Electrophysiological correlates of hierarchical stimulus processing: dissociation between onset and later stages of global and local target processing.

In the discussion of whether the processing of hierarchically structured stimuli proceeds from the more global to the more local level or vice versa, it is frequently assumed that the relative speed of global/local target identification (response time (RT) advantage) and the direction of interference from local/global distractors reflect the order of processing. Studies both in brain-injured patients and in normals, however, have demonstrated that RT advantage and interference are dissociable, leading to the conclusion that they do not provide a valid index of the order of global/local processing. The aim of the present event-related brain potential (ERP) study was to assess electrophysiological correlates of global/local processing and to determine how the relative speed of responding to global and local targets is related to these ERP measurements. In a divided-attention paradigm, subjects were asked to respond to hierarchically structured letter stimuli that contained a target letter either at the global or at the local level. The behavioral results confirmed a dissociation between RT advantage and interference. ERP analysis revealed an early posterior negative component (denoted as N250) as a sign of early global/local target perception. It was found that RT advantage is not a valid measure of the onset nor of the time course of this component. Furthermore, the N250 components to global and local targets exhibited a different time course and a different topographical distribution, suggesting that they are determined by separate processing structures. Together, the behavioral and electrophysiological results support the view that global and local target perception may be mediated by separate brain systems acting, at least initially, in parallel.