Resistance to variable-time schedules produced by spaced-response reinforcement schedules

Two experiments are reported in which rats were exposed to either a tandem fixed-ratio 1 differential-reinforcement-of-low-rate 10 sec schedule (tand FR1 DRL 10) (Experiment 1) or to a DRL 10 sec schedule (Experiment 2) prior to being exposed to a variable-time (VT) schedule. Response decrement was not universally found during the VT phase, one rat emmitting an increase in response rate relative to baseline (Experiment 1), while another showed neither an increase nor decrease during VT relative to baseline (Experiment2). The VT schedule induced less efficient responding in both experiments. Interresponse time (IRT) distributions (Experiment 2) indicated that the VT changed the pattern of responding. These results, when combined with others from our laboratory, indicate that although unsignalled variable delay of reinforcement may be a sufficient condition for producing resistance to response-independent reinforcement, it is not a necessary condition. It was concluded that a response competition model of the kind proposed by Henton and Iversen (1978) might have explanatory merit in this kind of experimental situation.