The justification of these physiological measures was originally derived from arousal theory, which holds that the stronger the stimulus or event, the stronger the psychological reaction, and the more pronounced these particular physiological responses. In studies of the influence of emotional disturbances on what he termed the “emergency reaction,” Cannon (1929) advanced the hypothesis that there is a diffuse, nonspecific sympathetic outflow through the interconnections in the sympathetic ganglia during emergency states and that this sympathetic discharge is integrated with behavioral states—the so-called “fight-or-flight” reaction. In Cannon’s formulation, autonomic and neuroendocrine activation associated with emotional disturbances serves to mobilize metabolic resources to support the requirements of fight or flight, thereby promoting the protection and survival of the organism.7

Although the intensity of autonomic, electrocortical, and behavioral reactions does tend to covary with the intensity of the evocative stimulus, the prediction of a general and diffuse physiological activation has failed empirical tests. Correlations among autonomic measures both within and between individuals are commonly found to be weak. Moreover, negative correlations have been found to occur within individuals during some tasks (e.g., between heart rate and skin conductance responses; see Lacey et al., 1963). Negative correlations have also been reported between electrocortical and autonomic measures of activation and between facial expressiveness and autonomic responses. Contrary to the notion that sympathetic nervous activation is global and diffuse, highly specific regional sympathetic activation has been observed in response to stressors (Johnson and Anderson, 1990), even in extreme conditions such as panic attacks (Wilkinson et al., 1998). Research also shows that the same excitatory stimulus (e.g., stressor) can have profoundly different effects on physiological activation across individuals or circumstances (Cacioppo et al., 2000; Kosslyn et al., 2002).

Cardiovascular, electrodermal, and respiratory activity respond in different ways to various psychological states and behaviors. The cardiovascular system responds to stimuli that may be considered arousing, and even to the anticipation of such stimuli. The responses are multiply determined, however, and there are individual differences in the direction and extent of cardiovascular response. For example, active coping tasks (i.e., those that require cognitive responses, such as test taking or interrogation) tend to increase blood pressure, but through different mechanisms (i.e., cardiac activation or vasoconstriction) for different kinds of tasks; moreover, individuals differ in the reactivity of these mechanisms. The evidence does not support the assumption that cardiovascular signals of arousal are consistent across individuals.

Electrodermal activity can be measured by skin conductance between

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