Conditioned reinforcement and discrimination in second-order schedules.

Pigeons were exposed to multiple second-order schedules in which responding on the "main key" was reinforced according to either a variable-interval or fixed-interval schedule by production of a brief stimulus on the "brief-stimulus key". A response was required to the brief stimulus during its fourth (final) presentation to produce food; responses to the earlier brief stimuli indicated the extent to which the final brief stimulus was discriminated from preceding ones. Main-key response rates were higher in early components of paired brief-stimulus schedules, in which each brief stimulus was the same as that paired with reinforcement, than in comparable unpaired brief-stimulus or tandem schedules. Poor discrimination occurred between paired brief stimuli (Experiment I). When chain stimuli on the main key induced a discrimination between the first two and second two brief stimuli, the response-rate enhancement in the paired brief-stimulus schedule persisted (Experiment II). Rate enhancement diminished when the initial link of the chain included the first three components (Experiment IV). Eliminating the contingency between responding and brief-stimulus production also diminished rate enhancement (Experiment III). The results show that the discriminative and conditioned reinforcing effects of food-paired brief stimuli may be selectively manipulated and suggest that the reinforcing effects are modulated by other reinforcers in the situation.