In real-life visual environments, where multiple objects compete for processing, new objects that require immediate attention often appear when attention is already focused elsewhere. The question of whether spatial attention can be directed independently to different locations in the visual field remains controversial. Serial models assume a unitary attentional focus that is directed to one object at a time and moves rapidly between objects. According to parallel models, attention can be simultaneously allocated to several visual objects, but the distribution of attention cannot change rapidly when new objects arrive. Here we demonstrate the existence of a fast and flexible mechanism of attentional object selection, where focal attention is allocated in parallel and independently to different target objects. Using event-related brain potential (ERP) markers of visual attention, we show that when two targets appear in rapid succession at different locations, two separate foci of attention are established, each with its own independent time course. Attention can be maintained at its previous location while it is simultaneously allocated to a new target object. Our results challenge the view that the attentional focus is always unitary and that the spatial selection of multiple visual objects operates in a strictly serial fashion.

■ The question whether attentional capture by salient but taskirrelevant visual stimuli is triggered in a bottom-up fashion or depends on top-down task settings is still unresolved. Strong support for bottom-up capture was obtained in the additional singleton task, in which search arrays were visible until response onset. Equally strong evidence for top-down control of attentional capture was obtained in spatial cueing experiments in which display durations were very brief. To demonstrate the critical role of temporal task demands on salience-driven attentional capture, we measured ERP indicators of capture by task-irrelevant color singletons in search arrays that could also contain a shape target. In Experiment 1, all displays were visible until response onset. In Experiment 2, display duration was limited to 200 msec. With long display durations, color singleton distractors elicited an N2pc component that was followed by a late Pd component, suggesting that they triggered attentional capture, which was later replaced by location-specific inhibition. When search arrays were visible for only 200 msec, the distractor-elicited N2pc was eliminated and was replaced by a Pd component in the same time range, indicative of rapid suppression of capture. Results show that attentional capture by salient distractors can be inhibited for short-duration search displays, in which it would interfere with target processing. They demonstrate that salience-driven capture is not a purely bottom-up phenomenon but is subject to top-down control. ■

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Previous work has demonstrated that when targets are defined by a constant feature, attention can be directed rapidly and in parallel to sequentially presented target objects at different locations. We assessed how fast attention is allocated to multiple objects when this process cannot be controlled by a unique color-specific attentional template. N2pc components were measured as temporal markers of the attentional selection of 2 color-defined targets that were presented in rapid succession. Both targets either had the same color (one color task) or differed in color (two color task). Although there were small but systematic delays of target selection in the two color task relative to the one color task, attention was allocated extremely rapidly to both target objects in the two color task, which is inconsistent with the hypothesis that their selection was based on a slow switch between different color templates. Two follow-up experiments demonstrated that these delays did not reflect template switch costs, but were the result of competitive interactions between simultaneously active attentional templates. These results show that the control of focal attention during multiple-feature search operates much faster and more flexibly than is usually assumed.