Further evidence of joint time-place control of rats' behavior: results from an 'open hopper' test.

Rats were trained on an interval time-place task. Food was intermittently available on each of four levers for 4 min in a 16-min session. After baseline training the rats received 'open hopper' sessions in which food was available on all levers for all of the 16-min sessions. Despite the absence of any contingencies for doing so, the rats continued to press the levers in the 'correct' sequence, for roughly the 'correct' amount of time. This confirms that the rat behavior was controlled, in part, by a representation of an elapsed interval of time. The rats responding was more variable in 'open hopper' sessions and error increased (in an exponential fashion) as the session proceeded. This finding suggests that the rats may have used shifts in the location of food availability to minimize the accumulation of error throughout baseline sessions.