General discussion
Two experiments tested the ability of human participants to learn the same category structures using either direct or indirect categorization tasks. The results showed that participants learned little about the category structures, while they made the same–different category judgments, unless the categories were separated by a one-dimensional category bound. Experiment 2 also showed that previous categorical
knowledge acquired during a direct classification task can be expressed in the same–different categorization task, but only when the categories can be separated by a
rule that is easily verbalized. These results suggest that there may be a limit to what can be learned indirectly about the world. It may be that much categorical knowledge can only be acquired directly. Furthermore, our results also suggest that much of this knowledge may be inaccessible to abstract reasoning. These two findings are consistent with the predictions made by COVIS (Ashby et al., 1998), which suggests that information-integration category structures are learned using a procedural-based system that has no separate category representation. In contrast, rulebased category structures are learned with an explicit hypothesis-testing system that has a separate category representation, which allows for transfer of category knowledge.