Rats' performance on an interval time-place task: increasing sequence complexity.

Rats were trained on an interval time-place learning (TPL) task in which the location of food availability depended on the time since the start of the session. Each of four levers (numbered 1, 2, 3, 4) provided food on an intermittent schedule for two nonconsecutive 3-min periods. The order in which the levers provided food was 1, 2, 4, 3, 2, 3, 1, 4. This order was consistent across sessions. Previous research conducted in our lab has shown that when only four "places" are used, rather than the eight in the present study, rats use a timing strategy to track the location of food. Pizzo and Crystal (2004) recently trained rats on an interval TPL in which each of eight arms of a radial arm maze provided food. They found evidence suggesting that rats used both spatial and temporal information. In the present study, in which a revisiting strategy was used (i.e., each lever provided food on more than one occasion), the rats tracked both the spatial and the temporal availability of food for the first half of the session. Interestingly, in the second half of the sessions, the rats appeared to be timing the availability of food even though they did not know where it would occur. That is, the rats knew the temporal, but not the spatial, contingencies for the second half of the session. It appears that the requirement of revisiting a previously reinforced lever resulted in rats' no longer being able to solve the spatial aspect of the task.