Although modeled on procedures used with nonhuman animals, some recent studies of perceptual learning in humans, using complex visual stimuli, differ in that they usually instruct participants to look for differences between the to-be-discriminated stimuli. This could encourage the use of mechanisms not available to animal subjects. To investigate the role of instructions, in 2 experiments, participants were given preexposure to checkerboards that were similar except for the presence of a small distinctive feature on each. For participants instructed to look for differences, performance on a same-different test was enhanced by preexposure in which the critical stimuli were presented on alternate trials-the usual perceptual learning effect. No such effect was found in 2 other preexposure conditions: when participants were told only to look at the stimuli and not explicitly told to look for differences; and when participants were instructed on an alternative task requiring attention to the stimuli. These results indicate a role for a learning process reinforced by success in finding stimulus differences; they challenge previous interpretations of results from studies using complex visual stimuli in the study of perceptual learning. (PsycINFO Database Record