Neuronal activity in early visual areas during global and local processing: a comment on Heinze, Hinrichs, Scholz, Burchert and Mangun.

In a paper published in the 10:4 issue of JOCN, Heinze et al. 1998) studied a directed attention task using hierarchically organized letter Navon stimuli. On the basis of negative PET and ERP findings, they argue that``early sensory inputs are not modulated to gate global vs. local visual information differentially into the two hemispheres .'' Rather, based on positive ERP findings (an N2 effect) in a divided-attention task, they argue that``later stages of cortical processing may be asymmetrically organized in the left and right hemispheres and operate in parallel to process global and local aspects of complex stimuli.'' The authors conclude that their re-sults``support models proposing that spatial frequency analysis is only asymmetric at higher stages of perceptual processing and not at the earliest stages of visual cortical analysis.'' In three previous functional-neuroimaging studies of directed-visual attention to complex-hierarchical stimuli of various types (letters made of letters, objects made of objects, and rectangles made of rectangles), we have repeatedly demonstrated hemispheric differences between global and local processing both in early visual processing areas (V2/V3). We also find that higher stages of perceptual processing that activate temporo-parietal cortex are implicated in a divided-attention task using such stimuli (Fink et al., 1996; Fink et al., 1997a). In addition, such hemispheric-differential activations at the early stages of visual processing are influenced by sti-Our PET findings are thus consistent with neuropsy-chological studies demonstrating a critical role of the temporo-parietal cortex in global/local processing (Ro-1988), functional-imaging data using fMRI (Martinez et al., 1997) showing hemispheric differences between global and local processing, and other functional-imaging data (using both PET and fMRI) on the attentional gating of early visual processing areas (Corbetta, Miezin, Dobmeyer, Shulman, & Petersen, 1990). Furthermore, electrophysiological data obtained in monkeys directly demonstrate enhanced neural activity in early visual cortex resulting from changes in attentional set The N2 effect observed by Heinze et al. (1998) is consistent with the widely accepted view that the local aspects of hierarchical letter Navon figures are preferentially processed in the left hemisphere while the global aspects of such figures are preferentially processed in the right hemisphere. Leaving aside the problem of the exact localization of ERP sources, it is important to consider whether their negative PET findings (Heinze et al., 1998) can be taken as evidence against our findings of early visual processing differences in similar experiments (Fink et al., 1997a; Fink et al., 1999). The failure of …