Attending to quadrants and ring-shaped regions: ERP effects of visual attention in different spatial selection tasks

Attentional modulations of event-related brain potentials (ERPs) were measured when subjects were cued to attend to a visual quadrant or to a ring-shaped region of visual space to detect infrequently presented targets within the attended region. Spatial attention directed to quadrants was reflected in modulations of sensory-evoked P1 and N1 components at lateral posterior sites and enhanced negativities (Nds) at midline electrodes that started around 150 ms poststimulus. When attention was directed to ring-shaped regions, no modulations of P1 and N1 amplitudes were found, and Nd effects observed at midline electrodes were delayed by about 50 ms. These findings indicate that behavioral effects observed both when attention is directed to contiguous regions and to general areas of visual space may be caused by different underlying processes. Intraperceptual "sensory gating" mechanisms operating in a way suggested by the notion of an attentional "zoom-lens" may be responsible for the selection of single regions, quadrants, or hemifields. When relevant regions are more complex, spatial selectivity will affect primarily postperceptual processes.