Place cells in the hippocampus become active at specific locations in the environment and are thought to be involved in spatial memory; reactivation of these neurons during sleep is thought to consolidate spatial memories. de Lavilléon et al. devised an experiment to create an artificial place–reward association (similar to the conditioned place preference paradigm) in sleeping mice: place cell spikes that were above a certain threshold triggered stimulation of the median forebrain bundle, the activation of which is thought to be rewarding. This protocol led to a marked place preference for the place field associated with the place cell that had been stimulated, thus directly illustrating that place cell reactivation during sleep is important for spatial memory and navigation.