them make better navigation decisions.
Gupta and his colleagues used electrodes attached to the rats to monitor certain neurons, called place cells, that fire in response to physical locations. Understanding how the activity of individual neurons enables the brain to perform cognitive functions may someday lead to mechanism-driven methods for treating neurocognitive disorders, Gupta said, while deeper knowledge of how animals navigate may provide roboticists clues for improving autonomous navigation systems.