This article reviews recent models of how the two hemispheres collabor
ate to facilitate efficient information processing. The strengths and
weaknesses of behavioral techniques and structural and functional neur
oimaging techniques are considered as they apply to these problems. Th
e role of the corpus callosum as an equilibrator of activation between
the two hemispheres is discussed and linked with its role as a dynami
c modulator for the mobilization of hemispheric resources. Issues rela
ted to metacontrol and dynamic fluctuations in hemispheric control are
also considered. (C) 1998 Academic Press.