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

Abstract

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.

Figure 1

Pilot behavioral data and skin conductance responses (SCRs). (A) Perceptual identification (black circles) indicated that the 494 nm stimulus was nearest the point of subjective equality (dashed line) between green and blue. Reaction times (white triangles) peaked around the intermediate spectral values. SCRs broken down by experimental phase for Group 1 (B) and Group 2 (B) showed low and undifferentiated SCRs across the color dimension prior to conditioning (white circles) and heightened SCRS for the CS+ versus CS− (gray bars) during fear conditioning. Generalization gradients in Group 1 (B) and Group 2 (B) revealed peak SCRs at an unreinforced value opposite the CS−. Error bars represent standard error of the mean. * p < .05; ** p < .015 (Bonferroni-corrected for multiple comparisons). For both (B) and (C), the transparent gray bars indicate which values served as the CS+ and the CS− and the arrow indicates the direction of the gradient shift.

Figure 2

Subjective ratings and reaction times. (A) Endorsements for the CS+ (494 nm) as “blue” decreased in Group 1 from baseline (white circles) to fear conditioning (gray triangle). Endorsements for the CS+ (494 nm) as “blue” increased in Group 2 from baseline (white circles) to fear conditioning (gray triangle) and from baseline to the generalization test (black circle). (B) Reaction times peaked at the CS+ in both groups. Gray bars reflect RTs to the CS− and CS+ during fear conditioning. (C) When asked to identify which stimulus was the CS+ at the end of the experiment, 56% of participants in Group 2 mistakenly identified a stimulus that was more blue than the actual CS+. Error bars represent standard error of the mean. * p < .05; ** p < .015 (Bonferroni-corrected for multiple comparisons).