Trends in Cognitive Sciences
Distracted and confused?: Selective attention under load
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 irrelevant 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.
Section snippets
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
Effects of perceptual load on distractor processing in the brain
Several neuroimaging studies show that high perceptual load in a relevant task modulates neural activity related to irrelevant distractors. In one study [10] neural activity in visual cortex associated with the perception of irrelevant motion distractors was determined by the level of load in a relevant task performed on words at fixation. Subjects were asked either to monitor a word's case (low load) or number of syllables (high load). Irrelevant motion background evoked responses in motion
How early is the gating of neural processing by load?
Effects of perceptual load in a relevant task can be found in V1 activity related to irrelevant stimuli. Schwartz et al. [17] assessed activity related to peripheral task-irrelevant checkerboard patterns presented while subjects performed a task of either low or high load on a rapid letter stream at fixation (Figure 2a). They found that visual cortex activity related to the task-irrelevant checkerboard was decreased by higher load in the central task. Importantly, retinotopic mapping revealed
Perceptual load and spatial attention
The effects of perceptual load require clear spatial separation between the target and distractor. When both target and distractor are parts of the same stimulus (e.g. a coloured word in the Stroop task [20]) high perceptual load (manipulated similarly to Figure 1b) can increase rather than decrease Stroop interference [21]. It is likely that when the distractor and target form parts of the same stimulus, paying more attention to the target (under high perceptual load) results in more attention
Crossmodal effects of perceptual load
It is important to determine whether attentional capacity is modality specific (such that, for example, auditory load should have no effects on the perception of visual distractors) or is shared between the modalities (such that load in one modality should determine distractor processing in another modality). With the prevailing emphasis in the last few decades on attention in vision, most load studies to date have been conducted in the visual modality, although a few studies have now examined
Improving distractor rejection in neuropsychological patients with perceptual load
Improvement in distractor rejection with a small increase in load is found for patients with a lesion to brain regions that are thought to mediate attentional capacities (see Box 2 for similar improvements in children and the elderly). Left neglect patients with a right parietal lesion are particularly distracted by stimuli in their right field. However, only a small increase of perceptual load in the task performed at fixation (increasing the number of letters in a search task from one to two)
Loading cognitive control processes
The effects of load on distractor processing depend crucially on the type of mental processing that is loaded. Load on executive cognitive control functions, such as working memory, that renders them unavailable to actively maintain stimulus-processing priorities throughout task performance has the opposite effect to perceptual load: it increases interference by irrelevant low-priority distractors rather than decreases it. Behavioural studies demonstrate that high working-memory load can
Conclusions
The studies reviewed here illustrate the importance of considering the level and type of load involved in the task performed to determine interference by task-irrelevant distractors. Simply instructing people to focus attention on a certain task is not sufficient to prevent distractor interference. A high perceptual load that engages full attention in the task is also needed. In contrast with the effects of perceptual load, high cognitive-control load increases distractor interference,
Acknowledgements
I thank Rocío J. Salavdor for assistance with the references. Special thanks to Gastón J. Madrid for assistance with the figures. I also thank Jon Driver and Vincent Walsh for helpful comments on an earlier draft.
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