Eysenck's incubation of fear hypothesis: an experimental test

The present experiment was designed to test Eysenck's hypothesis that repeated exposure to an unreinforced CS of brief duration following acquisition of a classical aversive CR may lead to a progressive increase in the strength of that CR, provided that the UCS is intense and the CR has drive-like properties. Using a between-groups design, normal human subjects were given identical classical acquisition trials, followed by extinction trials where CS duration was either 2, 8 or 16 sec. The UCS was of fixed high intensity. Dependent measures were tonic and phasic heart rate and skin conductance. No evidence of incubation was found as a function of CS duration. Nor was there any indication that CS duration differentially affected resistance to extinction. A small number of subjects showed evidence of incubation with heart rate measures during extinction. However, there was no indication that this enhancement was governed by the parameters suggested by Eysenck. UCR amplitude, which showed a positive correspondence with CS-bound activity throughout the trials, did not reliably predict incubation. Problems concerning both the definition and the demonstration of incubation are discussed.