Differential effects on test stress on the heart rates of extraverts and introverts.

A mental arithmetic task was administered to 39 subjects under conditions which imposed social stress. Using the relaxed state to provide a baseline, changes in heart rate and finger blood volume pulse were recorded. The subjects were divided to obtain groups with extreme scores on Eysenck's PEN scales, and the groups were compared on the two psychophysiological measures. Contrary to Eysenck's theory of Autonomic Lability as the neurological basis of N, the high N scoring group was not differentiated from the low N group by change in level of sympathetic activity as indicated by blood volume pulse, while, on the heart-rate measure, introverts showed a large increase and extraverts considerably less: there was no overlap between groups (p less than 0.001). It was concluded that extraverts exhibit greater parasympathetic activity relative to sympathetic arousal under this stress condition.