Interval timing under variations in the relative validity of temporal cues.

Two groups of pigeons were trained to respond on a white center key to a fixed-interval, 60-s schedule of reinforcement signaled by the onset of a side-key cue (S+ training). In additional training sessions, S+ trials alternated between S- trials in which a different side-key cue signaled nonreinforcement after 60 s (S+/S- training). For one group, S+/S- training sessions followed S+ training, and for the other group, S+/S- training preceded S+ training. Peak-time curves obtained from extended nonrewarded probe trials inserted among training trials showed loss of control by time during S+/S- training relative to S+ training. A follow-up experiment showed that this result was not caused by a difference in probability of reinforcement. We suggest that attention to time was weakened by the introduction of visual cues that were more valid predictors of trial outcomes.