Effects of discrimination training on fear generalization gradients and perceptual classification in humans.

To examine the effect of discriminative fear conditioning on the shape of the generalization gradient, two groups of participants first learned to discriminate between two color stimuli, one paired with an electrical shock (conditional stimulus, CS+) and the other explicitly unpaired (CS-). The CS+ was held constant as an intermediate (ambiguous) value along the blue-green color dimension while the CS- varied between groups as opposite endpoints along the blue-green color dimension. Postdiscrimination testing, using spectral wavelengths above and below the CS+, revealed opposing asymmetric gradients of conditioned skin conductance responses across training groups that skewed in a direction opposite the CS-. Moreover, perceptual ratings for the color of the CS+ were affected by discriminative conditioning, with the color value of the blue or green CS- inducing a shift in the frequency for ratings of the ambiguous CS+ as either "green" or "blue," respectively. These results extend findings on gradient shifts in the animal literature and suggest that postdiscrimination testing provides a more comprehensive estimate of the effects of discriminative fear conditioning than testing responses solely to the conditioned stimuli.