Within the posterior attentional system, the orientation and maintenance of visual spatial attention give rise to activity both in parietal regions that direct attentional signalling and in ventral occipital regions that are directed by attentional signals [Posner & Petersen 1990]. PET studies have associated activity in superior parietal lobule both with shifts of visual spatial attention [Corbetta & al. 1993; Vandenberghe & al. 2001] and with visual vigilance [Pardo & al. 1991; Vandenberghe & al. 1996], particularly during conditions that combine high perceptual demand with peripheral presentation of stimuli [Vandenberghe & al. 1996, 1997]. Additionally, there is some evidence of sex differences in patterns of parietal lobe activation associated with attention-demanding visual spatial tasks [Gur & al. 2000]. More recent fMRI work has focussed on inferior parietal cortex, where activation during shifts of attention has been mapped to anterior and posterior zones of the intraparietal sulcus [Corbetta & al. 1998, 2000; Wojciulik & Kanwisher 1999]. It has been suggested that intraparietal sulcus performs a more general role in attentional processing, functionally distinct from simple shifting and possibly related to the active suppression of distractors [Wojciulik & Kanwisher 1999].
In addition to parietal areas, visual spatial attention has effects on the ventral occipital regions that are targeted by attentional modulations. Visual attention augments activation in extrastriate cortex on the inferior surface of the occipital lobe, in a region centred on fusiform and lingual gyri [Pardo & al. 1991; Corbetta & al. 1993; Vandenberghe & al. 1996]. In the case of a lateral focus of spatial attention, this augmentation is contralateral to the attended hemifield [Heinze & al. 1994; Hillyard & al. 1997; Vandenberghe & al. 1997, 2000; Martínez & al. 1999; Hopfinger & al. 2000]. This well characterised, replicated, and lateralised effect of spatial attention in higher-order visual cortex can serve as a useful comparison for attentional phenomena in other brain regions. In order to examine differential responses of superior parietal, intraparietal, and ventral occipital cortices during attention in the presence of distractors, we applied fMRI during bilateral rapid serial visual presentation of stimuli in a visual spatial attention task.
Recent fMRI studies have focussed on event-related analysis of periods surrounding shifts of attention [Kastner & al. 1999; Corbetta & al. 2000; Hopfinger & al. 2000]. As our primary interest is not in shifts per se but in the effect of shifts on responses to subsequent stimuli, we have instead applied a blocked analysis comparing periods of attention to left and right hemifields. This blocked analysis of stimuli presented in continuous streams affords greater opportunity to examine steady-state visual response than would an event-related analysis of discrete, temporally isolated stimuli. In addition, there is evidence that at TR's of 2s or more, blocked analysis yields an advantage in BOLD signal strength [Buckner & al. 1996]. This paradigm of blocked comparison during rapid serial presentation has been previously applied in quantitative electroencephalographic studies of visual spatial attention in normal [Belmonte 1998] and patient [Belmonte 2000] populations.