Destruction or disconnection of specific neuronal structures or failure to activate those structures may impair brain function. Because the right hemisphere seems dominant for mediating arousal, which is an important determinant of the capacity for cerebral activation, we predicted that subjects with right hemisphere damage would have a greater reduction in the capacity for cerebral activation than subjects with left hemisphere damage. A paradigm requiring that two simple tasks be performed singly and simultaneously was used to assess the capacity for activation. Subjects with right hemisphere damage had significantly greater impairment in the capacity for cerebral activation than subjects with left hemisphere damage. This impairment may partly explain the associations between right hemisphere damage and decreased ability to perform certain analytic and linguistic tasks. © 1987 American Academy of Neurology.