Prioritization by transients in visual search

There is an ongoing debate as to whether prioritizing new objects over old objects (the so-calledpreview benefit) is the result of top-down inhibition of old objects (i.e., visual marking; Watson & Humphreys, 1997) or attentional allocation to new objects, presented with a luminance transient (Donk & Theeuwes, 2001). In the two experiments reported here, we tested whether prioritization by luminance transients alone can produce a subset-selective search similar to the preview effect. Subjects viewed multiobject displays while a subset of objects was briefly flashed. The subjects prioritized up to 14 flashed objects over at least 14 nonflashed objects. Since prioritization by luminance transients can produce a subsetselective search on its own, it may well play an important role in the preview benefit.     

Is it impossible to inhibit isoluminant items, or does it simply take longer? Evidence from preview search

Visual search can be facilitated when participants receive a preview of half the distractors (the preview benefit in search; Watson & Humphreys, 1997). Donk and Theeuwes (2001) have argued that preview-based benefits are abolished if the display items are isoluminant to a background. This is consistent with the preview benefit being due to onset capture by the new stimuli. In contrast, the present experiments challenge this suggestion and show that preview benefits can occur under isoluminant conditions, providing that they are given enough time to occur. In Experiment 1, we showed that a preview benefit can occur even with isoluminant stimuli, provided that the old items are previewed for a sufficient time. In Experiment 2, we tested and rejected the idea that this advantage is due to low-level sensory fatigue for the preview stimuli. These findings indicate that the preview effect is not caused solely by onset capture.     

Revisiting preview search at isoluminance: new onsets are not necessary for the preview advantage

It has been argued that search performance under preview conditions relies on automatic capture by luminance onsets (Donk & Theeuwes, 2001). We present three experiments in which preview search was examined with both isoluminant and nonisoluminant items (e.g., as defined by luminance onsets). Experiment 1 provided evidence against the automatic capture of attention by onsets. Search benefited when onset previews were followed by new onset stimuli, as compared with a full-set baseline matched for the number of new onsets but in which half the distractors appeared simultaneously at isoluminance. Furthermore, both Experiments 1 and 2 established a preview advantage when isoluminant targets followed onset previews, when compared with appropriate full-set baselines. Experiment 3 replicated this result, while showing that the preview benefit was disrupted by dual-task interference. The data indicate that new onsets are not necessary to generate a preview advantage in search. We discuss the data in terms of search's benefiting from active inhibition of old onset-defined stimuli.     

Offsets and prioritizing the selection of new elements in search displays: More evidence for attentional capture in the preview effect

The preview effect refers to the finding that new elements in a search display are prioritized over old elements. Donk and Theeuwes (2001) suggest that this prioritization is due to bottom-up attentional capture, as new elements that abruptly appear with increases in luminance produce a preview effect whereas new elements that are equiluminant to the background do not. The present study extends this research by examining if new elements that appear from offsets, with and without luminance changes, yield similar findings to onsets. This was indeed the case, as equiluminant offsets did not produce a preview effect but offsets with corresponding increases or decreases in luminance did show the prioritization of new elements over old elements. These findings show that the prioritization of new elements is sensitive to changes in luminance but not to the direction of such changes.     

Visual marking across eye blinks

Visual search for a conjunction target can be made efficient by presenting one initial set of distractors as a preview, prior to the onset of the other items in the search display Watson & Humphreys (Psychological Review 104:90–122, 1997). However, this “preview advantage” is lost if the initial items are offset for a brief period before onsetting again with the search display Kunar, Humphreys, & Smith (Psychological Science 14:181–185, 2003). Researchers have long disputed whether the preview advantage reflects a process of internally coding and suppressing the old items or of the onset of the new items capturing attention Donk & Theeuwes (Perception & Psychophysics 63:891–900, 2001). In this study, we assessed whether an internally driven blink (in which participants close their eyes) acts in the same manner as an external blink produced by offsetting and then onsetting the preview. In the novel blink conditions, participants searched feature, conjunction, and preview displays after being cued to blink their eyes. The search displays were presented during the eye blink, and so were immediately available once participants opened their eyes. Having participants make an eye blink generally slowed search but had no effect on the search slopes. In contrast, imposing an externally driven blink disrupted preview search. The data indicated that visual attention can compensate for internally driven blinks, and this does not lead to the loss of the representations of distractors across time. Moreover, efficient preview search occurred when the search items had no abrupt onsets, demonstrating that onsets of new search items are not critical for the preview benefit.     

The time course of preview search with color-defined, not luminance-defined, stimuli

We examined the time course of preview search, using stimuli that were defined by color, but not by luminance changes. We demonstrate that, under these conditions, search performance in a preview condition improved selectively over time, relative to a baseline condition in which all the items appeared together. The data confirm earlier reports from Humphreys, Kyllinsbaek, et al. (2004) and Watson and Humphreys (1997), who used luminance-defined stimuli and showed a long time course to preview search. The data contradict Donk and Verburg (2004), who argued that the preview benefit was instantaneous but did not include baseline conditions with which to test for any influence of distractors equivalent to the old items in preview search, even under nonpreview conditions. The data support the proposal that the prioritization of new items in preview search is a time-consuming business.     

Prioritizing new elements with a brief preview period: Evidence against visual marking

Watson and Humphreys (1997) have proposed that the ability to prioritize new elements over old elements involves a time-consuming process (of at least 400 msec) of active inhibition of the locations of the old elements, which they referred to asvisual marking. Recently, Donk and Theeuwes (2001) have suggested that prioritized selection of new over old elements is an instantaneous process related to the luminance change accompanying the appearance of the new elements. The aim of the present study was to test these two alternatives by investigating whether prioritized selection of new over old elements could be achieved with a very short preview of the old elements (50 msec). The results indicated that participants were able to prioritize selection of new over old elements when the new elements were presented with luminance onset whereas the old elements were not. New elements could not be prioritized if both the old and the new elements appeared with luminance onset. The results indicated that prioritization of new elements is based on an instantaneous process, rather than on a time-consuming process.     

Resisting change: The influence of luminance changes on visual marking and the preview benefit

Visual search can benefit when one set of distractors is presented as a preview, prior to the appearance the second set of distractors plus the target (Watson & Humphreys, 1997). It has been shown that changing the shape of the old, previewed stimuli when the new items appear causes the old stimuli to recompete for selection with the new ones. In contrast, changing the luminance or color of the old stimuli has no detrimental effects. Here, we present five experiments that reassessed the effect of luminance changes in preview search. We show (1) that preview search is remarkably resistant to large changes in the absolute luminance of the old stimuli, even when those changes would ordinarily be sufficient to signal the appearance of a new object and draw attention (Experiments 1 and 2), and (2) that resistance to luminance changes can be bolstered by feature-based inhibitory processes (Experiments 3–5). These findings are discussed in terms of the possible ecological properties of time-based visual selection and possible mechanisms underlying the preview benefit.     

Eye movements and time-based selection: Where do the eyes go in preview search?

In visual search tasks, presenting one set of distractors (previewing them) before a second set which contains the target, improves search efficiency compared to when all items appear simultaneously. It has been proposed that this preview benefit reflects an attentional bias against old information and toward new information. Here we tested directly whether there was such a bias by measuring eye movement behavior. The main findings were that fixations were biased against, and overall dwell times were shorter on, old stimuli during search in the preview condition. In addition, the initial onset of search was delayed in the preview condition and saccades made during the preview period did not disrupt the ability to prioritize new items. The data demonstrate directly that preview search results in an attentional bias toward new items and against old items.     

Faster target selection in preview visual search depends on luminance onsets: behavioral and electrophysiological evidence

To investigate how target detection in visual search is modulated when a subset of distractors is presented in advance (preview search), we measured search performance and the N2pc component as an electrophysiological marker of attentional target selection. Targets defined by a color/shape conjunction were detected faster and the N2pc emerged earlier in preview search relative to a condition in which all items were presented simultaneously. Behavioral and electrophysiological preview benefits disappeared when stimuli were equiluminant with their background, in spite of the fact that targets were feature singletons among the new items in preview search. The results demonstrate that previewing distractors expedites the spatial selection of targets at early sensory-perceptual stages, and that these preview benefits depend on rapid attentional capture by luminance onsets.     

Selecting from dynamic environments: attention distinguishes between blinking and moving

Pinto, Olivers, and Theeuwes (2006) showed that a static target can be efficiently found among different types of dynamically changing distractors. They hypothesized that attention employs a broad division between static and dynamic information, a hypothesis that conforms with earlier research. In the present study, we investigated whether attention can only make use of this crude division or can exploit more subtle discriminations within the dynamic domain. In Experiment 1, participants were able to efficiently find a blinking target among moving distractors and moving targets among blinking distractors, although all items changed at the same rate and produced the same change in local luminance. In Experiment 2, search for a dynamic target among dynamic distractors was aided when we gave the distractors additional dynamic cues. Experiment 3 showed that making the displays equiluminant affected search efficiency for a static target among moving distractors, but not among blinking distractors. The findings refute the broad division hypothesis and suggest that object continuity plays an important role in selection.     

Object-based inhibitory priming in preview search: Evidence from the “top-up” procedure

Visual search in a conjunction task can be facilitated if half the distractor items are previewed prior to the other half of the distractors and the target item. Here, we investigate the nature of this preview by using a top-up procedure, which presents an initial preview followed by a secondary preview after a period of time (the offset period). In Experiment 1, we demonstrate that increasing the time of the offset period decreases search efficiency. If the offset period is increased to 2 sec, the previewed items are searched to a greater extent than when the offset period is 450 msec. This holds even when the old items remain in the same positions across presentations and when they differ in color from new search stimuli. However, when the offset intervals are reduced, the preview can be discounted from search even when the old items change locations between exposures (Experiment 2) and when they are not distinguished from search displays by their color (Experiment 3). The last result occurs only as long as the preview items can be grouped in terms of form. When the preview stimuli are heterogeneous, they are no longer discounted from search if their locations change across the offset period (Experiment 4). We interpret the data in terms of object-based priming, which enables the repeated form of the old distractors to be filtered more easily from search.     

Visual marking: the effects of irrelevant changes on preview search

Visual search is facilitated when participants receive a preview of half of the distractors before presentation of the second distractor set (Watson & Humphreys, 1997). In seven experiments, we examined the effects of irrelevant change on this preview benefit. Experiments 1-4 showed that the benefit was not disrupted by the abrupt appearance of irrelevant distractors during the preview period unless they were the same color as the new items. However, blinking off-and-on irrelevant distractors that were present at the start of the preview period disrupted the preview benefit irrespective of their feature overlap with other items (Experiments 5-7). The data are consistent with the inhibition of old stimuli (visual marking) via a location-based template along with an anticipatory feature-based set for new stimuli.     

Visual change with moving displays: more evidence for color feature map inhibition during preview search

Preview search with moving stimuli was investigated. The stimuli moved in multiple directions, and preview items could change either their color or their shape before onset of the new (search) displays. In Experiments 1 and 2, the authors found that (a) a preview benefit occurred even when more than 5 moving items had to be ignored, and (b) color change, but not shape change, disrupted preview search in moving stimuli. In contrast, shape change, but not color change, disrupted preview search in static stimuli (Experiments 3 and 4). Results suggest that preview search with moving displays is influenced by inhibition of a color map, whereas preview search with static displays is influenced by inhibition of locations of old distractors.     

Preview search and contextual cuing

We investigated the effect of contextual cuing (M. M. Chun & Y. Jiang, 1998) within the preview paradigm (D. G. Watson & G. W. Humphreys, 1997). Contextual cuing was shown with a 10-item letter search but not with more crowded 20-item displays. However, contextual learning did occur in a preview procedure in which 10 preview items were followed by 10 new items. Repeating the new items alone did not generate contextual learning, but repeating the preview items alone did, as long as they had a consistent spatial relation with the target. This was not merely due to the onset of the preview items being associated with the target location. No learning effect took place with a preview of homogeneous items that competed less for selection with new stimuli. The results provide evidence for old items being processed in preview search and providing a context for subsequent search of new items.     

Top-up search and the attentional blink: A two-stage account of the preview effect in search

An inefficient visual search task can be facilitated if half the distractor items are presented as a preview prior to the presentation of the remaining distractor items and target. This benefit in search is termed the preview effect. Recent research has found that a preview effect can still occur if the previewed items disappear before reappearing again just before the search items (the “top-up” procedure). In this paper we investigate the attentional demands of processing during the preview and the top-up periods. Experiment 1 found that if attention is withdrawn from the top-up stage then no preview effect occurs. Likewise if attention is withdrawn from the initial preview period then the preview effect is reduced (Experiment 2). The data suggest that the preview effect is dependent on attention being paid both to the initial display and also to the re-presentation of the old display before the search display appears. The data counter accounts of preview search in terms of automatic attention capture by new items or by inhibition of return. We discuss alternative accounts of the results, and in particular suggest an amalgamation of a temporal grouping and a visual marking account of preview search.     

Visual marking inhibits singleton capture

This paper is concerned with how we prioritize the selection of new objects in visual scenes. We present four experiments investigating the effects of distractor previews on visual search through new objects. Participants viewed a set of to-be-ignored nontargets, with the task being to search for a target in a second set, added to the first after 1000ms. This second set could contain a salient feature singleton, defined in terms of its color, orientation, or both color and orientation. When the singleton was a distractor, search was slowed relative to when there was no singleton. Search was facilitated when the singleton was a target. Interestingly, both the interference and facilitation effects were modulated when the preview shared features with the singleton. Follow-up experiments showed that this reduction of singleton effects was not due to: (i) low-level sensory aspects of the displays, (ii) increased heterogeneity in the search set in the preview condition, or (iii) color-based grouping of old and new items. Instead, we suggest that there is an inhibitory carry-over from the first to the second set of items based on feature similarity. We suggest the suppression stems from a process termed visual marking, which suppresses irrelevant visual objects in anticipation of more relevant new objects (Watson & Humphreys, 1997). The findings argue against alternative explanations such as the automatic capture by abrupt new onsets account.     

旧客体抑制和新客体捕获视角下预览效应的机制

预览效应是一种在已经存在的多个旧客体中优先选择多个新客体, 对新客体表现出优先加工的现象。文章综述了预览效应认知神经机制的两种主要观点：(1)旧客体抑制观点, 认为预览效应是由于对旧客体的抑制; (2)新客体突现捕获注意观点, 认为预览效应是由于新客体出现伴随着亮度突然增加捕获注意。这两种观点之间的主要争论在于：预览效应与新旧客体亮度变化的关系; 新旧客体的颜色关系是否影响对目标的搜索; 在一些因素的作用下, 旧客体形状变化是否还会破坏预览效应。文章指出, 预览效应是旧客体抑制机制与新客体突现捕获注意机制共同作用的结果。     

Do new objects capture attention?

The visual system relies on several heuristics to direct attention to important locations and objects. One of these mechanisms directs attention to sudden changes in the environment. Although a substantial body of research suggests that this capture of attention occurs only for the abrupt appearance of a new perceptual object, more recent evidence shows that some luminance-based transients (e.g., motion and looming) and some types of brightness change also capture attention. These findings show that new objects are not necessary for attention capture. The present study tested whether they are even sufficient. That is, does a new object attract attention because the visual system is sensitive to new objects or because it is sensitive to the transients that new objects create? In two experiments using a visual search task, new objects did not capture attention unless they created a strong local luminance transient.     

When a reappearance is old news: visual marking survives occlusion

Performance in a visual search task becomes more efficient if half of the distractors are presented before the rest of the stimuli. This "preview benefit" may partly be due to inhibition of the old (previewed) items. The preview effect is abolished, however, if the old items offset briefly before reappearing (D. G. Watson & G. W. Humphreys, 1997). The authors examined whether this offset effect still occurred if the old items undergo occlusion. Results show that a preview benefit was found when the old items were occluded but not otherwise, consistent with the idea of top-down attentional inhibition being applied to the old items. The preview benefit is attenuated, however, by movement of the irrelevant stimuli in the displays.