Operant behavior is organized in bouts that are particularly visible under variable-interval (VI) schedules of reinforcement. Previous research showed that increasing the work required to produce a response decreases the rate at which bouts are emitted and increases the minimum interresponse time (IRT). In the current study, the minimum effective IRT was directly manipulated by changing the minimum duration of effective lever presses reinforced on a VI 40-s schedule. Contrary to assumptions of previous models, response durations were variable. Response durations were typically 0.5 s greater than the minimum duration threshold; durations that exceeded this threshold were approximately log-normally distributed. As the required duration threshold increased, rats emitted fewer but longer bouts. This effect may reflect an effort-induced reduction in motivation and a duration-induced facilitation of a response–outcome association.

操作性行为以回合的形式组织，在可变间隔 (VI) 强化计划下尤为明显。先前的研究表明，增加产生响应所需的工作会降低发出回合的速度，并增加最小交互响应时间 (IRT)。在当前的研究中，最小有效 IRT 是通过改变以 VI 40 秒计划强化的有效杠杆按压的最短持续时间来直接操纵的。与先前模型的假设相反，响应持续时间是可变的。响应持续时间通常比最小持续时间阈值大 0.5 秒；超过此阈值的持续时间近似呈对数正态分布。随着所需持续时间阈值的增加，大鼠发出的次数更少但时间更长。