Perceptual units in the acquisition of visual categories.

A series of experiments provided evidence that the representational structure of categories comprising dot patterns is based on pattern parts and pattern configuration rather than on pattern elements. We found that similarity judgments and postacquisition classification data could not be explained in terms of element-level perceptual units, even for categories of dot patterns with seven of their eight dots in the exact same relative location. The importance of higher order perceptual units was indicated by evidence that the long-term retention of information specific to previously learned category exemplars, which is typical of natural objects, can also be obtained for artificial dot patterns, providing their structure reflects the perceptual characteristics identified in Tversky and Hemenway's (1984) study of natural objects: Members of the same category had to be perceptually distinctive at the level of pattern configuration and perceptually similar at the level of pattern parts. The level of within-category similarity for a set of categories (relative to between-categories similarity) did not predict whether item-specific information would be retained; long-term retention appears to require both within-category similarity and dissimilarity, but at different levels of perceptual structure.