Effects of preexposure on stimulus discrimination: an investigation of the mechanisms responsible for human perceptual learning.

The effect of preexposure on human perceptual learning was investigated in four experiments. In Experiments 1a and 1b, participants were preexposed to one pair of visual stimuli on an intermixed schedule (AX/BX) and one on a blocked schedule (CX_DX). The ability to discriminate between AX and BX and between CX and DX was then assessed by examining the extent to which key presses assigned to each member of a stimulus pair generalized to the other member (Experiment 1a) and by looking at the accuracy of same-different responses (Experiment 1b). Stimuli were more easily discriminated following intermixed than following blocked preexposure on both the generalization and same-different tasks. This suggests that two stimuli are more perceptually distinct after intermixed preexposure. Experiments 2a and 2b investigated the mechanisms responsible for perceptual learning using same-different tasks. The results support the suggestion that the enhanced discrimination observed after intermixed preexposure is due to increases in the salience of the unique elements.