Effect of between-category similarity on basic level superiority in pigeons.

Children categorize stimuli at the basic level faster than at the superordinate level. We hypothesized that between-category similarity may affect this basic level superiority effect. Dissimilar categories may be easy to distinguish at the basic level but be difficult to group at the superordinate level, whereas similar categories may be easy to group at the superordinate level but be difficult to distinguish at the basic level. Consequently, similar basic level categories may produce a superordinate-before-basic learning trend, whereas dissimilar basic level categories may result in a basic-before-superordinate learning trend. We tested this hypothesis in pigeons by constructing superordinate level categories out of basic level categories with known similarity. In Experiment 1, we experimentally evaluated the between-category similarity of four basic level photographic categories using multiple fixed interval-extinction training (Astley and Wasserman, 1992). We used the resultant similarity matrices in Experiment 2 to construct two superordinate level categories from basic level categories with high between-category similarity (cars and persons; chairs and flowers). We then trained pigeons to concurrently classify those photographs into either the proper basic level category or the proper superordinate level category. Under these conditions, the pigeons learned the superordinate level discrimination faster than the basic level discrimination, confirming our hypothesis that basic level superiority is affected by between-category similarity.