Individual Differences in Phasic Cardiac Reactivity to Psychological Stress and the Law of Initial Value

The validity of the law of initial value (LIV), which predicts relationships between prestimulus level and phasic response, was evaluated for two cardiac measures, heart rate (HR) and electrocardiographic T‐wave amplitude (TWA). HR acceleration and TWA attenuation were produced in undergraduates by the performance of the backward digit span task. The between‐subjects correlations of prestimulus level with the magnitude of the phasic responses were significant, with signs opposite to that which would be predicted by the LIV. The within‐subjects prestimulus level‐phasic response correlations, however, were consistent with the LIV. It appears that individual differences in psychologically‐elicited phasic cardiac reactivity negate LIV predictions. These results emphasize the importance of sharply distinguishing the between‐ vs. within‐subjects formulations of the LIV, and are contrary to recent psychophysiology textbook accounts of the prestimulus level‐phasic response relationship for cardiac variables.