Four experiments explored the factors controlling human responding on random interval (RI) schedules of reinforcement. All experiments identified 2 types of responding: "bout-initiation" and "within-bout" responding. Responding on RI schedules was related to the interval value rates, being higher on an RI-30s than on an RI-60s or RI-120s schedule, which impacted bout-initiation responding to the greater degree (Experiments 1 and 3). Experiment 2 found similar overall response rates on random ratio (RR) and random interval with a linear feedback loop (RI+) schedules, with both higher than on an RI schedule. Bout-initiation rates were similar across all schedules, but within-bout responding differed. Experiments 3 and 4 examined the impact of a response cost and noted greater bout-initiation responding but not greater within-bout rates with low costs. Overall, these experiments suggest that bout-initiation responding may be subject to control by factors that increase the strength of conditioning to the context, whereas within-bout responding is less sensitive to these influences. (PsycINFO Database Record

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