FIGURE 12.5. Orthostatic versus psychological stress. Left: Group mean responses to orthostatic and psychological stressors depicted as response vectors on the autonomic plane, from prestress baseline (intersection of horizontal and vertical dotted lines) to the stress conditions (arrowheads). Axes depict ms of heart period change related to sympathetic and parasympathetic control. Right: Individual response vectors (N = 9) to the psychological stressors revealing individual differences in the direction of response. Individual differences were stable, as evidenced by standard deviation bars at the arrowheads, reflecting deviations across the three psychological stressors (mental arithmetic, speech stress, and reaction time). Note that responses for a given individual were generally consistent across stressors. From "Autonomic Cardiac Control. III. Psychological Stress and Cardiac Response in Autonomic Space as Revealed by Pharmacological Blockades," by G. G. Berntson, J. T. Cacioppo, and K. S. Quigley, 1994, Psychophysiology, 31, 599-608. Copyright 1994 by Blackwell Publishing, Ltd. Reprinted with permission.

the change score along the sympathetic and parasympathetic axes. Results are illustrated in Figure 12.5, which displays response vectors from baseline (intersecting dotted lines). Both classes of stressors yielded an overall reciprocal pattern of sympathetic activation and parasympathetic withdrawal.

This similarity at the group level, however, belies a fundamental difference between the two classes of stressors. In accord with the reciprocal pattern of control in many autonomic reflexes, there was a significant negative correlation between the responses of the autonomic branches across subjects with the orthostatic stressor. Greater increases in sympathetic control were associated with larger decreases in parasympathetic control. All subjects showed similar response vectors, differing only in magnitude. In contrast, there was no correlation between the autonomic branches to the psychological stressors. Rather, as illustrated in Figure 12.5 (right), there were notable individual dif ferences in responses to psychological stress. Some subjects showed primarily parasympathetic withdrawal, others reciprocal sympathetic activation and parasympathetic withdrawal, and still others primarily sympathetic activation. This was not attributable simply to an increase in error variance to psychological stress, as individual response vectors were stable across the psychological stressors. This can be seen in the error bars at the arrowheads of Figure 12.5 (right), which depict the standard errors for the response vectors under the different psychological stressors.

Why does it matter? Without independent measures of sympathetic and parasympathetic control, lawful differences between orthostatic and psychological stressors would not have been apparent, and individual differences in the response to psychological stress would not have been discerned. In accord with the corollary of proximity, psychophysiological mapping in this case was improved by the deployment of a more appropriate analytical method that assessed autonomic control at a more proximal level than could be derived from the end organ response. This is important not only for basic studies of psychophysiological relations, but also because different modes of autonomic control may have distinct health implications.

There have been reports of a relation between cardiac reactivity to stressors and negative health status, including diminished immune functions, although the predictive power of heart rate is small and not always significant (see Cacioppo, 1994). This is likely attributable to the use of heart rate measures, as Cacioppo (1994) found no relation between overall heart rate reactivity and the immune response to vaccine, but did find a significant relation between immune status and the sympathetically mediated component of heart rate reactivity. Multilevel analysis, capitalizing on more proximal mappings, revealed order in psychosomatic relations where none was apparent with more distal mappings.

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