13 Ss (aged 18–28 yrs) received 17 painful electrical stimulations of medium intensity, alternated with 3 strong stimulations. 13 matched controls received 20 strong stimulations, which followed a predictable pattern because they were of constant intensity. Subjective fear ratings and autonomic responses of skin conductance response, heart rate (HR), and respiration to a warning signal that preceded the unconditioned stimulus/stimuli (UCS) were higher in the experimental condition than in the control condition, even though the control group received more stimuli of high intensity. Experimental Ss had HR acceleration after the warning stimulus, while the control Ss continued to show a deceleration. Results support the hypothesis that sudden increases in intensity of a UCS contribute to fear of the UCS.
Arntz, A., Van Eck, M., & de Jong, P. J. (1992). Unpredictable sudden increases in intensity of pain and acquired fear. Journal of Psychophysiology, 6(1), 54-64.