In an area of the brain known as the hippocampus, there are special neurons known as place cells. These fire up whenever an animal enters a particular place in its environment. The firing up of these cells constructs a “cognitive map” in the brain, and has allowed scientists to work out where a rat is based on which neurons are active. Now, researchers have also managed to determine where the rat will go next.
As reported in the journal Neuron, the team noticed that the activation of a particular place cell could be linked to either where the rat was or where it was going. They placed rats in an eight-arm maze, where three arms contained hidden food. The team used this set up to test both reference memory, which allows the rat to remember which arms contain food and which don't, and working memory, which allows the rat to remember which arms it has visited and which it still needs to explore.
The team was able to study the firing of place cells during the tasks. In reference memory tests, the sequence of place cells firing up gave the researchers an idea of where the rat was going to go next.
"This gives us an insight into what the animal is thinking about space," senior author Jozsef Csicsvari, from the Institute of Science and Technology Austria, said in a statement. "We used this concept to understand how rats think during tasks that test their spatial memory. The animal is thinking about a different place than the one it is in. In fact, we can predict which arm the rat will enter next."
The team could predict more than just where the rat was going. They could also tell when a rat was about to go down the wrong path. "When the rat makes a mistake, it replays a random route," Csicsvari added. "Based on the place cells, we can predict that the rat will make a mistake before it commits it."
While the research is very exciting with respect to reference memory, it’s not applicable to working memory. In working memory tasks, the firing patterns only corresponded to the last arm visited by the rat. These two approaches are accessed in different ways by the brain.
"With reference memory, the brain truly navigates and remembers that 'this is a location I have to go to'," said Csicsvari. "This uses the hippocampus, which is important for spatial tasks. Working memory is more abstract, each location is an item on the animal's list of places to visit. The hippocampus probably signals to the prefrontal cortex where the rat was, and the prefrontal cortex keeps track of which items it can tick off"."