Lateralization and interhemispheric integration of directed attention]

Directed attention is a function to direct and shift the focus of awareness adequately to behaviorally relevant sensory events. Healthy subjects direct attention evenly to right and left hemispaces. Unilateral spatial neglect is a failure to respond normally to stimuli on the side opposite a cerebral lesion, which is considered to represent a unilateral disruption of directed attention. The established clinical observation that neglect usually occurs after right hemisphere lesions and the results of functional imaging studies suggest the right hemisphere dominance for directed attention. It is hypothesized that the right hemisphere distributes attention to space bilaterally, whereas the left hemisphere distributes attention primarily to right hemispace. However, patients with callosotomy show no apparent neglect with either right or left hand. Ishiai et al. (2001) reported detailed analyses of eye movements when a patient with a callosal infarction bisected lines. Left unilateral spatial neglect may appear, when use of the right hand induces a rightward bias in the attentional control of the left hemisphere and damage to its cingulate gyrus inhibits interhemispheric integration of attention. By contrast, the disconnected but intact right hemisphere may bisect a line accurately by integrating attention to the extents perceived in the left and right visual fields.