Superordinate category formation in pigeons: association with a common delay or probability of food reinforcement makes perceptually dissimilar stimuli functionally equivalent.

Training associated pairs of perceptually dissimilar stimulus classes with a common delay or probability of food reinforcement in pigeons. Then, different choice responses were trained to 1 component class in each pair. In a choice test, the untrained class in each pair occasioned the same response as did the choice-trained class. In a 3rd experiment, 2 classes had reinforcement delays of 1 s and 15 s, respectively, and 2 other classes had reinforcement probabilities of 0.1 and 0.9. Then, 1 choice response was reinforced to a class previously associated with a better condition of reinforcement (e.g., 1-s delay or 1.0 probability), and a different response was reinforced to a class previously associated with a worse condition of reinforcement (0.1 probability or 0-s delay). Testing with all classes suggested that categorization was based on the relative reinforcement or hedonic value and not on the parametric details of reinforcement.