Evidence for time-place learning in the Morris water maze without food restriction but with increased response cost.

Time-place learning is the ability to distinguish between resources that vary in location at different times of day. Only one previous report has demonstrated successful time-place learning without using food as reward. In this experiment, satiated rats failed to form time-place discriminations in a Morris water maze while food deprived rats did, leading to the conclusion that food system activation is necessary for time-place learning. However, in addition to food system activation, response cost was also increased, which previously has been demonstrated to be effective in allowing the formation of time-place discriminations. The purpose of these two experiments is to test whether food system activation or heightened response cost allowed for time-place learning in the Morris water maze. In the first experiment, we replicate the failure to find time-place discriminations in the Morris water maze without food restriction and without increased response cost. In the second experiment, we found that increased response cost without food restriction was effective in allowing the formation of a time-place discrimination. The implications of this result are discussed in light of the timing mechanism used for time-place discriminations, the nature of the response cost, and the event-time-place tripartite association.