WITHIN-SESSION CHANGES IN RESPONDING DURING CONCURRENT SCHEDULES WITHDIFFERENT REINFORCERS IN THE COMPONENTS

Rats and pigeons responded on several concurrent schedules that provid ed different reinforcers in the two components (food and water for rat s, Experiment 1; wheat and mixed grain for pigeons, Experiment 2). The rate of responding and the time spent responding on each component us ually changed within the session. The within-session changes in respon se rates and time spent responding usually followed different patterns for the two components of a concurrent schedule. For most subjects, t he bias and sensitivity to reinforcement parameters of the generalized matching law, as well as the percentage of the variance accounted for , decreased within the session. Negative sensitivity parameters were s ometimes found late in the session for the concurrent food-water sched ules. These results imply that within-session changes in responding co uld cause problems for assessing the validity of quantitative theories of concurrent-schedule responding when the components provide differe nt reinforcers. They question changes in a general motivational state, such as arousal, as a complete explanation for within-session changes in responding. The results are compatible with satiation for, or sens itization-habituation to, the reinforcers as explanations.