Visual-spatial selective attention is an essential brain function that enables human observers to select and process relevant information in the visual fields with high efficiency. Several brain areas have been implicated in the control of attention, and although animal studies have contributed significant information about attentional mechanisms little is known about the level(s) of the visual afferent pathways at which incoming information is modulated or biased by selective attention in humans. Previous studies using non-invasive scalp recordings of event-related potentials (ERPs) have shown that visual stimuli at attended locations are selected and processed preferentially as early as 80-90 msec after stimulus onset. We have proposed that these effects occur in the extrastriate visual cortex (Mangun et al., 1993a), but ERP methods do not yet permit a precise localization of the visual cortical areas(s) involved in the generation of scalp-recorded electrical activity. To further investigate spatial attention mechanisms we have combined functional neuroimaging (positron emission tomography--PET) with ERP recordings in order to analyze both the anatomical sites (from PET) and time course (from ERPs) of attentional selection processes in the visual pathways. Based on these studies we hypothesize that incoming sensory signals are first modulated within the ventral lateral extrastriate visual cortex (fusiform gyrus), and that this attention-related activity has a post- stimulus latency as early as 80-130 msec. Here we propose to follow-up these initial studies using improved electrical mapping and intracranial electrical modeling, and higher resolution PET imaging (3-D volume acquisition with 15-O labeled water coregistered with MRI scans). Two important questions about these spatial attention mechanisms will be addressed: First, we will investigate the extent to which attention- related activity in the fusiform gyrus is related to the direction of attention in the visual fields as opposed to other processes involved in the discrimination of complex stimuli. Second, we will investigate the relationship between the extrastriate attention effects we have observed, and the activation of parietal cortex during attention shifting that was recently reported by Corbetta et al. (1993). The proposed experiments have direct theoretical consequences for current models of attentional selection by attempting to elucidate both the anatomical locus and time course of attention-related modulations of neural processing in the intact human brain. The proposed approach also represents a novel combination of complementary techniques (PET and ERP recording) in order to address an important aspect of higher mental function - visual selective attention.