Selective reorienting response of the left hemisphere to invalid visual targets in the right side of space. Relevance for the spatial neglect syndrome

In humans, damage in the right hemisphere often provokes the striking inability to attend the left side of space, i.e. left spatial neglect. For years the leading hypothesis for the higher epidemiological incidence of left over right spatial neglect has been that the right hemisphere can orient attention to both sides of space while the left hemisphere only to the right side. Because of this hemispheric specialization, patients with lesion in the left hemisphere do not usually suffer right spatial neglect. Though comprehensive and very influential, to date, this explanation only found clear support in a MEG investigation into stimulus driven shifts of auditory attention. In contrast, no evidence for the same hypothesis was ever gathered from studies of visual attention run with behavioral paradigms that have importantly influenced the interpretation and modeling of attentional deficits in left spatial neglect. Here we report the first fMRI evidence directly supporting this long-standing hypothesis on the hemispheric lateralisation of mechanisms regulating orienting of spatial attention in the human brain. By demonstrating that the left hemisphere selectively reorients attention to invalidly cued visual targets in the right side of space we provide a more comprehensive account for deficits of spatial attention suffered by right brain damaged patients with left unilateral neglect.