A comparison of three approaches for the determination of baseline levels of physiological activity.

The traditional means of determining baseline levels of physiological activity in research evaluating responses to stressful stimuli has been to record activity during rest periods either before or interspersed with periods of stimulation. Subjects may, however, anticipate the impending stimulation, thus elevating their levels of physiological activity even during periods of presumed rest. Eight physiological responses--forehead muscle tension, heart rate, heart rate variability, skin resistance level and non-specific responses, systolic and diastolic blood pressure, and pulse transit time--were recorded from 39 subjects during sessions which included both task and rest periods, and during sessions with only rest periods and periods of viewing slides of restful scenery. The results indicate a significant decrease in levels of physiological activity in most of the 8 physiological measures when activity was recorded during sessions in which no tasks were presented. The lowest levels of physiological activity were recorded while subjects viewed slides. These results suggest that the traditional approach to the determination of baseline may yield levels of physiological activity contaminated by anticipation of upcoming stimuli.