Two experiments were conducted in order to explore the relationship between extraversion and brain activity. In Experiment 1, subjects were randomly assigned to receive either caffeine or placebo. Higher levels of extraversion were associated with more frequent eyeblinking, which is an indirect index of central dopaminergic activity. The strength of the association between eyeblinking and extraversion was stronger among females than among males, and did not differ appreciably between the caffeine and placebo conditions. Higher levels of extraversion also tended, on average, to be associated with greater relative left hemisphere activation, as measured by the Chimeric Faces Task. However, the strength of the association appeared to vary as a function of both gender and caffeine/placebo condition. Experiment 2 attempted to replicate the finding that extraversion is associated with hemispatial bias. Subjects completed the Chimeric Faces Task in the evening and then again the following morning. The correlations between hemispatial bias and extraversion were generally rather strong for both males and females, with more extraverted individuals exhibiting greater relative left hemisphere activation. The strength of the association did not vary appreciably as a function of the time of day at which subjects completed the Chimeric Faces Task.
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