The effect of variability of unattended information on global and local processing: evidence for lateralization at early stages of processing

Visual objects can often be analyzed as hierarchical in structure, composed of local elements that are spatially arranged to form a global shape. The brain mechanisms involved in the analysis of hierarchical figures have been under considerable scrutiny in recent years, and one of the many interesti ng features that have emerged is that there is an asymmetry across the two hemispheres for global (right hemisphere) vs local (left hemisphere) proces sing. Event-related potentials (ERP) were used to examine selective attenti on to global or local levels of hierarchical figures to determine the stage of processing at which the asymmetry first emerges. Two conditions were te sted, one in which unattended information was variable from trial to trial, and one in which it was not. The variability of unattended information inf luenced the lateralization of processing. Presentation of invariable, neutr al distracters resulted in global/local processing asymmetries at early sta ges (P1). In contrast, presentation of variable, task-relevant distractors resulted in processing asymmetries that occurred at much later stages (N2). Our hypothesis is that lateralized enhancement of neural populations in ex trastriate cortex results from both selective attention to locations in the visual field, as well as selective attention to global or local informatio n. We suggest that unattended information that varies from trial to trial i s processed in parallel with attended information, masking hemisphere biase s for local vs global information at early stages of processing. (C) 2000 E lsevier Science Ltd. All rights reserved.