In a key memory study, after running a maze, rats mentally replay their actions - but backward, like a film played in reverse.

While running, the animal's hippocampal cells fired in order, corresponding with the animal's position on the track. When the animal stopped, many of the same cells fired again, but the sequence of cell activation was in reverse order and spanned the entire track. This replay was literally instant - it took less than a second to replay up to 30 seconds of running.