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The impact of item clustering on visual search: It all depends on the nature of the visual search.

Decades of vision research on how people search for a target item among distractor items have always avoided item clustering. Instead, researchers made sure that items were evenly distributed in search displays. This, however, is rarely the case in our everyday visual environment. Consequently, it is largely unknown how item clustering may impact visual search performance. In this study, I manipulated item clustering in search displays. In an easy feature search, observers looked for a target letter "T" among distractor letters "Os" and reported whether the target was pointing to the left or to the right. In a difficult spatial configuration search, observers searched and reported the orientation of the target letter "T" among distractor letters "Ls". The two types of searches thus had the same target but different distractors. In two experiments, I found that while item clustering slowed down the easy feature search, it speeded up the difficult spatial configuration search. Together, these results show that item clustering significantly affects visual search performance and its exact impact (negative or positive) depends on the nature of the visual search.