Spatial attention: more than intrinsic alerting?

It has been proposed that the right hemisphere alerting network co-activates, either directly or via the brainstem, the attention system in the parietal cortex involved in spatial attention. The observation that impaired alertness and sustained attention can predict the outcome of neglect might suggest such a relationship, too. In the present fMRI study, we intended to analyse and compare the functional anatomy of two attentional conditions both involving intrinsic (endogenous) alerting and fixation but differing with respect to the degree of spatially distributed attention by using the same paradigm under two different attentional conditions. In a group of ten participants, both a focused and a distributed visuospatial attention condition evoked similar patterns of activation in dorsolateral prefrontal regions, in the anterior cingulate gyrus, in the superior and inferior parietal cortex as well as in the superior temporal gyrus and in the thalamus. These activation foci were stronger in the right hemisphere under both conditions. After subtraction of the alertness condition with focused spatial attention, distributed spatial attention with stimuli appearing at unpredictable locations within both visual fields induced additional bilateral activations only in the left and right superior parietal cortex and in the right precuneus suggesting that these regions are specific for a more widespread dispersion of spatial attention.