sudden lowering of blood pressure (e.g., during assumption of an upright posture) (Berntson, Cacioppo, and Quigley, 1991).

The baroreflex displays the essential characteristics of a feedback-regulated, homeostatic servomechanism that responds to perturbations and acts to restore basal blood pressure. But blood pressure regulation is far more complex. Indeed, blood pressure changes can be seen in anticipation of a perturbation, before any change in baroreceptor afference. Examples include the increased blood pressure just prior to assumption of an upright posture or in anticipation of threat or danger. To some extent, these likely reflect simple Pavlovian conditioning, in which stimuli (environmental or cognitive) that predict an impending perturbation can serve as conditioned stimuli for an anticipatory, compensatory adjustment (Dworkin, 2000).

Not only can sympathetic and parasympathetic activation within the autonomic nervous system be reciprocal—as implied by arousal theories—but it can also be uncoupled, coactivated or coinhibited (e.g., Berntson, Cacioppo, and Quigley, 1991, 1993). Reciprocal activation fosters a rapid and dramatic change in effector status (e.g., heart rate); uncoupled activation affords more fine tuning (e.g., vagal withdrawal in response to mild exercise); and coactivation or coinhibition can regulate or mute the functional consequences of underlying neural adjustments.

Importantly for the interpretation of polygraph data, individual differences in the mode of autonomic activation to acute psychological stressors have been identified, with some individuals showing primarily sympathetic increases, others primarily vagal withdrawal, and others showing reciprocal sympathetic activation (Berntson et al., 1994; Cacioppo et al., 1994). In addition, Obrist, Light, and colleagues demonstrated that active coping tasks (those with which one copes by doing something, e.g., mental arithmetic) tend to elicit beta-adrenergic (e.g., cardiac) activation and increased blood pressure, whereas passive coping tasks (those with which one copes by enduring; e.g., cold pressor) tend to elicit alpha-adrenergic (e.g., vasomotor) activation (e.g., Light, Girdler, and Hinderliter, in press) and increased blood pressure. Individual differences have been found in these cardiovascular patterns as well, with some individuals showing greater cardiac reactivity and others greater vasomotor reactivity (Light et al., 1993; Kasprowicz et al., 1990; Sherwood, Dolan, and Light, 1990).

In sum, cardiovascular responses to stimuli that may be considered arousing are multiply determined, and there are individual differences in terms of the direction and extent of cardiovascular reactivity that is observed. These findings call into question assumptions about cardiovascular signals of arousal that are consistent across individuals.