Statistical regularities modulate attentional capture independent of search strategy

An earlier study using the additional singleton task showed that statistical regularities regarding the distractor location can cause an attentional bias that affects the amount of attentional capture by distractors and the efficiency of selection of targets. The distractor singleton was systematically present more often in one location than in all other locations. The present study investigated whether this bias also occurs when observers adopt a feature search mode, i.e., when they search for a specific feature (circle) between elements with different shapes, while ignoring a colored distractor singleton. It is assumed that in feature search, observers can ignore distractors in a top-down way and as such one expects that statistical regularities about the distractor location should not play a role. Contrary to this prediction, we found that even in feature search, both attentional capture by the distractors and the efficiency of selecting the target were impacted by these statistical regularities. Moreover, statistical regularities regarding the feature value of the distractor (its color) had no effect on the amount of capture or the efficiency of selection. We claim that statistical regularities cause passive lingering biases of attention such that on the priority map, the location containing a high probability distractor competes less for attention than locations that are less likely to contain distractors.     

Lesioning a connectionist model of visual search: selective effects on distractor grouping.

A connectionist model of visual search is presented, in which search is determined by patterns of grouping between distractors and between the target and the distractors. Grouping is based on conjunctive relations between simple form elements (corners, line-end terminators), and uses principles of similarity and spatial proximity. In its normal, 'unlesioned' state, the model simulates the search data generated by human subjects when they search for simple form conjunctions amongst either homogeneous or heterogeneous distractors (Humphreys & Muller, in press). In this paper, the performance of the model is examined after the model is subject to various types of 'lesion'. 'Lesioning' is produced either by increasing the internal noise on the activation functions governing the interactions between processing units, or by eliminating processing units from different loci in the model. Increasing the internal noise within the model, or lesioning units within 'high-level' processing stages, can generate the selective effects on search found in a detailed single case study of an agnosic patient (Humphreys, Riddoch, Quinlan, Price & Donnelly, this volume): Namely, there is selective disruption of search for a form conjunction amongst homogeneous distractors relative to search for the same target amongst heterogeneous distractors. This selective effect can be attributed to the 'lesion' disrupting grouping between distractors, which normally facilitates search with homogeneous distractors. The simulations demonstrate that (1) search processes can be disrupted by adding internal noise to search functions, (2) that there can be selective effects on grouping processes, and (3) that these selective effects can be associated with different types of lesion to different stages in the model. The implications of the simulations for understanding visual processing impairments in agnosia are discussed.     

Efficient visual search without top-down or bottom-up guidance

Two types of mechanisms have dominated theoretical accounts of efficient visual search. The first are bottom-up processes related to the characteristics of retinotopic feature maps. The second are top-down mechanisms related to feature selection. To expose the potential involvement of other mechanisms, we introduce a new search paradigm whereby a target is defined only in a context-dependent manner by multiple conjunctions of feature dimensions. Because targets in a multiconjunction task cannot be distinguished from distractors either by bottom-up guidance or top-down guidance, current theories of visual search predict inefficient search. While inefficient search does occur for the multiple conjunctions of orientation with color or luminance, we find efficient search for multiple conjunctions of luminance/size, luminance/shape, and luminance/topology. We also show that repeated presentations of either targets or a set of distractors result in much faster performance and that bottom-up feature extraction and top-down selection cannot account for efficient search on their own. In light of this, we discuss the possible role of perceptual organization in visual search. Furthermore, multiconjunction search could provide a new method for investigating perceptual grouping in visual search.     

Searching from the top down: Ageing and attentional guidance during singleton detection

Previous investigations of adult age differences in visual search suggest that an age-related decline may exist in attentional processes dependent on the observer's knowledge of task-relevant features (top-down processing). The present experiments were conducted to examine age-related changes in top-down attentional guidance during a highly efficient form of search, singleton detection. In Experiment 1 reaction times to detect targets were lower when target features were constant (feature condition) than when target features were allowed to vary between trials (mixed condition), and this reaction time benefit was similar for younger and older adults. Experiments 2 and 3 investigated possible interactions between top-down and bottom-up (stimulus-driven) processes. Experiment 2 demonstrated that search times for both age groups could be improved when targets varied on an additional feature from distractors (double-feature condition) but only when top-down control was available (feature search). In Experiment 3, the availability of top-down guidance enabled both younger and older adults to override the distracting effects of a noninformative spatial location cue. These findings indicate that top-down attentional control mechanisms interact with bottom-up processes to guide search for targets, and that in the context of singleton detection these mechanisms of top-down control are preserved for older adults.     

Nonspatial interdimensional attentional capture

Accuracy in identifying a target is impaired by a task-irrelevant singleton distractor even when the target and distractor appear in the same location. However, whether this impairment, known as a nonspatial interdimensional attentional capture, is contingent on a top-down attentional set or determined by stimulus-driven signals from distractors is unclear. To examine whether interdimensional attentional capture is affected by a top-down attentional set, the present study explicitly manipulated observers’ search strategies (the singleton detection or feature search modes) and the number of objects consisting of the search items. The results indicated that interdimensional attentional capture occurred even under the feature search mode but that the capture effect decreased under this search mode irrespective of the number of distractors, suggesting that top-down knowledge was effective in modulating nonspatial interdimensional capture.     

Attentional selection of a peripheral ring overrules the central attentional bias

In visual search, distractors that fall between fixation and the target are more disruptive than distractors that fall at the same eccentricity as the target (Wolfe, O'Neill, & Bennett, 1998). This sensitivity to the relative eccentricity of target and distractor elements originates from a space-based bias favoring stimuli closer to the fovea. We show that this spatial bias can be overruled by cuing attention to a ring-shaped object. We rule out various space-based explanations of these findings, including (1) attention to fronto-parallel planes in depth and (2) serial attention to different portions of a ring. We suggest that attentional selection of a ring-shaped object operates independently of, and can overrule, spatial biases in selection.     

Color grouping in space and time: evidence from negative color-based carryover effects in preview search

Five experiments addressed the role of color grouping in preview search (D. G. Watson & G. W. Humphreys, 1997). Experiment 1 used opposite color ratios of distractors in preview and second search displays, creating equal numbers of distractors in each color group in the final display. There was selective slowing for new targets carrying the majority color of the old items. This effect held when there was no bias in the preview and only the second search set had an uneven color ratio (Experiment 2). In Experiment 3, participants had foreknowledge of the target color, and effects were shown over and above those due to color biases. Experiment 4 demonstrated negative color carryover even when previews changed color. Experiment 5 showed reduced color carryover effects when previews were presented more briefly. Collectively, the results provide evidence for inhibitory carryover effects in preview search based on feature grouping.     

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.     

Parallel pattern processing and visual agnosia.

A series of visual search experiments are reported examining pattern processing in a visual agnosic patient. We examined search for targets defined by: (I) the combination of their features relative to homogeneous distractors; (2) the combination of their features relative to heterogeneous distractors; and (3) a single feature difference relative to the distractors (their orientation). Normal subjects demonstrate evidence of spatially parallel search when combined-feature targets are detected against homogeneous distractors, and when targets are defined by a salient feature difference. There are non-linear effects of the number of distractors present, and absent responses can be as fast as present. In contrast, search times for combined-feature targets amongst heterogeneous distractors increase linearly with display size, with the slope for absent responses about twice that for present. The contrast between search for combined-feature targets amongst homogeneous and heterogeneous distractors can be attributed to the effects of grouping between distractors and between distractors and targets (Duncan & Humphreys, 1989, 1992; Humphreys & Muller, in press). Grouping between homogeneous distractors facilitates search. An agnosic patient, HJA, showed normal search functions for single-feature targets and for combined-feature targets amongst heterogeneous distractors. However, he was impaired at search for combined-feature targets amongst homogeneous distractors. This suggests that HJA is selectively impaired at grouping conjunctions of form features. The relations between HJA's agnosia and his problem in the parallel grouping of form conjunctions are discussed, as are the implications of the work for understanding normal vision.     

Attentional repulsion effect despite a colour-based control set

A salient distractor can have a twofold effect on concurrent visual processes; it can both reduce the processing efficiency of the relevant target (e.g., increasing response time) and distort the spatial representation of the display (e.g., misperception of a target location). Previous work has shown that knowledge of the key feature of visual targets can eliminate the effect of salient distractors on processing efficiency. For instance, knowing that the target of interest is red (i.e., having an attentional control set for red) can eliminate the cost of green distractors on the speed of response to the target. The present study shows that the second mark of irrelevant salient distractors, i.e., distortions in spatial representation, is resistant to such top-down control. Using the attentional repulsion effect, we examined the influence of salient distractors on target localization. Observers had a colour-based control set and the distractors either matched or mismatched with the control set. In the first two experiments, we found systematic mislocalization of targets away from the peripheral distractors (i.e., an attentional repulsion effect). Critically, the effect was caused by distractors that both matched and mismatched the control set. A third experiment, using the same stimuli, found that processing efficiency was perfectly resistant to distractors that did not match the control set, consistent with previous work. Together, the present findings suggest that although top-down control can eliminate the cost of a salient distractor on processing efficiency, it does so without eliminating the distractor's influence on the spatial representation of the display.     

Repetition effects in visual search

Maljkovic and Nakayama (1994) demonstrated an automatic benefit of repeating the defining feature of the target in search guided by salience. Thus, repetition influences target selection in search guided by bottom-up factors. Four experiments demonstrate this repetition effect in search guided by top-down factors, and so the repetition effect is not merely part of the mechanism for determining what display elements are salient. The effect is replicated in singleton search and in three situations requiring different degrees of top-down guidance: when the feature defining the target is less salient than the feature defining the response, when there is more than one singleton in the defining dimension, and when the target is defined by a conjunction of features. Repetition does not change the priorities of targets, relative to distractors: Display size affects search equally whether the target is repeated or changed. More than one mechanism may underlie the repetition effect in different experiments, but assuming that there is a unitary mechanism, a short-term episodic memory mechanism is proposed.     

A search asymmetry reversed by figure-ground assignment

We report evidence demonstrating that a search asymmetry favoring concave over convex targets can be reversed by altering the figure-ground assignment of edges in shapes. Visual search for a concave target among convex distractors is faster than search for a convex target among concave distractors (a search asymmetry). By using shapes with ambiguous local figure-ground relations, we demonstrated that search can be efficient (with search slopes around 10 ms/item) or inefficient (with search slopes around 30–40 ms/item) with the same stimuli, depending on whether edges are assigned to concave or convex "figures." This assignment process can operate in a top-down manner, according to the task set. The results suggest that attention is allocated to spatial regions following the computation of figure-ground relations in parallel across the elements present. This computation can also be modulated by top-down processes.     

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.     

Working memory and target-related distractor effects on visual search

We examined two forms of top-down effects on visual selection: (1) information held in working memory (WM) and (2) the semantic relations between targets and distractors. We found that items held in WM affected search for a different target. This WM-based interference effect generalized across different exemplars, even though participants could remember the specific exemplar on the trial. This argues against a memory top-up account of performance. In addition, there was interference from distractors that were not held in WM but were semantically related to the target. The effects of WM capture and the effects of capture by a distractor related to the target combined additively. The data suggest that task-irrelevant information in WM and task-relevant templates for targets compete separately for selection. The implications for understanding top-down processes in search are discussed.     

Overriding age differences in attentional capture with top-down processing

Two experiments investigated the influence of top-down information on adult age differences in the ability to search for singleton targets using spatial cues. In Experiment 1, both younger and older adults were equally able to use target-related top-down information (target feature predictability) to avoid attentional capture by uninformative (25% valid) cues. However, during informative (75% valid) cue conditions, older adults demonstrated less efficient use of this cue-related top-down information. The authors extended these findings in Experiment 2 using cues that were either consistent or inconsistent with top-down feature settings. Results from this second experiment showed that although older adults were capable of avoiding attentional capture when provided with top-down information related to target features, capture effects for older adults were notably larger than those of younger adults when only bottom-up information was available. The authors suggest that older adults' ability to use top-down information during search to avoid or attend to cues may be resource-limited.     

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.     

Driving attention with the top down: the relative contribution of target templates to the linear separability effect in the size dimension

Bauer, Jolicoeur, and Cowan (1996a, 1996b, 1998) have shown that visual search for a target among distractors is apparently serial if the target is nonlinearly separable from the distractors in a particular feature space (e.g., color or size). In contrast, if the target is linearly separable from the distractors, search is relatively easy and seemingly spatially parallel. We examined the contribution of top-down knowledge of the target to the linear separability effect on search. Two visual search experiments were conducted using small, medium, or large circles as targets. In the first experiment, participants could use knowledge of the target to guide search, whereas, in the second, the target was unknown on each trial. Search for a medium (nonlinearly separable) target among small or large distractors benefited least from knowledge of the target as compared with search for a small or large target. Thus, the linear separability effect can be determined in part by use of top-down knowledge to facilitate the detection of targets at the ends of a continuum defining the stimuli.     

Synergy of stimulus-driven salience and goal-directed prioritization: Evidence from the spatial blink

In the spatial blink paradigm, participants search for a target of a designated color in a rapidly presented stream of letters at fixation. Target identification is typically impaired if a peripheral distractor appears shortly before the target, inducing a spatial blink, but impairment is observed only when the distractor also shares the sought-for color. Such results reveal an important top-down influence on the capture of attention. In the present experiments, we examined the influence of the bottom-up transients associated with the appearance and disappearance of distractors in the spatial blink paradigm. Onsets and offsets alone are incapable of inducing a spatial blink, but we found that the presence of such transients did enhance the effects of target-color-matched distractors. The results reveal important synergistic interactions between top-down and bottom-up factors involved in attentional capture.