Perceptual fluency can be used as a cue for categorization decisions. 
Learning in the prototype distortion task is thought to involve perceptual learning in which category members experience an enhanced visual response (Ashby & Maddox.  Annual Review of Psychology, 56, 149-178, 2005).  This response likely leads to more-efficient processing, which in turn may result in a feeling of perceptual fluency for category members.  We examined the perceptual-fluency hypothesis by manipulating fluency independently from category membership.  We predicted that when perceptual fluency was induced using subliminal priming, this fluency would be misattributed to category membership and would affect categorization decisions.  In a prototype distortion task, the participants were more likely to judge stimuli that were not members of the category as category members when the nonmembers were made perceptually fluent with a matching subliminal prime.  This result suggests that perceptual fluency can be used as a cue during some categorization decisions.  In addition, the results provided converging evidence that some types of categorization are based on perceptual learning.