LUMINANCE AND SPATIAL ATTENTION EFFECTS ON EARLY VISUAL PROCESSING

Event-related potentials (ERPs) were recorded from healthy subjects in response to unilaterally flashed high and low luminance bar stimuli p resented randomly to left and right field locations. Their task was to covertly and selectively attend to either the left or right stimulus locations (separate blocks) in order to detect infrequent shorter targ et bars of either luminance. Independent of attention, higher stimulus luminance resulted in higher ERP amplitudes for the posterior N95 (80 -110 ms), occipital P1 (110-140 ms), and parietal N1 (130-180 ms). Bri ghter stimuli also resulted in shorter peak latency for the occipital N1 component (135-220 ms); this effect was not observed for the N1 com ponents over parietal, central or frontal regions. Significant attenti on-related amplitude modulations were obtained for the occipital P1, o ccipital, parietal and central N1, the occipital and parietal P2, and the parietal N2 components; these components were larger to stimuli at the attended location. In contrast to the relatively short latencies of both spatial attention and luminance effects, the first interaction between luminance and spatial attention effects was observed for the P3 component to the target stimuli (350-750 ms). This suggests that in teractions of spatial attention and stimulus luminance previously repo rted for reaction time measures may not reflect the earliest stages of sensory/perceptual processing. Differences in the way in which lumina nce and attention affected the occipital P1, occipital N1 and parietal N1 components suggest dissociations among these ERPs in the mechanism s of visual and attentional processing they reflect, Nonetheless, scal p current density mappings of the attention effects throughout the lat ency ranges of the P1 and N1 components show the most prominent attent ion-related activity to be in lateral occipital scalp areas. Such a pa ttern is consistent with the spatially selective filtering of informat ion into the ventral stream of visual processing which is responsible for complex feature analysis and object identification.