Bejarano, R. & Hackenberg, T.D. (2007).
IRT–stimulus contingencies in chained schedules: Implications for the concept of conditioned reinforcement.
Journal of the Experimental Analysis of Behavior, 88, 215-227.
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.
Key words: chained schedules, conditioned reinforcement, interresponse times, key pecks, pigeons