Motoneuronal insensitivity in extraverts as revealed by the startle response paradigm

Abstract

The present research was designed both to examine possible differences between extraverts and introverts in motoneuronal sensitivity to gradations in stimulus intensity, and to investigate the utility of the startle response paradigm in the area of individual differences. Startle stimuli, 85 dB(A) SPL and 60 dB(A) SPL noise bursts, were presented with and without a 50 dB(A) SPL prepulse to subjects low and high in extraversion, as defined by scores on the Eysenck Personality Inventory (Eysenck & Eysenck, 1968; Manual for the Eysenck Personality Inventory. San Diego, CA: ITS). Startle response latency revealed that introverts responded more quickly to the 85 dB(A) SPL stimuli than to the 60 dB(A) SPL stimuli, whereas extraverts responded equally rapidly to stimuli at both levels of intensity. There was no effect of extraversion on response amplitude or probability. Response amplitude was significantly inhibited by the prepulse in the 85 dB(A) SPL condition, but not in the 60 dB(A) SPL condition. Response probability was inhibited more in the 85 dB(A) SPL condition than in the 60 dB(A) SPL condition. Response latency was not affected by the prepulse. The results of the present study are discussed in terms of indiscriminate polysynaptic motoneuronal responding to different levels of stimulus intensity processed at the brainstem in extraverts, as well as in terms of the results found by Pivik, Stelmack and Bylsma (1988; Psychophysiology, 25, 16–24), showing reduced motoneuronal excitability in extraverts.