Diversity and substitutability of adjunctive activities under fixed-interval schedules of food reinforcement

Six rats received food contingent on pressing a lever on fixed-ratio 1, fixed-interval 30-second, and fixed-interval 60-second schedules, with concurrent access to a drinking spout, a running wheel, and a block of wood. Drinking, running, and chewing were monitored automatically, and these and other activities were observed directly during selected sessions. Because all sessions ended after delivery of 60 pellets, total time available for activities other than eating increased over the three schedules. Time spent contacting the lever and visiting the food tray increased in proportion to total available time, whereas the time spent in other activities changed in a complex manner such that drinking was the dominant adjunctive behavior in the 30-second condition, and running or chewing the dominant adjunctive behavior in five of six rats in the 60-second condition. General activity and grooming also occupied significant amounts of time. In a subsequent part of the experiment, running and chewing were prevented, and the majority of other activities, especially drinking and grooming, increased. The results show that (a) FI schedules of food reinforcement are accompanied by a wide variety of adjunctive activities; (b) the preferred activity differs according to the schedule duration; and (c) the extent to which activities substitute for one another is limited by the tendency for different activities to occupy different parts of the interreinforcement interval.