The concept of brain disconnection derives from work in which the two cerebral hemispheres are surgically separated by division of the corpus callosum. The patient behaves as if his two half-brains function to some degree independently. The syndrome of brain disconnection is exemplified by such split-brain cases (Geschwind, 1965). Neither hemisphere shows an awareness of the functions of the other, and there is a marked failure to cross-match stimuli across the midline of the body. The integrity of the corpus callosum is essential to normal integration between the hemispheres.

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Vigilance is an important facet of animal life andparticularly so in relation to three areas: external danger, the exploitation of environmental resources, and intraspecific communication. The second of these involves, in addition to vigilance, an active search of the environment. The organization of the nervous system, however, imposes certain limitations on the effectiveness of vigilance and these are described, with particular reference to experimental studies of vigilance in man. Three types of strategy are then described which function to offset the neural limitations on vigilance. These are: neural mechanisms, individual behavioural strategies, and social strategies. These are considered in relation to those areas of life already defined as requiring a watchkeeping function.

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