Distracted and confused?: Selective attention under load Nilli Lavie Department of Psychology and Institute of Cognitive Neuroscience, University College, Gower Street, London WC1E 6BT, UK The ability to remain focused on goal-relevant stimuli in the presence of potentially interfering distractors is crucial for any coherent cognitive function. However, simply instructing people to ignore goal-irrelevant stimuli is not sufficient for preventing their processing. Recent research reveals that distractor processing depends critically on the level and type of load involved in the processing of goal-relevant information. Whereas high perceptual load can eliminate distractor proces- sing, high load on ‘frontal’ cognitive control processes increases distractor processing. These findings provide a resolution to the long-standing early and late selection debate within a load theory of attention that accommo- dates behavioural and neuroimaging data within a framework that integrates attention research with executive function. Introduction The ability to remain focused on a task is vital for any coherent cognitive function, especially when there might be potential interference from distractors that are irrele- vant for the task. However, people are often distracted by task-irrelevant stimuli. Daily life provides numerous examples: a fly hovering about might distract you while reading this article, an attractive bill-board can distract a driver, and so forth. In the laboratory, research that looked at the extent to which distractor processing can be prevented led to an enduring controversy. Mixed results as to whether focusing attention on task-relevant stimuli can exclude distractors from early perceptual processing (an ‘early’ selection effect) or can only prevent distractors from controlling behaviour and memory (a ‘late’ selection effect) has fuelled a longstanding debate between early- and late-selection views of attention [1]. Recent research on the role of load in the processing of task-relevant information in determining the processing of task-irrelevant distractors offers a possible resolution. This research indicates that distractor perception can be prevented (early selection) when processing of task- relevant stimuli involves high perceptual load, and that although distractors are perceived in tasks of low perceptual load (late selection), their impact on behaviour depends on other types of load, such as that on working memory. These results have therefore provided better understanding of the circumstances under which people can achieve coherent goal-focused behavior with minimal intrusions of goal-irrelevant information. Perceptual load studies: behavioural experiments Research on the role of perceptual load in selective attention was triggered by the hypothesis that perception has limited capacity (as in early-selection views) but processes all stimuli in an automatic mandatory fashion (as in late-selection views) until it runs out of capacity [2,3]. This led to the predictions that high perceptual load that engages full capacity in relevant processing would leave no spare capacity for perception of task-irrelevant stimuli. In situations of low perceptual load, however, any capacity not taken up in perception of task-relevant stimuli would involuntarily ‘spill over’ to the perception of task-irrelevant distractors. These predictions were tested in experiments that assessed the effects on distractor perception of varying perceptual load in the task-relevant processing [3–5]. Increased perceptual load means that either the number of different-identity items that need to be perceived is increased, or that for the same number of items perceptual identification is more demanding on attention [3–5] (see Figure 1). These experiments found that increased perceptual load reduces, indeed typically eliminates, any distractor interference effects, in support of the perceptual load hypothesis. Reduced distractor interference under conditions of high perceptual load is not simply the result of the general increase in task difficulty with load and the associated slowing of performance. Manipulations of extreme sensory degradation (e.g. reducing the target size or contrast so much so that it is barely seen) that cannot be compensated for by applying more attention – in other words subjecting target identification to sensory ‘data limits’ rather than attentional ‘resource limits’ [6] – increase the general task difficulty (i.e. reduce speed and accuracy, compared with an intact target) but do not reduce distractor interference [7]. Alternative accounts to perceptual load in terms of general task difficulty or slowing are also ruled out by the findings (reviewed later) that increasing load on cognitive control processes (e.g. working memory) increases task difficulty but has the opposite effect to perceptual load, resulting in an increase (rather than a decrease) in distractor interference. The studies mentioned so far assessed perception of the distractor identity in the ‘response competition’ paradigm (Figure 1). Other paradigms used since have included Corresponding author: Lavie, N. (n.lavie@ucl.ac.uk). Available online 5 January 2005 Review TRENDS in Cognitive Sciences Vol.9 No.2 February 2005 www.sciencedirect.com 1364-6613/$ - see front matter Q 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.tics.2004.12.004