Determining baseline and adaptation periods in stress research

Investigations pertaining to psychophysiological stress responses typically employ a preexperimental 'baseline' period. The appropriateness of this methodology was examined in two studies using heart rate (HR) measures. In study 1, HR decreased significantly from the beginning to the end of a 15-min adaptation period, suggesting that the optimal length for a prestress adaptation period must be at least 15 min for HR when a stressor is anticipated. Study 2 compared the customary prestress 'baseline' measure to four other possible candidates: two recovery periods between stressor presentations, a postexperimental time period, and a recording taken on a different day when no stressors were presented. The prestress measure was significantly higher than recovery and postexperimental measures. The postexperimental measure was retained as the best option for the computation of a baseline score. These studies highlight the factors pertinent to the selection of a suitable time frame for a baseline measure.