It has long been debated whether or not a salient stimulus automatically attracts people's attention in visual search. Recent findings showed that a salient stimulus is likely to capture attention especially when the search process was inefficient due to high levels of competition between the target and distractors. Expanding these studies, the present study proposes that a specific nature of visual search, as well as search efficiency, determines whether or not a salient, task-irrelevant singleton stimulus captures attention. To test this proposition, we conducted three experiments, in which participants performed two visual search tasks whose underlying mechanisms are known to be different: orientation-feature search and Landolt-C search tasks. We found that color singleton distractors captured attention when participants performed the orientation-feature search task. The magnitude of this capture effect increased as search efficiency decreased. On the contrary, the capture by singleton distractors was not observed under the Landolt-C search task. This differential pattern of capture effect was not due to differences in search efficiency across the search tasks; even when search efficiency was controlled for, stimulus-driven capture of attention by a salient distractor was found only under the feature search. Based on these results, the present study suggests that in addition to search efficiency, the nature of search strategy and the extent to which attentional control is strained play crucial roles in observing stimulus-driven attentional capture in visual search.

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When one searches for a specific target in a cluttered visual scene, a perceptually salient stimulus or a stimulus that matches working memory's contents is prioritized for attentional selection. In the present study, we aimed at clarifying under which circumstance stimulus-driven attention or memory-driven attention is more pronounced. We hypothesized that one crucial factor affecting stimulus-driven versus memory-driven attention is how a concurrent visual search task is performed. To address this issue, we employed two visual search tasks whose underlying mechanisms are known to be different: Landolt-C search and orientation feature search. One group of participants performed visual search tasks containing a memory-matching stimulus, and the other group conducted searches in the presence of a salient singleton distractor. The results showed that the effects of stimulus-driven and memory-driven attention differed, depending on the cognitive mechanisms underlying the visual search tasks. A memory-matching stimulus captured attention when participants performed the Landolt-C search, whereas this capture was diminished under feature search. In contrast, capture by the salient singleton distractor was found only under feature search. These results demonstrate that the nature of the underlying visual search tasks is an important factor for observing stimulus-driven versus memory-driven attention. Our results also provide a potential solution to resolve current debate regarding memory-driven attention in visual search.