Simultaneous presentation of similar stimuli produces perceptual learning in human picture processing

Human participants received unsupervised exposure to difficult-to-discriminate stimuli (e.g., A and A'), created with a morphing procedure from photographs of faces, before learning a discrimination between them. Experiments 1 and 2 demonstrated that prior exposure enhanced later discrimination and that intermixed exposure (A, A', A, A' ...) resulted in better subsequent discrimination than blocked exposure (B, B, ... B', B' ...). Experiments 3 and 4 showed that simultaneous exposure to 2 similar stimuli facilitated the later acquisition of both a simultaneous and a successive discrimination, and this effect was observed even though simultaneous exposure to 2 stimuli fostered the development of an excitatory association between them (Experiment 5). The findings of Experiments 1 and 2 revealed a perceptual learning effect with pictures of faces, and the findings of Experiments 3-5 ate difficult to reconcile with associative analyses of perceptual learning. Keywords: perceptual learning, humans, unsupervised, associative, faces