Conditioned inhibition of classical conditioning was investigated with the startle reflex and the skin conductance response (SCR) in humans using a serial presentation of the conditioned inhibitor (X) and of the conditioned stimulus (CS). The unconditioned stimulus (US) was a shock. During conditioning, participants were presented with two different reinforced CS (A, B) and with X preceding A (noted X --> A). During X --> A, A was not reinforced with the US. During the summation test, B, X --> B, and Y --> B were presented (Y was a new stimulus that tested the specificity of the inhibitory properties of X). B was not reinforced during the summation test. A, B, X, and Y were lights of different colors. Participants were divided into a low and a high anxious group based on the TPQ (C.R. Cloninger, 1987). In the low anxious group, conditioned startle potentiation and SCR responses to A were inhibited when X preceded A (noted A(XA)). This differential responding to A and AXA emerged earlier with the SCR than with startle. During the summation test, the inhibitory properties of X did not transfer to B. In the high anxious group, there was only a differential SCR to A and AXA. X did not inhibit startle potentiation to A.
