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FIG. 5. (A) Sample stimuli from study by Anllo-Vento and Hillyard (31). Stimuli were presented to the left and the right visual field (LVF and RVF, respectively) in random order. Each stimulus consisted of a pair of briefly flashed adjacent squares separated by a short time interval (50 or 150 ms), which was perceived as a square moving in the direction of the arrow. Infrequent target pairs (150-ms separation) seem to move more slowly than standard pairs (50-ms separation). Stimulus color (red, blue), field of presentation (left, right) and movement direction (vertical, horizontal) were all randomized. (B) Subtracted ERP difference waves reflecting the hierarchical selection of location, feature, and target under attend-color (Left) and attend-motion (Right) conditions. Selection of the relevant visual field location (Top) was reflected in the difference wave formed by subtracting the ERP to the unattended field from the ERP to the attended field: difference ERPs shown are from contralateral occipito-temporal sites and show attentional modulation of the P1 and N1 components starting at 80 ms. Selection by feature (Middle) was indexed by the SN component beginning at about 150 ms and was seen in the difference waves formed by subtracting the ERP to the unattended feature value from the ERP to the attended feature value for stimuli at the attended location (thick line), but not at the unattended location (thin line). Selection of targets (movement speed) was reflected in N2 and LPC components beginning at 250–300 ms and was apparent in the difference waves formed by subtracting the ERPs to the nontarget (fast-moving) stimuli from the target (slow-moving) stimuli presented at the attended location and having the attended feature value. Mean motor reaction time (RT) is shown on time base for each condition.

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.

CONCLUSION

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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