Spatial Organization of Distractors in Visual Search

Abstract This paper investigates visual search in displays in which distractors are spatially grouped. Detection of targets at the border of two regions where distractors are coherent is easier than can be predicted by models that do not take spatial organization into account. Thus, models of visual search need to incorporate effects of spatial organization in addition to bottom-up difference calculations. People can quickly locate items of potential interest in a visual scene, despite the fact that these items, and the other items in the scene, are rarely fully known in advance. Because this rapid guidance of visual inspection cannot be completely planned in advance, it is believed that it occurs primarily bottom-up, that is, guided by visual input rather than by expectations. In support, models of visual processing that include local difference calculations have been successful at explaining many aspects of tasks such as texture segregation Julesz, 1981; Nothdurft, 1992, 1993) and search (Bergen & Julesz, 1983; Nothdurft, 1992, 1993; Wolfe, 1994). On the other hand, not all phenomena in visual search can be explained by local accounts, and in this paper we report evidence for non-local influences on search in spatially organized displays. The spatial organization of items in a visual display can affect search efficiency (Banks & Prinzmetal, 1976; Bundesen & Pedersen, 1983; Farmer & Taylor, 1980; Poisson & Wilkinson, 1992; Theeuwes, 1996; Treisman, 1982). For example, Treisman (1982, Experiment 1) arranged 36 distractors of two types into different numbers of coherent groups. Fewer, larger groups yielded faster search times for a conjunction target (a green H among red Hs and green xs, where the target shared one feature with each of the two types of distractor) than more, smaller groups. One important question is the effect on search of target position relative to the boundary between groups. Are targets in the middle of coherent groups (with homogeneous neighbours) detected faster than those near group boundaries (with more heterogeneous neighbours)? Treisman (1982, Experiment 4) investigated the effect of target position relative to coherent groups within the display, using one-dimensional arrays of letters. The arrays consisted of three distractors of one type on the left, three distractors of another type on the right, and a target either within one of the distractor groups (surrounded by only one kind of distractor) or in the middle between the two groups (surrounded by two kinds of distractor). A conjunction target was detected fastest if it was within one of the groups, rather than between the two groups. Treisman investigated only the role of spatial position relative to groups in one-dimensional stimuli, however. ' In these stimuli, local neighbourhood is confounded with group membership, because targets within a group of distractors are surrounded by homogeneous distractors and targets between groups are surrounded by heterogeneous distractors. This paper investigates the effects of different local neighbourhoods for targets that were within, rather than between, groups of distractors. We examined the effect on detection of target position within a spatially grouped two-dimensional display containing two distractor types. We compared detection in grouped displays with detection in two kinds of control display, homogeneous (one distractor type) and ungrouped (two distractor types). We were interested in whether detection in grouped displays would be more similar to efficient detection in homogeneous displays than to inefficient detection in ungrouped displays. We found relatively efficient detection in grouped displays. In the General Discussion, we consider extensively whether models failing to take into account the spatial organization of the display can account for our results, and we conclude that they cannot. Chun and Jiang (1998) demonstrated that the spatial configuration of a search display can assist search, using a task in which the observer had to determine which of two targets was present in the display. …