IMPULSIVITY, CAFFEINE, AND TASK-DIFFICULTY - A WITHIN-SUBJECTS TEST OF THE YERKES-DODSON LAW

According to the Yerkes-Dodson law, performance is an inverted-U function of arousal with a negative relationship between optimal arousal and task difficulty. Both easy (letter cancellation) and difficult (verbal abilities) tasks were completed during the morning by 100 subjects differing in impulsivity; each subject was tested with five different doses of caffeine. Data were subjected to a traditional analysis of variance; in addition, data from individual subjects were analyzed. Group-level results indicated that performance was an interactive function of task, caffeine, and impulsivity (P < 0.05): As predicted by the Yerkes-Dodson law, performance on the easy task tended to improve as caffeine dosage increased, but on the difficult task, (less aroused) impulsive subjects improved while (more aroused) nonimpulsive subjects first improved and then deteriorated. Moreover, analyses of the performance of individual subjects strongly supported the inverted-U hypothesis (P < 0.001). The hypothesis that easier tasks require higher levels of arousal for optimal performance than more difficult tasks received limited support at the individual level. Thus, despite methodological and probabilistic biases against the inverted-U and task-difficulty hypotheses, both group and individual level analyses yielded results consistent with the Yerkes-Dodson law.