When a rat is placed in a certain environment, several hippocampal cells fire. These place cells (hippocampal neurons responsive to a particular location) remain inactive until the rat passes through that place again.
Recordings taken from these cells during NERM sleep (non-random eye movement), during food-search and again during NREM sleep after food-searching showed correlations.
Thus, the blanks can be filled with place cell; NERM.