Factors controlling the micro-structure of human free-operant behaviour: Bout-initiation and within-bout responses are effected by different aspects of the schedule

Two experiments examined factors controlling human free-operant performance in relation to predictions based on the nature of bout-initiation and within-bout responding. Overall, responding was higher for a random ratio (RR) than a random interval (RI) schedule, with equal rates of reinforcement. Bout-initiation rates were not different across the two schedules, but within-bout rates were higher on the RR schedule. Response cost reduced overall rates of responding, but tended to suppress bout-initiation responding more than within-bout responding (Experiments 1 & 2). In contrast, reinforcement magnitude increased all forms of responding (Experiment 2). One explanation consistent with these effects is that bout-initiation responses are controlled by overall rates of reinforcement through their impact on the context (i.e. are stimulus-driven), but that within-bout responses are controlled by response reinforcement (i.e. are goal-directed). These current findings are discussed in the light of these theoretical suggestions.