Abstract
Perceptual load theory (Lavie, 1995) contends that distractor processing is dependent upon the availability of perceptual resources: distractor identities are processed under conditions of low perceptual load, but not under conditions of high perceptual load. However, recent evidence has shown that salient distractors – i.e., irrelevant color singletons – may sometimes be processed regardless of perceptual load, as indicated by distractor interference (Biggs & Gibson, 2010). But, whereas Biggs and Gibson observed that the salience of the distractor dominated load when the distractor appeared within the task-relevant search array, the present study examined whether the salience of the distractor would also dominate load under conditions in which the distractor was spatially separated from the task-relevant search display. Contrary to the findings reported by Biggs and Gibson, these findings suggested that the identity of the distractor was processed when perceptual load was low, but not when perceptual load was high. However, although the identity of the distractor did not appear to be processed in the high-load condition, the mere presence of a salient distractor nevertheless incurred a significant RT cost, suggesting a greater filtering cost for singleton distractors. Note that this cost was obtained even though observers had full knowledge of the color of both the search array and the distractor. Two subsequent experiments were conducted to investigate the nature of this filtering cost, one in which observers had only color knowledge of the search array and one in which they had only color knowledge of the distractor. Surprisingly, the results showed that the filtering cost arose only when observers had color knowledge of the distractor, suggesting that observers processed the distractor only when they had irrelevant knowledge about it. Thus, these findings are important because they show that some forms of knowledge can have detrimental effects on the control of attention.