ELECTROPHYSIOLOGICAL EVIDENCE OF A PERCEPTUAL PRECEDENCE OF GLOBAL VS. LOCAL VISUAL INFORMATION

Aim of the present study was to investigate the mechanisms of attentio nal selection of hierarchically organized visual patterns (compound le tter stimuli), while subjects were engaged in target selection at eith er the global or local level. Event-related brain potentials (ERPs) we re recorded using a high density electrode montage. Reaction times (RT s) to target stimuli were also recorded. RT data indicated the interfe rence effect of global incongruent information with the local one. ERP data were consistent with behavioral data. In fact, the early sensory N115 component recorded at the primary visual areas exhibited smaller responses to locally attended elements when the global configuration was incongruent rather than congruent, suggesting an interference effe ct of the global with the local level. Conversely, no interference eff ect was found for globally attended configurations. These results stro ngly support the view of a perceptual advantage of globally conveyed i nformation, very likely mediated by low spatial frequency channels. At later processing levels, N1 and P3 components were faster and larger when attention was paid to the global configuration. The difference be tween target and nontarget responses, indexing the attentional target selection, yielded a broad occipital-temporal negativity focused onto the left hemisphere in the attend-local, and over the right hemisphere in the attend-global condition. The present findings indicate a hemis pheric asymmetry in cerebral activation during local/global processing . In addition, they provide robust evidence of a sensory precedence of global information. (C) 1998 Elsevier Science B.V.