To test this hypothesis, we placed rats on a straight track with a T-junction ahead while recording place cells from their hippocampus. Access to the junction – as well as the left and right hand arms beyond it – was prevented by a transparent barrier.
One of the arms had food at the end, while the other side was empty. After observing the food the rats were put in a sleep chamber for an hour. Finally, the barrier was removed and the rats were returned to the track and allowed to run across the junction and on to the arms.
During the rest period, data showed that the place cells that would later provide an internal map of the food arm were active. Cells representing the empty arm were not activated in this way. The map was sequentially activated consistent with trajectories leading to and from the food, indicating that the hippocampus was simulating or preparing future paths leading to a desired goal.