A NEUROCHEMICAL MODEL UNDERLYING DIFFERENCES IN REACTION-TIMES BETWEEN INTROVERTS AND EXTROVERTS

Based on a model for action of the mesostriatal dopamine (DA) system the assumption is made that DA activity modulates the threshold for behavioral responses to sensory input. This modulating function of DA activity on the transmission of sensory input into motor output is most likely reflected in reaction time tasks. This study attempted to relate a model of DA activation of differences in reaction times between introverts and extraverts. Experiment 1 addressed the question of whether haloperidol-induced deviations from the physiological level of DA activity would differentially affect reaction times of extraverts and introverts. The DA receptor blocker haloperidol caused a marked impairment in reaction time performance in both, extraverts and introverts, but no relationship between Extraversion and reaction time performance was found. In Experiment 2, biosynthesis of DA, as measured by biochemical analysis of urine samples, was significantly reduced by alpha-methyl-p-tyrosine (AMPT). Although there were no differences in DA activity between extraverts and introverts after treatment with AMPT, reaction time performance was markedly decreased in introverts, but not in extraverts, as compared to the respective placebo groups. The outcome of the two experiments suggests individual differences between extraverts and introverts in responsivity to experimentally induced changes in DA activity. Introverts appear to be more sensitive to changes in DA activity than extraverts whereas the latter can be characterized by more effective neuronal compensatory mechanisms for maintaining homeostasis in neurotransmission. Furthermore, responsivity to changes in DA activity seems to be mediated by up-regulation of postsynaptic receptors.