Hemispheric asymmetries in vigilance and cerebral arousal mechanisms in younger and older adults

Younger and older adults responded to an asterisk presented to either the l eft or right visual field, with delay interval between onset of a fixation cross and onset of the asterisk varied. At delay intervals longer than 3 s, reaction time was faster when the same visual field was stimulated on 2 su ccessive trials than when different visual fields were stimulated. This pre field effect was larger in the left visual field (right hemisphere), consis tent with hypotheses of hemispheric asymmetry for arousal and vigilance. Fo r older adults, these results were obtained only when participants responde d with the left hand, suggesting that some aspects of hemispheric asymmetry for attention-related processing or the efficiency of interhemispheric tra nsfer may change in an age-related fashion.