Generalization of feature- and rule-based learning in the categorization of dimensional stimuli: evidence for dual processes under cognitive control.

Participants in 2 experiments classified face stimuli into 2 categories determined by the gender of the faces. Although the category rule was simple, the stimuli, created by morphing male and female faces, made the explicit identification of the rule difficult. Participants were classified as rule users or nonusers depending on whether they explicitly identified the gender rule on a postexperiment questionnaire. Nonusers displayed a generalization decrement to new category exemplars that were more obviously male and female but less similar to the trained exemplars. Rule users, instead, generalized their category judgments perfectly to new category members. However, when an exception to the rule was introduced, generalization decrement for stimuli related to the exception was evident in both groups. All participants displayed a near-perfect ability to reverse their category judgments, regardless of whether learning occurred in the presence or absence of the explicit gender rule. The results highlight a distinction between rule- and feature-based category learning but show that both processes are subject to cognitive control.