In four experiments, rats were trained to leverpress for food according to variable ratio (VR) schedules of reinforcement. Some subjects received presentations of a brief (500 ms), response-contingent stimulus. For subjects trained on a VR-30 schedule, presenting the stimulus uncorrelated with reinforcement (Experiments 1 and 2), or midway through the ratio (Experiments 2 and 3), produced response rates lower than those supported by a VR-30 schedule with no stimulus, and closely similar to those supported by a simple VR-15 schedule (Experiment 3). In Experiment 4 the stimulus was presented midway through a VR-200 schedule, and in this case its effect was to elevate response rates so that they approximated to those supported by a VR-100 schedule. Experiments 3 and 4 further showed that a procedure in which the stimulus was presented both midway through the ratio and along with food produced effects identical to those generated when the stimulus occurred only in midratio. Since these two procedures were found (in Experiment 3) to generate stimuli with differing Pavlovian properties it as concluded that these properties cannot account for their (similar) effects on operant performance. An interpretation that emphasizes the discriminative function of the stimuli is offered. © 1989.
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