Perceptual learning of faces and textures is tuned to trained identities

Perceptual learning, although typically specific to properties of the trained stimulus, will transfer across stimuli under some conditions. Transfer of learning is sometimes obtained with complex visual stimuli, suggesting that, in such cases, learning alters processing at higher stages of the visual pathway. Although our previous work has shown category-specific enhancements of identification performance for faces and textures, whether such enhancements are indifferent to changes in stimulus identity has not been investigated. We addressed this question by using ten-alternative forced-choice texture and face identification tasks that were performed by separate groups of observers on two consecutive days. The faces were cropped to display only internal features, and equated for spatial-frequency content. Textures were band-limited noise patterns. Two sets of each stimulus category with equivalent spatial attributes were created, differing only in stimulus identities across sets. All stimuli were displayed at one of seven contrasts, in one of three external noise levels for a total of 21 stimulus conditions. For each stimulus category, one group of observers performed the task with the same stimulus set on both days. The other group was exposed to one set on the first day and the other on the second day. Method of constant stimuli was used to estimate 50% correct identification thresholds; learning was defined as a reduction in contrast thresholds from the first day to the next. With both textures and faces, performance improvements were substantially greater for the group that was exposed to identical stimuli on both days; thus, perceptual learning did not transfer across changes to stimulus identity. It appears that in both cases, the learning mechanism effectively localises informative regions of the stimulus that most efficiently enable individuation from other members of the same stimulus category. Surprisingly, fluency from prior experience with faces did not facilitate transfer of learning.