or relational stimulus processing requires an organism to appreciate the interrelations between or among two or more stimuli (e.g., same or different, less than or greater than). In the current study, we explored the role of concrete and abstract information processing in pigeons performing a visual categorization task which could be solved by attending to either the specific objects presented or the relation among the objects. In Experiment 1, we gave pigeons three training phases in which we gradually increased the variability (that is, the number of object arrays) in the training set. In Experiment 2, we trained a second group of pigeons with an even larger number of object arrays from the outset. We found that, the larger the variability in the training exemplars, the lesser the pigeons' attention to object-specific information and the greater their attention to relational information; nevertheless, the contribution of object-specific information to categorization performance was never completely eliminated. This pervasive influence of object-specific information is not peculiar to animals, but has been observed in young children and human adults as well.