Predictable and unpredictable shock: Behavioral measures of aversion and physiological measures of stress.

Previous articles have reviewed the behavioral literature and concluded that predictable shock conditions are less aversive than unpredictable shock conditions. This article reviews the literature on predictable and unpredictable shock conditions relative to physiological measures of stressfulness and considers the possible role of stress-induced analgesia in both the physiological and behavioral effects. It finds that unpredictable conditions are physiologically more stressful than predictable conditions when subjects are exposed to them for one or a few sessions and parameters of stress are relatively severe. However, predictable conditions may be more stressful than unpredictable conditions when sessions are long and extend over days and parameters of stress are less severe. The effect of extended stress appears to depend upon the physiological measure used. These findings are discussed in terms of the phasic versus chronic nature of predictable versus unpredictable stress and the organism's ability to adapt physiologically to these conditions. Finally, the data on stress-induced analgesia are reviewed. We conclude that stress-induced analgesia does not significantly contribute to either preference for predictable over unpredictable stress or their differential physiological effects.