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Selective attention has been hypothesized to reduce distractor interference at both perceptual and postperceptual levels (Lavie, 2005), respectively, by focusing perceptual resources on the attended location and by blocking at postperceptual levels distractors that survive perceptual selection. This study measured the impact of load on these selection mechanisms using a flanker paradigm (Eriksen & St. James, 1986) and indexing distractor interference as a function of separation. It distinguished changes in the extent of focus of the distractor-interference function of separation (reflecting perceptual selection) from changes in the amplitude of distractor interference not accompanied by changes in focus (reflecting postperceptual selection). It showed that: (1) the spatial profile of perceptual resources is shaped like a "Mexican hat" (Müller et al., 2005); (2) increasing perceptual load focuses perceptual resources (Caparos & Linnell, 2009); (3) increasing cognitive load defocuses perceptual resources; and (4) participants with reduced working-memory span show reduced postperceptual blocking of distractors. While these findings are consistent with two levels of selective attention, they show that the first perceptual level is affected not only by perceptual but also by cognitive-control mechanisms.

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Exposure to the urban environment has been shown dramatically to increase the tendency to process contextual information. To further our understanding of this effect of urbanization, we compared performance on a local-selection task of a remote people, the Himba, living traditionally or relocated to town. We showed that (a) spatial attention was defocused in urbanized Himba but focused in traditional Himba (Experiment 1), despite urbanized Himba performing better on a working memory task (Experiment 3); (b) imposing a cognitive load made attention as defocused in traditional as in urbanized Himba (Experiment 2); and (c) using engaging stimuli/tasks made attention as focused in urbanized Himba, and British, as in traditional Himba (Experiments 4 and 5). We propose that urban environments prioritize exploration at the expense of attentional engagement and cognitive control of attentional selection.