ABSTRACTTo determine whether fluctuations of EEG asymmetry in the human cerebral hemispheres influenced overt performance, 14 male and 14 female subjects were tested in verbal and nonverbal target detection tasks and reaction time (RT) was recorded. RT was longest in both word and pattern tasks when the hand used for responding was contralateral to the hemisphere presumably most relevant to the task. The task variable did not affect EEG asymmetry, but females had lower R/L alpha ratios than males and EEG power increased and frequency decreased over the course of testing in association with decreased RT. RT also varied as a function of EEG asymmetry. In contrast to original expectations, RT was longest to verbal targets when R/L alpha power ratios were highest and longest to nonverbal targets when R/L ratios were lowest. The results were compatible with the notion of an inverted U function relating performance and arousal, and supported previous studies indicating that alpha asymmetry influences overt performance.