Use of multiple dimensions 115 Use of multiple dimensions in learned discriminations

Many naturally occurring categories vary across multiple stimulus dimensions (e.g. size, color, texture). When humans cat-egorize multidimensional stimuli on the basis of a single dimension this has been taken to indicate use of a rule that could be verbalized. Sorting on the basis of all the stimulus dimensions (‘overall similarity ’ or ‘family resemblance’) has been taken to indicate a more basic, implicit, automatic, perhaps associative process. However, a review of the literature on animal discrimination learning shows that animals often discriminate on the basis of one dominant dimension. In recent experi-ments, situations conducive to more complex cognitive processes have increased family resemblance sorting in humans. In an effort to resolve this apparent paradox, experiments were conducted in which humans and pigeons were exposed to multidimensional category discrimination tasks under closely similar conditions. Preliminary results show no evidence that even a non-verbal rule can be said to be involved in pigeons ’ choices in these conditions, despite the fact that under some conditions a single dimension may dominate their behavior