The hippocampus, a part of the brain essential for memory, has long been known to "replay" recently experienced events. Previously, replay was believed to be a simple process of reviewing recent experiences in order to help consolidate them into long-term memory. However, University of Minnesota Medical School researcher A. David Redish, Ph.D., along with his colleagues Anoopum Gupta and David S. Touretzky, Ph.D., from the Center for the Neural Basis of Cognition at Carnegie Mellon University, and Matthijs van der Meer, Ph.D., also at the University of Minnesota, have discovered that the replay function of the hippocampus is actually a much more complex, cognitive process.
This discovery gives insight into the decision-making process and Redish hopes that this insight will help researchers learn more about what is happening in the brain when this decision-making process goes wrong.
By studying brain activity in rats, Redish, Gupta, and colleagues discovered that the information played out during this replay process depends on the task the animal is facing. They found that it was not the more recent experiences that were played back in the hippocampus, but instead, the animals were most frequently playing back the experiences they had encountered the least. They also discovered that some of the sequences played out by the animal were ones they had never before experienced. These observations suggest that this hippocampal process plays an important role in an animal's active learning process and may also play a role in maintaining the animal's internal representation of the world, or its cognitive map.
"The point of the cognitive map is flexibility. It gives animals the ability to plan novel paths within their environment," said Redish. "This replay process may be an animal's way of learning how the world is interconnected, so it can plan new routes or paths."
Redish and his team have been studying decision-making in rats by putting electrode "hats" on the animals and recording their brain activity. The hats detect when individual neurons in the brain fire. Certain neurons, called place cells, fire in response to the animal's current physical location and create the animal's internal, cognitive map of their environment. Through their study of the animal's brain activity, Redish and his team can identify where the rat is located simply by observing which place cells are firing.