CONDITIONED REINFORCEMENT AND CHOICE WITH DELAYED AND UNCERTAIN PRIMARY REINFORCERS

In an adjusting-delay choice procedure, pigeons could peck on either a red key or a green key. A peck on the red key always led to a delay a ssociated with red houselights and then food. The delay was adjusted o ver trials to estimate an indifference point-a delay at which the two keys were chosen about equally often. In some conditions, a peck on th e green key led to food on all trials after delays of either 10 a or 3 0 s, and green houselights were lit during the delays. In other condit ions, food was presented on only half of the green-key trials. If the green houselights continued to occur on both reinforcement and nonrein forcement trials, preference for the green key always decreased. Prefe rence for the green key also decreased if half of the trials had 30-s houselights followed by food and the other half had no green houseligh ts and no food. However, preference for the green key actually increas ed if half of the trials had 10-s green houselights followed by food a nd the other half had no green houselights followed by no food. The la tter condition therefore demonstrated a case in which preference for a n alternative increased when food was removed from half of the trials. The results suggest that the red and green houselights served as cond itioned reinforcers. A hyperbolic decay model (Mazur, 1989) provided g ood predictions for all conditions by assuming that the strength of a conditioned reinforcer is inversely related to the total time spent in its presence before food is delivered.