Search efficiency is not sufficient: The nature of search modulates stimulus-driven attention

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.     

Bottom-up and top-down control in visual search

Previous research suggests that the allocation of attention is largely controlled either in a stimulus-driven or in a goal-driven manner. To date, few studies have systematically manipulated variables affecting stimulus-driven and goal-driven selection independently in order to investigate how both manners of control interrelate and affect performance in visual search. In the present study observers were presented with search displays consisting of an array of line segments rotated at various orientations. The task of observers was to indicate the presence or absence of a vertical line segment (the target) presented amongst a series of nontargets and possibly one distractor. By varying the absolute differences in orientation between the target, nontargets, and distractors, relative target-distractor salience and target-distractor similarity were independently manipulated to investigate the contribution of stimulus-driven and goal-driven control. The major result was that relative target-distractor salience and target-distractor similarity affected search performance independently. Performance was better in cases where the irrelevant distractor was not a salient item in the search display and did not look similar to the target. The results are discussed in terms of models of attentional control.     

Coordination of voluntary and stimulus-driven attentional control in human cortex

Visual attention may be voluntarily directed to particular locations or features (voluntary control), or it may be captured by salient stimuli, such as the abrupt appearance of a new perceptual object (stimulus-driven control). Most often, however, the deployment of attention is the result of a dynamic interplay between voluntary attentional control settings (e.g., based on prior knowledge about a target's location or color) and the degree to which stimuli in the visual scene match these voluntary control settings. Consequently, nontarget items in the scene that share a defining feature with the target of visual search can capture attention, a phenomenon termed contingent attentional capture. We used functional magnetic resonance imaging to show that attentional capture by target-colored distractors is accompanied by increased cortical activity in corresponding regions of retinotopically organized visual cortex. Concurrent activation in the temporoparietal junction and ventral frontal cortex suggests that these regions coordinate voluntary and stimulus-driven attentional control settings to determine which stimuli effectively compete for attention.     

Stimulus-determined discrimination mechanisms for color search.

Visual attention can be goal driven, stimulus driven, or a combination of the two. Here we report evidence for an unexpectedly stimulus-driven component of visual search for a target defined by color. Observers demonstrated a surprisingly cost-free ability to incorporate multiple classifiers in search for a target of one color from among distractors of other colors. A target color was presented among distractors that could change from trial to trial (intermixed presentation) or that remained constant across all trials in a block (blocked presentation). For blocked presentation, a single search classifier (a mechanism that segregates the target from distractors in color space) could be adopted, whereas for intermixed presentation different classifiers had to be used when the distractor colors changed. The benefit of blocked presentation was very small, suggesting that the appropriate classifier was determined very quickly in trials for which the classifier changed. The results suggest that the stimulus-driven activation of an appropriate stimulus classifier can be very efficient.     

Stimulus-determined discrimination mechanisms for color search

Visual attention can be goal driven, stimulus driven, or a combination of the two. Here we report evidence for an unexpectedly stimulus-driven component of visual search for a target defined by color. Observers demonstrated a surprisingly cost-free ability to incorporate multiple classifiers in search for a target of one color from among distractors of other colors. A target color was presented among distractors that could change from trial to trial (intermixed presentation) or that remained constant across all trials in a block (blocked presentation). For blocked presentation, a single search classifier (a mechanism that segregates the target from distractors in color space) could be adopted, whereas for intermixed presentation different classifiers had to be used when the distractor colors changed. The benefit of blocked presentation was very small, suggesting that the appropriate classifier was determined very quickly in trials for which the classifier changed. The results suggest that the stimulus-driven activation of an appropriate stimulus classifier can be very efficient.     

Salience-controlled visual search: Are the brightest and the least bright targets found by different processes?

It has been proposed that maximum and minimum targets (like the brightest or the least-bright item of a sample) are found by different search processes, the latter but not the former being sensitive to the withdrawal of focal attention. The present study could not establish such a systematic difference and suggests that variations in search performance are due to variations in target salience. Bright targets among dim distractors were generally more salient than dim targets among bright dis-tractors (Experiment 1). In both conditions, search performance was deteriorated when attention was withdrawn (Experiment 2). Performance became identical when maximum and minimum targets were matched in salience; this was also confirmed for dark items on bright background (Experiment 3). The data underline the importance of salience in visual search but do not support the assumption that maximum and minimum target searches are qualitatively different in their demands on focal attention. Salience and attention rather seem to resemble complementary functions in visual search; the less salient a target is the more attention is required to detect it.     

Interactions between top-down and bottom-up attention in barn owls (<Emphasis Type="Italic">Tyto alba</Emphasis>)

Selective attention, the prioritization of behaviorally relevant stimuli for behavioral control, is commonly divided into two processes: bottom-up, stimulus-driven selection and top-down, task-driven selection. Here, we tested two barn owls in a visual search task that examines attentional capture of the top-down task by bottom-up mechanisms. We trained barn owls to search for a vertical Gabor patch embedded in a circular array of differently oriented Gabor distractors (top-down guided search). To track the point of gaze, a lightweight wireless video camera was mounted on the owl’s head. Three experiments were conducted in which the owls were tested in the following conditions: (1) five distractors; (2) nine distractors; (3) five distractors with one distractor surrounded by a red circle; or (4) five distractors with a brief sound at the initiation of the stimulus. Search times and number of head saccades to reach the target were measured and compared between the different conditions. It was found that search time and number of saccades to the target increased when the number of distractors was larger (condition 2) and when an additional irrelevant salient stimulus, auditory or visual, was added to the scene (conditions 3 and 4). These results demonstrate that in barn owls, bottom-up attention interacts with top-down attention to shape behavior in ways similar to human attentional capture. The findings suggest similar attentional principles in taxa that have been evolutionarily separated for 300 million years.     

Goal-directed and stimulus-driven attention in cross-dimensional texture segregation

Goal-directed and stimulus-driven control of attention was examined in a visual texture segregation task. Recent published reports have debated the existence and efficiency of goal-directed guidance of attention. Some of this research has focused on the apparent stimulus-driven attentional priority given to salient distractors, even when they are known to be irrelevant to the task. In the present study, subjects searched a texture array for targets defined along one dimension. These displays also included distractors created by variation in an irrelevant dimension. Targets were of three different overall shapes. On each trial, distractors could be the same shape as the target or one of the other two shapes. In two experiments subjects were informed of the overall shape of the target prior to stimulus presentation. In these experiments, distractors that did not match the overall shape of the target caused less interference than distractors that matched the target’s shape. In the third experiment, subjects were not informed of the overall shape of the target. In this experiment all distractors caused roughly equal interference. The results of these experiments demonstrate that if subjects are given information about the overall shape of the target, they are able to use this information to reduce interference from distractors that do not match the overall target shape. While acknowledging some stimulus-driven interference, this illustrates a previously unexplored source of goal-directed guidance that can reduce interfering effects of even salient distractors and argues against purely stimulus-driven control of attention.     

跨感觉通道任务中刺激驱动注意捕获的不对称性

通过两个实验探讨了来自不同感觉通道或不同空间方位的奇异刺激在视听搜索范式中的注意捕获。实验一使用定位任务探讨刺激驱动注意捕获中跨通道效应且及其不对称性, 结果发现, 当靶子在听觉通道时, 只有来自听觉同侧空间方位的奇异刺激成功的捕获了注意; 而来自视觉同侧空间和异侧空间的奇异刺激并没有捕获注意。实验二使用检测任务进一步探讨刺激驱动的注意捕获中的跨通道效应且具有不对称性, 结果发现, 来自同侧和异侧的听觉奇异刺激均捕获了注意, 但是视觉奇异刺激却不能捕获注意。研究表明, 视听搜索范式中的奇异刺激的注意捕获具有不对称性, 且存在着一种超感觉通道的注意资源。     

Rewards shape attentional search modes

Visual attention can be configured for specific stimulus features (feature search mode) or it can be non-specifically set for salient pop-outs (singleton detection mode). Additionally, monetary rewards have been shown to bias attention toward specific features, but it is unknown whether secondary reinforcers (images of US$) can shape global attention via search modes. In a between-group study, we trained participants to value one search mode over the other. In a testing phase, a salient distractor captured the attention of the value-singleton group; however, the value feature group was completely unaffected. This suggests that rewards automatically bias global attention mechanisms and potentially mediate the handoff between stimulus-driven and goal-directed attentional control.     

Limitations in attending to a feature value for overriding stimulus-driven interference

Six experiments were conducted to examine the effect of knowledge of a target for overriding stimulus-driven interference in simple search tasks (Experiments 1-3) and compound search tasks (Experiments 4-6). In simple search when the target differed from nontargets in orientation, a singleton distractor that had an orientation equivalent to that of a target interfered with search for the target. When the singleton distractor was less salient than the target with respect to the target-defining feature, it still caused interference. Such within-dimensional, nonsaliency-based interference also occurred in compound search tasks. In contrast, no interference occurred when a singleton distractor was defined in cross-dimension in a simple search task. When a compound search task was used, the salient distractor interfered with the search for a less salient target. These results are discussed in terms of their applicability to existing models and the limitations of top-down penetrability of a feature processing stage.     

Effects of search mode and intertrial priming on singleton search

There is no consensus as to what information guides search for a singleton target. Does the most salient display element capture attention, regardless of the observer's attentional set? Do observers adopt a default salience-based search mode? Does knowledge of the target's defining featural property (when available) affect search? Finally, can intertrial contingencies account for the disparate results in the literature? We investigated search for a shape singleton when (1) the target and nontarget shapes switched unpredictably from trial to trial, (2) the target feature remained fixed, and (3) the target was a singleton on only one third of the trials. We examined overall reaction times, search slopes, errors, and the magnitude of the slowing caused by a cross-dimensional singleton distractor. Our results argue against the idea that search is guided solely by stimulus-driven factors or that subjects adopt a singleton detection mode that is blind to feature information. They show also that intertrial contingencies, although potent, cannot account for the variety of results in the literature.     

Overriding stimulus-driven attentional capture

Theeuwes (1992) found a distracting effect of irrelevant-dimension singletons in a task involving search for a known target. He argued from this that selectivity is determined solely by stimulus salience; the parallel stage of visual processing cannot provide top-down guidance to the attentive stage sufficient to permit completely selective use of task-relevant information. We argue that in the task used by Theeuwes, subjects may have adopted the strategy of searching for an odd form even though the specific target form was known. In Experiment 1, we replicated Theeuwes’s findings. Search for a circle target among diamond nontargets was disrupted by the presence of a diamond nontarget that was uniquely colored. In two subsequent experiments, we discouraged the singleton detection strategy, forcing subjects to search for the target feature. There was no distracting effect of a color singleton in these experiments, even with displays physically identical to those of Experiment 1, demonstrating that top-down selectivity is indeed possible during visual search. We conclude that goal-directed selection of a specific known featural identity may override stimulus-driven capture by salient featural singletons.     

Searching for stimulus-driven shifts of attention

Several types of dynamic cues (e.g., abrupt onsets, motion) draw attention in visual search tasks even when they are irrelevant. Although these stimuli appear to capture attention in a stimulus-driven fashion, typical visual search tasks might induce an intentional strategy to focus on dynamic events. Because observers can only begin their search when the search display suddenly appears, they might orient to any dynamic display change (Folk, Remington, & Johnston, 1992; Gibson & Kelsey, 1998). If so, the appearance of capture might result from task-induced biases rather than from the properties of the stimulus. In fact, such biases can even create the appearance of stimulus-driven capture by stimuli that typically do not capture attention (Gibson & Kelsey, 1998). The possibility of task-induced, topdown biases plagues the interpretation of all previous studies claiming stimulus-driven attention capture by dynamic stimuli. In two experiments, we attempt to eliminate potential task-induced biases by removing any need to monitor for display changes. In the first experiment, search displays did not change on most trials. In the second experiment, although new search displays appeared on each trial, we ensured that observers never saw the changes, by making them during large saccades. In both cases, dynamic events still received search priority, suggesting that some dynamic stimuli capture attention in a stimulus-driven fashion.     

Grouping effects on spatial attention in visual search.

In visual search tasks, spatial attention selects the locations containing a target or a distractor with one of the target's features, implying that spatial attention is driven by target features (M.-S. Kim & K. R. Cave, 1995). The authors measured the effects of location-based grouping processes in visual search. In searches for a color-shape combination (conjunction search), spatial probes indicated that a cluster of same-color or same-shape elements surrounding the target were grouped and selected together. However, in searches for a shape target (feature search), evidence for grouping by an irrelevant feature dimension was weaker or nonexistent. Grouping processes aided search for a visual target by selecting groups of locations that shared a common feature, although there was little or no grouping by an irrelevant feature when the target was defined by a unique salient feature.     

Grouping Effects on Spatial Attention in Visual Search

In visual search tasks, spatial attention selects the locations containing a target or a distractor with one of the target's features, implying that spatial attention is driven by target features (M.S. Kim & K. R. Cave, 1995). The authors measured the effects of location-based grouping processes in visual search. In searches for a color-shape combination (conjunction search), spatial probes indicated that a cluster of same-color or same-shape elements surrounding the target were grouped and selected together. However, in searches for a shape target (feature search), evidence for grouping by an irrelevant feature dimension was weaker or nonexistent. Grouping processes aided search for a visual target by selecting groups of locations that shared a common feature, although there was little or no grouping by an irrelevant feature when the target was defined by a unique salient feature.     

Target-nontarget similarity decreases search efficiency and increases stimulus-driven control in visual search

Some points of criticism against the idea that attentional selection is controlled by bottom-up processing were dispelled by the attentional window account. The attentional window account claims that saliency computations during visual search are only performed for stimuli inside the attentional window. Therefore, a small attentional window may avoid attentional capture by salient distractors because it is likely that the salient distractor is located outside the window. In contrast, a large attentional window increases the chances of attentional capture by a salient distractor. Large and small attentional windows have been associated with efficient (parallel) and inefficient (serial) search, respectively. We compared the effect of a salient color singleton on visual search for a shape singleton during efficient and inefficient search. To vary search efficiency, the nontarget shapes were either similar or dissimilar with respect to the shape singleton. We found that interference from the color singleton was larger with inefficient than efficient search, which contradicts the attentional window account. While inconsistent with the attentional window account, our results are predicted by computational models of visual search. Because of target–nontarget similarity, the target was less salient with inefficient than efficient search. Consequently, the relative saliency of the color distractor was higher with inefficient than with efficient search. Accordingly, stronger attentional capture resulted. Overall, the present results show that bottom-up control by stimulus saliency is stronger when search is difficult, which is inconsistent with the attentional window account.     

Attentional capture by color without any relevant attentional set

The aim of the present study was to investigate mechanisms underlying attentional capture by color. Previous work has shown that a color singleton is able to summon attention only in the presence of a relevant attentional set, whereas when a color singleton is not useful for a task, evidence for purely stimulus-driven attentional capture is controversial. Three visual search experiments (T-L task) were conducted using a method different from that based on set sizes, consisting of monitoring target-singleton distance in a unique display size. In Experiment 1, we demonstrated that attention can be summoned in a real stimulus-driven manner by an irrelevant color singleton. Experiment 2A extended this observation, showing that the color singleton attracted attention even when capture was detrimental. However, Experiment 2B showed that such capture can be strategically prevented. Finally, in Experiment 3, we examined whether such a capture was due to a spatial shift or to a filtering cost, providing evidence supporting the shift hypothesis. Stimulus-driven capture was observed when color was neither the defining nor the reported target attribute (Yantis, 1993) and when subjects naive of visual search tasks were used. The present results give experimental support to many contemporary models of visual attention.     

Attentional capture is contingent on the interaction between task demand and stimulus salience

The aim of this research was to investigate the potential impacts of task demand and stimulus salience on the stimulus-driven attentional capture effect. The participants performed an inefficient visual search task while an irrelevant luminance singleton was present. In Experiment 1, the task demand was manipulated while the stimulus salience of the irrelevant singleton was fixed. With the same salient singleton, the attentional capture effect was observed in the low-difficulty condition but disappeared in the high-difficulty condition. In Experiment 2, the stimulus salience was manipulated while the task demand was fixed. With the same task, the highly salient singleton captured attention, whereas the relatively lowly salient singleton could not. In Experiment 3, both task demand and stimulus salience were manipulated simultaneously. The stimulus-driven attentional capture effect by the irrelevant singleton increased not only as the task demand decreased but also as the stimulus salience increased. The present study might provide a way to reconcile conflicting findings in the attentional capture literature; the underlying neural mechanism is discussed. 2009 The Psychonomic Society, Inc.     

Cross-dimensional interference and cross-trial inhibition

In two experiments, we examined whether one source of cross-dimensional interference in visual search involves cross-trial position priming. In Experiment 1, we demonstrated cross-dimensional interference in search for an orientation-defined target: Search for a left-tilted target among right-tilted non-targets was disrupted by the presence of a singleton color distractor. In all conditions, search was facilitated when a target was presented at the same position as a target in the previous trial (positive position priming). In addition, there were negative effects of position priming on orientation targets that fell on the same side as singleton distractors on the previous trial. In Experiment 2, to examine the impact of negative position priming on cross-dimensional interference, trials with and without singleton distractors were presented in a single trial block. The chance of a singleton distractor's being present on a preceding trial was then equated across displays when the distractor was and when it was not subsequently present. Cross-dimensional interference was eliminated under this mixed presentation condition, suggesting that the cost of cross-dimensional interference was not determined by the stimulus-driven factors in the current trial, at least when a limited number of target and distractor locations was used. We conclude that top-down selection is possible during visual search, and this leads to inhibition of the location of salient distractors. The cost of this is slowed detection of targets at inhibited locations on subsequent trials.