A new study shows that a specific pattern of activity in one part of the brain occurs when rats in a maze are playing out memories that help them decide which way to go. This demonstrates that rats rely on memories when making decisions.

After a rat travels the different pathways in a maze, spatial trajectories are recorded in the hippocampus. A pattern of brain activity called sharp-wave ripple (SWR) events represents the possible paths that the rat can take from its current location. These SWRs are 'reactivated' when the rat must decide which path to take. In this way, SWRs play an important role in the evaluation of upcoming choices.

Loren M. Frank (Massachusetts Institute of Technology, Cambridge) and colleagues asked how SWR reactivation aided memory-guided spatial decisions in Long-Evans rats (Neuron 77, 1163–1173; 2013). Rats were placed in a W-shaped maze and rewarded for visiting the different arms of the maze in the following order: center, left, center, right, center, left, etc. Frank's team was most interested in SWR activity when the animals were in the center arm, because at that point the animals must remember whether they previously visited the left or right arm in order to know which arm they should visit next to earn the reward. The researchers found that the number of SWRs was greatest when the rats were still—that is, when they were considering their options. These memory reactivation events represented sequences that proceeded away from the animal's current location when the rats chose the correct path.

The researchers measured the brain activity of the rats while they were learning to perform this task and after they had perfected it. They found that there was more SWR reactivation before the rats correctly chose which way to go. The proportion of activated cells could predict whether the rat chose the correct path or the incorrect path. In other words, when the rat remembered the possibilities better, it made better decisions.

Credit: iStockphoto.com/lculig

Interestingly, the trajectories represented by the SWRs included both the correct choice and the incorrect choice. When the rats had mastered the task (>85% correct trials), there was a bias toward reactivating the representation of the correct pathway. The scientists suggested that perhaps the hippocampus becomes biased toward reactivating the correct action sequence in order to increase the likelihood of the rat carrying out that action sequence.