Psychophysiology
In the generalization phase, the analysis for SCR returned a significant difference between the stimuli, F (5,424.03) = 22.93, p< .001, R 2 = .213 but neither a difference between the groups, F (2,85.00) = 1.20, p = .307, R 2 = .027, nor a significant Stimulus x Group interaction, F (10, 424.03) = 0.91, p = .528,R 2 = .021. Simple contrast models demonstrated that participants’ SCR response to CS- differed significantly as compared to CS+, b(CS-, CS+) = 0.03, SE = 0.004,t (434.00) = 8.24, p < .001, GS1,b(CS-, GS1) = 0.02, SE = 0.004, t (434.00) = 6.60, p < .001, GS2, b(CS-, GS2) = 0.01, SE = 0.004, t (434.00) = 2.89, p = .004, but did not differ to GS3, b(CS-, GS3) = 0.00, SE = 0.004, t (434.00) = 1.07, p = .287, and GS4,b(CS-, GS4) = 0.00, SE = 0.004, t (434.00) = 0.38, p = .706 (Bonferroni correction α < .010). Trend analysis revealed both a significant Linear, F (1,437.00) = 108.72, p < .001 and a Quadratic trend across stimuli and groups, F (1,433.04) = 7.98, p = .005. The linearity of the overall SCR generalization gradient was characterized by a monotonic decrease from CS+ to CS-; while the curvature of the gradient was characterized by a strong generalized responding from CS+ to GS1 and GS2.
For ssVEPs, the main effect of Stimulus just failed to reach the significance level, F (5, 425) = 2.15, p = .059,R 2 = .025, and there was no main effect of Group, F (2,85) = 1.43, p = .244,R 2 = .033, or Stimulus x Group interaction,F (10, 425) = 1.12, p = .341, R 2 = .026, indicating that participants’ visuocortical engagement was almost similar for the six stimuli in the three groups. We exploratorily followed the nearly significant main effect of Stimulus since it was one of our main hypotheses. Simple contrast models indicated that only GS2 differed significantly from CS-, b(CS-, GS2) =0.10, SE = 0.04, t (435.00) = 2.34, p = .019, but it did not survive Bonferroni correction (α < .010). All other stimuli showed no significant differences from CS- (CS+:b(CS-, CS+) = 0.03, SE = 0.04, t (435.00) = 0.77, p = .441 , GS1: b(CS-, GS1) = 0.04, SE = 0.04, t (435.00) = 0.98, p = .326, GS3:b(CS-, GS3) = - 0.01, SE = 0.04, t (435.00) = 0.21, p = .838, GS4: b(CS-, GS4) = - 0.02, SE = 0.04, t (435.00) = 0.50, p = .616. Further trend analyses returned neither a Linear, F (1, 438) = 3.34,p = .068 nor a Quadratic trend across the test stimuli,F (1, 438) = 0.63, p = .428. Mean signal topographies for ssVEPs in generalization can be found in Figure 4.