A mouse is allowed to move about freely within an enclosed space while an electrode records the activity of a place cell in one of its hippocampi. A video camera records the mouse’s movements so that its position can be correlated with the activity of this neuron.

The images below show the activity patterns for four successive sessions for (1) a normal mouse and (2) a mutant mouse whose Ca2+/calmodulin-dependent kinase gene is constantly active (its activity cannot be modulated).

Before each session, each mouse is removed from the enclosure for a few moments and then put back inside. In the normal mouse, the place cell’s activity pattern is the same in all four sessions (the cell is always active when the mouse goes to a particular area in the enclosure). But in the mutant mouse, this is not the case.