The personality dimension of introversion/extraversion is one of the few personality dimensions that can be reliably identified from study to study and investigator to investigator. The importance of this demension within personality theory is due both to the stability of the trait and the influential theory of H. J. Eysenck. The basic assumption in Eysenck's theory of introversion/extraversion is that the personality differences between introverts and extraverts reflect some basic difference in the resting level of cortical arousal or activation. Assuming that there is a curvilinear relationship (an inverted U) between levels of stress and performance leads to a test of this arousal theory. That is, moderate increases in stress should hinder the performance of introverts who are presumably already highly aroused. However, the same moderate increase in stress might help the performance of the presumably underaroused extraverts. Revelle, Amaral, and Turriff reported that the administration of moderate doses of caffeine hindered the performance of introverts and helped the performance of extraverts on a cognitive task similar to the verbal test of the Graduate Record Examination. Assuming that caffeine increases arousal, this interaction between introversion/extraversion and drug condition supports Eysenck's theory. This interaction was explored in a series of experiments designed to replicate, extend, and test the generality of the original finding. The interaction between personality and drug condition was replicated and extended to additional cognitive performance tasks. However, these interactions were affected by time of day and stage of practice, and the subscales of introversion/extraversion, impulsivity, and sociability, were differentially affected. In the morning of the first day, low impulsives were hindered and high impulsives helped by caffeine. This pattern reversed in the evening of the first day, and it reversed again in the evening of Day 2. We concluded that the results from the first day of testing require a revision of Eysenck's theory. Instead of a stable difference in arousal between low and high impulsives, it appeared that these groups differed in the phase of their diurnal arousal rhythms. The result is that low impulsives are more aroused in the morning and less aroused in the evening than are the high impulsives. A variety of peripheral or strategic explanations (differences in caffeine consumption, guessing strategies, distraction, etc.) for the observed performance increments and decrements were proposed and tentatively rejected. It seems probable that some fundamental change in the efficiency with which information is processes is responsible for these performance changes.
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