were attending to color, a stimulus property mediated by the ventral visual pathway, or direction of motion, a feature presumably processed by the dorsal visual pathway. However, differences in scalp distribution between the SNs associated with color and motion selection were indicative of separate cortical origins.
These results suggest that the registration and processing of an object’s features in both the ventral and dorsal pathways can be strongly gated by spatial attention. More specifically, the selective processing of nonspatial features reflected in the SN, N2, and LPC components is strongly dependent upon the prior selection for location, reflected in the P1 and N1 components. This hierarchical relationship supports early selection theories that propose attentional control over perceptual processing and seems to conflict with the late selection view that different stimulus attributes are processed in parallel at all locations (54, 55). Moreover, the different ERP configurations associated with spatial and nonspatial selections provide strong evidence that attention to location operates via qualitatively different mechanisms from attention to other stimulus features. In sum, it seems that attention to location is indeed “special” (52, 53) and plays a unique role in feature integration.
The studies reviewed above illustrate how ERP and neuroimaging data can be combined to reveal both the spatial and temporal properties of neural activity during selective attention. As we have noted, such physiological data can supply converging operations for testing alternative psychological models of attention derived from behavioral studies (21). Information about the physiological bases of attention in humans also provides an essential link with the rapidly expanding literature on animal studies of attention (16, 18, 19). By comparing the spatio-temporal configurations of neural activity in homologous brain regions during the performance of comparable tasks across species, the validity of animal models of human attentional processes can be properly established. A close interplay between human and animal investigations will be required to learn how stimulus information is encoded, transformed, and selectively processed by the brain’s attentional systems.
We thank our current and former colleagues who have made essential contributions to the work reported here: Vince Clark, Hajo Heinze, Hermann Hinrichs, Steve Luck, Ron Mangun, Tom Münte, and Marty Woldorff. Jon Hansen, Matt Marlow, Carlos Nava, and Theresa Rubin provided valuable technical assistance. This research was supported by Office of Naval Research Grant N00014–93–I–0942, National Institute of Mental Health Grant MH25594, National Institutes of Health Grant NS 17778, and the San Diego McDonnell-Pew Center for Cognitive Neuroscience.
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