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The idea of two separate attention networks in the human brain for the voluntary deployment of attention and the reorientation to unexpected events, respectively, has inspired an enormous amount of research over the past years. In this review, we will reconcile these theoretical ideas on the dorsal and ventral attentional system with recent empirical findings from human neuroimaging experiments and studies in stroke patients. We will highlight how novel methods-such as the analysis of effective connectivity or the combination of neurostimulation with functional magnetic resonance imaging-have contributed to our understanding of the functionality and interaction of the two systems. We conclude that neither of the two networks controls attentional processes in isolation and that the flexible interaction between both systems enables the dynamic control of attention in relation to top-down goals and bottom-up sensory stimulation. We discuss which brain regions potentially govern this interaction according to current task demands.

Nicotine enhances attentional functions. Since chronic nicotine exposure through smoking induces neuroadaptive changes in the brain at a structural and molecular level, the present functional MRI (fMRI) study aimed at investigating the neural mechanisms underlying visuospatial and sustained attention in smokers and non-smokers. Visuospatial attention was assessed with a location-cueing paradigm, while sustained attention was measured by changes in response speed over time. During invalid trials, neural activity within the basal forebrain was selectively enhanced in smokers and higher basal forebrain activity was associated with increased parietal cortex activation. Moreover, higher levels of expired carbon monoxide in smokers before scanning were associated with higher parietal cortex activation and faster responses to invalidly cued targets. Smokers showed a slowing of responses and additionally recruited an area within the right supramarginal gyrus with increasing time on task. Activity decreases over time were observed in visual areas in smokers. The data provide evidence for altered attentional functions in smokers as compared with non-smokers, which were partly modulated by residual nicotine levels and were observed at a behavioural level for sustained and at a neural level for spatial and sustained attention.

The deficit to reorient attention from ipsilesional to contralesional space is one key feature of the spatial neglect syndrome. As previous studies suggest that reorienting of visuospatial attention is modulated by cholinergic neurotransmission, we investigated whether cholinergic stimulation with nicotine (Nicorette 2 mg, Pharmacia/Pfizer, Helsingborg, Sweden) facilitates attentional reorienting in spatial neglect patients. Nine nonsmoking patients with stable neglect symptoms were investigated in a within-subject cross-over design. We used a location-cueing paradigm and analysed reaction time (RT) differences between validly and invalidly cued, as well as between neutrally cued and uncued targets as a function of hemifield and drug. Moreover, since the nicotine effect is mediated by parietal brain areas in healthy subjects, we tested whether lesion location influences the pharmacological effect. Nicotine speeded RTs in valid and invalid trials nonspecifically, without modulating the validity effect in the location-cueing task in the whole group of patients. Lesion-symptom mapping revealed a relationship between lesion site and the pharmacological effect on reorienting to contralesional space in right parietal and temporal brain regions. We conclude that in patients with chronic spatial neglect the performance in the location-cueing paradigm can be modulated by a cholinergic stimulant provided that the lesion spares right parietal and temporal cortex.