IRT-stimulus contingencies in chained schedules: implications for the concept of conditioned reinforcement

Two experiments with pigeons investigated the effects of contingencies between interresponse times (IRTs) and the transitions between the components of 2- and 4-component chained schedules (Experiments 1 and 2, respectively). The probability of component transitions varied directly with the most recent (Lag 0) IRT in some experimental conditions and with the 4th (Lag 4) IRT preceding the most recent one in others. Mean component durations were constant across conditions, so the reinforcing effect of stimulus change was dissociated from that of delay to food. IRTs were longer in the Lag-0 than in the Lag-4 conditions of both experiments, thus demonstrating that stimulus change functioned as a reinforcer. In the Lag-0 conditions of Experiment 2, the Component-1 IRTs increased more than the Component-2 IRTs, which in turn increased more than the Component-3 IRTs. This finding runs counter to the conditioned-positive-reinforcement account of chained-schedule responding, which holds that the reinforcing effect of stimulus change should vary in strength as an inverse function of the delay to the unconditioned reinforcer at the end of the chain because conditioned reinforcement is due to first- or higher-order classical conditioning. Therefore, we present other possible explanations for this effect.