Hierarchical visual stimuli: Electrophysiological evidence for separate left hemispheric global and local processing mechanisms in humans

Using reaction time measurements it has been shown that the neural processing of hierarchically composed visual stimuli may be mediated by partly distinctive mechanisms of global and local feature analysis. Patients with focal left hemisphere brain lesions showed an impairment of local and patients with right hemisphere lesions of global stimulus perception. Our experiment was designed to extend the investigations of global/local feature analysis using the event-related brain potential (ERP) technique in healthy subjects. Thirteen subjects viewed a series of hierarchically composed non- linguistic stimuli in a divided attention condition in order to detect target stimuli which could be present at both local and global feature level. ERP analysis showed a posterior negative component (Ne) to be the correlate of early global/local information processing. When local targets were presented there was an effect of the global (non-target) level (termed 'distractor level') upon the onset of Ne but not vice versa. This supports the view of separate cortical mechanisms of global/local feature analysis. At odds with the previous patient data was the finding that both global and local level information was predominantly processed from the left hemisphere.