Brief article Rating the similarity of simple perceptual stimuli: asymmetries induced by manipulating exposure frequency Thad A. Polk * , Charles Behensky, Richard Gonzalez, Edward E. Smith Department of Psychology, University of Michigan, 525 E. University, Ann Arbor, MI 48109-1109, USA Received 20 March 2001; accepted 27 June 2001 Abstract When judging the similarity of two stimuli, people’s ratings often differ depending on the order in which the comparison is presented (A vs. B or B vs. A). Such directional asymmetries have typically been demonstrated using complex concepts that have a large number of semantic features and a standard explanation is that different sets of features are emphasized depending on the direction of the comparison. In this study, we show that directional asym- metries in the similarity of simple perceptual stimuli can be predictably manipulated merely by presenting each member of a pair with different frequency. Participants rated the similarity of color patches before and after performing an irrelevant training task in which a subset of colors was presented ten times more frequently than others. The similarity ratings after training were significantly more asymmetric than the ratings before training. We discuss the implications of these findings for models of similarity judgment and propose a computa- tionally explicit explanation based on asymmetries in representational stability. q 2002 Else- vier Science B.V. All rights reserved. Keywords: Similarity judgment; Neural network; Color perception; Prototypical; Reference points; Frequency; Attractor; Simulation; Contrast model T.A. Polk et al. / Cognition 82 (2002) B75–B88 B75 Cognition 82 (2002) B75–B88 www.elsevier.com/locate/cognit 0010-0277/02/$ - see front matter q 2002 Elsevier Science B.V. All rights reserved. PII: S0010-0277(01)00151-2 COGNITION * Corresponding author. Fax: 11-734-763-7480. E-mail address: tpolk@umich.edu (T.A. Polk).