NEURAL MECHANISMS OF GLOBAL AND LOCAL PROCESSING - A COMBINED PET ANDERP STUDY

The neural mechanisms of hierarchical stimulus processing were investi gated using a combined event-related potentials (ERPs) and positron em ission tomography (PET) approach. Healthy subjects were tested under t wo conditions that involved selective or divided attention between loc al and global levels of hierarchical letter stimuli in order to determ ine whether and where hemispheric differences might exist in the proce ssing of local versus global information. When attention was divided b etween global and local levels, the N2 component of the ERPs (260- to 360-msec latency) elicited by the target stimuli showed asymmetries in amplitude over the two hemispheres. The N2 to local targets was large r over the left hemisphere, but the N2 to global targets tended to be slightly larger over the right hemisphere. However, the shorter-latenc y, sensory-evoked P1 component (90- to 150-msec latency) was not diffe rent for global versus local targets under conditions of divided atten tion. In contrast, during selective attention to either global or loca l targets, asymmetries in the N2 component were not observed. But unde r selective attention conditions, the sensory-evoked P1 components in the extrastriate cortex were enlarged for global versus local attentio n. Increased regional cerebral blood flow in the posterior fusiform gy rus bilaterally was observed in the PET data during selective attentio n to either global or local targets, but neither these nor the P1 comp onent showed any tendency toward hemispheric difference for global ver sus local attention. Neither were there any activations observed in th e parietal lobe during selective attention to global versus local targ ets. Together these data indicate that early sensory inputs are not mo dulated to gate global versus local information differentially into th e two hemispheres. Rather, later stages of processing that may be asym metrically organized in the left and right hemispheres operate in para llel to process global and local aspects of complex stimuli (i.e., the N2 effect of the ERPs). This pattern of results supports models propo sing that spatial frequency analysis is only asymmetric at higher stag es of perceptual processing and not at the earliest stages of visual c ortical analysis.