Response summation in the pigeon

Three experiments were performed to determine the effects of schedule of reinforcement on additive summation using pigeons as subjects. In the first experiment the subjects were reinforced on a VI 1-min schedule whenever either of two stimuli was present. Their simultaneous presentation produced response summation. In the second experiment key color and the schedule of reinforcement were changed in alternate sessions. When the reinforcement schedule was VI 30 sec, only one of the four subjects showed response summation. When the reinforcement schedule was VI 3 min, all four subjects showed response summation. In the third experiment, the subjects were reinforced on a VI 30-sec schedule whenever either of two stimuli was present. Their simultaneous presentation produced response summation in only one of three subjects. The results suggest that either high reinforcement frequency or high response rate sets a ceiling beyond which response summation is not observed.