A load theory of attention in which distractor rejection depends on the level and type of load involved in current processing was tested. A series of experiments demonstrates that whereas high perceptual load reduces distractor interference, working memory load or dual-task coordination load increases distractor interference. These findings suggest 2 selective attention mechanisms: a perceptual selection mechanism serving to reduce distractor perception in situations of high perceptual load that exhaust perceptual capacity in processing relevant stimuli and a cognitive control mechanism that reduces interference from perceived distractors as long as cognitive control functions are available to maintain current priorities (low cognitive load). This theory resolves the long-standing early versus late selection debate and clarifies the role of cognitive control in selective attention.

The hypothesis that working memory is crucial for reducing distraction by maintaining the prioritization of relevant information was tested in neuroimaging and psychological experiments with humans. Participants performed a selective attention task that required them to ignore distractor faces while holding in working memory a sequence of digits that were in the same order (low memory load) or a different order (high memory load) on every trial. Higher memory load, associated with increased prefrontal activity, resulted in greater interference effects on behavioral performance from the distractor faces, plus increased face-related activity in the visual cortex. These findings confirm a major role for working memory in the control of visual selective attention.Despite a vast body of research on visual attention and on working memory, the interaction between the two has seldom been addressed. There have been a few recent suggestions that working memory may play a role in the control of selective attention (1, 2), but evidence for a specific role has been scarce. Here we show a direct causal role for working memory in the control of selective attention.The most enduring issue in the study of attention is the extent to which distractor processing can be prevented (3). Lavie recently proposed that the level of perceptual load in a display is a crucial factor (4). Several studies have shown that distractors that could not be ignored in situations of low perceptual load (for example, when just a few task-relevant stimuli were presented) were successfully ignored in situations of high perceptual load (for example, when many relevant stimuli were present). Thus, less distrac-tor processing was found in the harder task.Here we propose that the effect of task difficulty on distractor processing depends critically on the mental process that is loaded. We suggest that directing attention appropriately requires the active maintenance of stimulus priorities in working memory, specifying which stimuli are currently relevant. A high load on working memory should therefore lead to reduced differentiation between high-and low-priority stimuli (that is, between targets versus distractors in a selective attention task). This leads to the counterintuitive prediction of an opposite effect to that found for perceptual load: Higher working memory load should increase distractor processing.

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The effects of perceptual load and those of target-stimulusdegradation on distractor processing were contrasted. Targets either had to be found among several nontargets (high perceptual load) or were presented alone and were intact (low perceptual load), had reduced size and contrast (Experiment 1), had reduced duration and were followed by a mask (Experiment 2), or had reduced visual acuity owing to position eccentricity (Experiment 3) in the degraded low-load condition. The results showed that both high perceptual load and target degradation increased general task difficulty, as is reflected by overall reaction times and accuracy. However, whereas high perceptual load reduced response-competition effects of irrelevant distractors, target degradation increased distractor effects. These results support the hypothesis that distractor processing depends on the extent to which high perceptual load exhausts attention in relevant processing, and provide a dissociation between perceptual load and general task difficulty and processing speed.

Recent reports have claimed that observers show accurate knowledge of the mean size of a group of similar objects, a finding that has been interpreted to suggest that sets of multiple objects are represented in terms of their statistical properties, such as mean size (Ariely, 2001;Chong & Treisman, 2003, 2005a, 2005b. In the present study, we directed visual attention to a single set member and found that mean estimations were modulated according to the size of the attended item, regardless of whether size was the relevant search criterion (Experiment 1) or not (Experiment 2). These findings suggest that observers do not always accurately average together the entire set, and that instead the average is either biased by the features of the attended item, or based on a short-cut strategy of extracting the mean of a smaller subset.

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