The influence of temporal spacing on time-place discrimination.

The conditions under which circadian and interval-timing mechanisms are used in time-place discrimination were investigated. Rats earned the first daily meal by pressing a lever beginning 3.5 h after the start of the session and a second daily meal by pressing another lever. The second meal started 3.5 or 7 h (Experiment 1) or 0.75 or 1.75 h (Experiment 2) after the start of the first meal, using independent groups. In Experiment 1, approximately half of the rats used an interval-timing mechanism, and the other half used a circadian mechanism. In Experiment 2, the rats timed two intervals, one from the start of the session until the first meal and the other from the first to the second meal. A circadian mechanism is relevant to timing intervals in the range of 1.75-3.5 h, and aninterval-timing mechanism can be used to time intervals from 0.75-7 h.