Standing out in a small crowd: The role of display size in attracting attention

Search mode, not the attentional window, determines the magnitude of attentional capture

A salient color distractor is known to capture attention during search for a less salient shape target, but the mechanisms underlying attentional capture are debated. Theeuwes (2004, Psychonomic Bulletin & Review, 11(1), 65-70) argued that attentional capture depends on the size of the attentional window. If the attentional window is large, search is efficient and attentional capture should be stronger because the distractor is more likely to be inside the window. Consistently, we found higher search efficiency and more attentional capture in singleton than in feature search. However, differences in attentional capture only occurred when singleton and feature search were performed by different groups of participants, but not when singleton and feature search occurred unpredictably in the same group of participants. This result contradicts the attentional window account because search efficiency was always higher in singleton than in feature search. Rather, the results support search mode theory, which claims that participants looked for the most salient stimulus in singleton search ("singleton detection mode"), which resulted in more capture by the salient color distractor. When search types varied unpredictably, it was impossible to apply a consistent search strategy, which eliminated the differences between singleton and feature search. Further, we manipulated search efficiency by target-nontarget similarity. With dissimilar nontargets, the target was salient and search efficiency was high. Therefore, the attentional window account predicts more capture. However, we found the opposite result in singleton search and no difference in feature search. Taken together, these observations are inconsistent with the attentional window account but support search mode theory.

Terms of debate: Consensus definitions to guide the scientific discourse on visual distraction

Hypothesis-driven research rests on clearly articulated scientific theories. The building blocks for communicating these theories are scientific terms. Obviously, communication - and thus, scientific progress - is hampered if the meaning of these terms varies idiosyncratically across (sub)fields and even across individual researchers within the same subfield. We have formed an international group of experts representing various theoretical stances with the goal to homogenize the use of the terms that are most relevant to fundamental research on visual distraction in visual search. Our discussions revealed striking heterogeneity and we had to invest much time and effort to increase our mutual understanding of each other's use of central terms, which turned out to be strongly related to our respective theoretical positions. We present the outcomes of these discussions in a glossary and provide some context in several essays. Specifically, we explicate how central terms are used in the distraction literature and consensually sharpen their definitions in order to enable communication across theoretical standpoints. Where applicable, we also explain how the respective constructs can be measured. We believe that this novel type of adversarial collaboration can serve as a model for other fields of psychological research that strive to build a solid groundwork for theorizing and communicating by establishing a common language. For the field of visual distraction, the present paper should facilitate communication across theoretical standpoints and may serve as an introduction and reference text for newcomers.

Feature-Specific Salience Maps in Human Cortex

Priority map theory is a leading framework for understanding how various aspects of stimulus displays and task demands guide visual attention. Per this theory, the visual system computes a priority map, which is a representation of visual space indexing the relative importance, or priority, of locations in the environment. Priority is computed based on both salience, defined based on image-computable properties; and relevance, defined by an individual's current goals, and is used to direct attention to the highest-priority locations for further processing. Computational theories suggest that priority maps identify salient locations based on individual feature dimensions (e.g., color, motion), which are integrated into an aggregate priority map. While widely accepted, a core assumption of this framework, the existence of independent feature dimension maps in visual cortex, remains untested. Here, we tested the hypothesis that retinotopic regions selective for specific feature dimensions (color or motion) in human cortex act as neural feature dimension maps, indexing salient locations based on their preferred feature. We used fMRI activation patterns to reconstruct spatial maps while male and female human participants viewed stimuli with salient regions defined by relative color or motion direction. Activation in reconstructed spatial maps was localized to the salient stimulus position in the display. Moreover, the strength of the stimulus representation was strongest in the ROI selective for the salience-defining feature. Together, these results suggest that feature-selective extrastriate visual regions highlight salient locations based on local feature contrast within their preferred feature dimensions, supporting their role as neural feature dimension maps.SIGNIFICANCE STATEMENT Identifying salient information is important for navigating the world. For example, it is critical to detect a quickly approaching car when crossing the street. Leading models of computer vision and visual search rely on compartmentalized salience computations based on individual features; however, there has been no direct empirical demonstration identifying neural regions as responsible for performing these dissociable operations. Here, we provide evidence of a critical double dissociation that neural activation patterns from color-selective regions prioritize the location of color-defined salience while minimally representing motion-defined salience, whereas motion-selective regions show the complementary result. These findings reveal that specialized cortical regions act as neural "feature dimension maps" that are used to index salient locations based on specific features to guide attention.

The role of salience in the suppression of distracting stimuli

Researchers have long debated whether salient distractors have the power to automatically capture attention. Recent research has suggested a potential resolution, called the signal suppression hypothesis, whereby salient distractors produce a bottom-up salience signal, but can be suppressed to prevent visual distraction. This account, however, has been criticized on the grounds that previous studies may have used distractors that were only weakly salient. This claim has been difficult to empirically test because there are currently no well-established measures of salience. The current study addresses this by introducing a psychophysical technique to measure salience. First, we generated displays that aimed to manipulate the salience of two color singletons via color contrast. We then verified that this manipulation was successful using a psychophysical technique to determine the minimum exposure duration required to detect each color singleton. The key finding was that high-contrast singletons were detected at briefer exposure thresholds than low-contrast singletons, suggesting that high-contrast singletons were more salient. Next, we evaluated the participants' ability to ignore these singletons in a task in which they were task irrelevant. The results showed that, if anything, high-salience singletons were more strongly suppressed than low-salience singletons. These results generally support the signal suppression hypothesis and refute claims that highly salient singletons cannot be ignored.

The Attentional Capture Debate: When Can We Avoid Salient Distractors and When Not?

There has been a long-standing debate concerning whether we are able to resist attention capture by salient distractors. The so-called "signal suppression hypothesis" of Gaspelin and Luck (2018) claimed to have resolved this debate. According to this view, salient stimuli "naturally attempt to capture attention", yet attention capture may be prevented by a top-down inhibitory mechanism. The current paper describes the conditions in which attention capture by salient distractors can be avoided. Capture by salient items can be avoided when the target is non-salient and therefore difficult to find. Because fine discrimination is needed, a small attentional window is adapted resulting in serial (or partly serial) search. Salient signals outside the focused attentional window do not capture attention anymore not because they are suppressed but because they are ignored. We argue that in studies that have provided evidence for signal suppression, search was likely serial or at least partly serial. When the target is salient, search will be conducted in parallel, and in those cases the salient singleton cannot be ignored nor suppressed but instead will capture attention. We argue that the "signal suppression" account (Gaspelin & Luck, 2018) that seeks to explain resistance to attentional capture has many parallels to classic visual search models such as the "feature integration theory" (Treisman & Gelade, 1980), "feature inhibition" account (Treisman & Sato, 1990), and "guided search" (Wolfe et al, 1989); all models that explain how the serial deployment of attention is guided by the output of earlier parallel processes.

Irrelevant sights and sounds require spatial suppression: ERP evidence

Both real-world experience and behavioral laboratory research suggest that entirely irrelevant stimuli (distractors) can interfere with a primary task. However, it is as yet unknown whether such interference reflects competition for spatial attention - indeed, prominent theories of attention predict that this should not be the case. Whilst electrophysiological indices of spatial capture and spatial suppression have been well-investigated, experiments have primarily utilized distractors which share a degree of task-relevance with targets, and are limited to the visual domain. The present research measured behavioral and ERP responses to test the ability of salient yet entirely task-irrelevant visual and auditory distractors to compete for spatial attention during a visual task, while also testing for potentially enhanced competition from multisensory distractors. Participants completed a central letter search task, while ignoring lateralized visual (e.g., image of a dog), auditory (e.g., barking), or multisensory (e.g., image + barking) distractors. Results showed that visual and multisensory distractors elicited a P_D component indicative of active lateralized suppression. We also establish for the first time an auditory analog of the P_D component, the P_AD , elicited by auditory and multisensory distractors. Interestingly, there was no evidence to suggest enhanced ability of multisensory distractors to compete for attentional selection, despite previous proposals of a "special" saliency status for such items. Our findings hence suggest that irrelevant multisensory and unisensory distractors are similarly capable of eliciting a spatial "attend-to-me" signal - a precursor of spatial attentional capture - but at least in the present data set did not elicit full spatial attentional capture.

A new technique for estimating the probability of attentional capture

Latency-based metrics of attentional capture are limited: They indicate whether or not capture occurred, but they do not indicate how often capture occurred. The present study introduces a new technique for estimating the probability of capture. In a spatial cueing paradigm, participants searched for a target letter defined by color while attempting to ignore salient cues that were drawn in either a relevant or irrelevant color. The results demonstrated the typical contingent capture effect: larger cue validity effects from relevant cues than irrelevant cues. Importantly, using a novel analytical approach, we were able to estimate the probability that the salient cue captured attention. This approach revealed a surprisingly low probability of attentional capture in the spatial cuing paradigm. Relevant cues are thought to be one of the strongest attractors of attention, yet they were estimated to capture attention on only about 30% of trials. This new metric provides an index of capture strength that can be meaningfully compared across different experimental contexts, which was not possible until now.

Attentional capture is modulated by stimulus saliency in visual search as evidenced by event-related potentials and alpha oscillations

This study used a typical four-item search display to investigate top-down control over attentional capture in an additional singleton paradigm. By manipulating target and distractor color and shape, stimulus saliency relative to the remaining items was systematically varied. One group of participants discriminated the side of a dot within a salient orange target (ST group) presented with green circles (fillers) and a green diamond distractor. A second group discriminated the side of the dot within a green diamond target presented with green circle fillers and a salient orange square distractor (SD group). Results showed faster reaction times and a shorter latency of the N2pc component in the event-related potential (ERP) to the more salient targets in the ST group. Both salient and less salient distractors elicited Pd components of equal amplitude. Behaviorally, no task interference was observed with the less salient distractor, indicating the prevention of attentional capture. However, reaction times were slower in the presence of the salient distractor, which conflicts with the hypothesis that the Pd reflects proactive distractor suppression. Contrary to recent proposals that elicitation of the Pd requires competitive interactions with a target, we found a greater Pd amplitude when the distractor was presented alone. Alpha-band amplitudes decreased during target processing (event-related desynchronization), but no significant amplitude enhancement was observed at electrodes contralateral to distractors regardless of their saliency. The results demonstrate independent neural mechanisms for target and distractor processing and support the view that top-down guidance of attention can be offset (counteracted) by relative stimulus saliency.

Learned distractor rejection persists across target search in a different dimension

Attention is guided by several factors, including task-relevant target features, which attract attention, but also statistical regularities associated distractors, which repel attention away from themselves. However, whether feature-based distractor regularities (e.g., color) are extracted automatically from a feature dimension orthogonal to the target-guiding dimension (e.g., shape) remains to be tested. In two experiments, we tested if learned distractor rejection by color operated when color was not part of the attentional control settings, specifically, while attention was guided by a shape-based target template. Participants performed a visual search task for a task-relevant shape in displays containing two unsegregated colors. These displays allowed us to manipulate target guidance (based on shape) independently from distractor-based regularities (based on color). In both experiments we found clear evidence for learned distractor rejection: faster mean response times to locate the target when a consistent distractor color was present than when it was absent. Critically, these task-irrelevant learned distractor rejection effects were robust despite strong target guidance by an orthogonal search dimension. These findings corroborate recent demonstrations of learned distractor rejection during strong target guidance, indicating that learned distractor rejection and target guidance can operate on separate feature dimensions.

Electrophysiological Evidence for the Suppression of Highly Salient Distractors

There has been a longstanding debate as to whether salient stimuli have the power to involuntarily capture attention. As a potential resolution to this debate, the signal suppression hypothesis proposes that salient items generate a bottom-up signal that automatically attracts attention, but that salient items can be suppressed by top-down mechanisms to prevent attentional capture. Despite much support, the signal suppression hypothesis has been challenged on the grounds that many prior studies may have used color singletons with relatively low salience that are too weak to capture attention. The current study addressed this by using previous methods to study suppression but increased the set size to improve the relative salience of the color singletons. To assess whether salient distractors captured attention, electrophysiological markers of attentional allocation (the N2pc component) and suppression (the PD component) were measured. The results provided no evidence of attentional capture, but instead indicated suppression of the highly salient singleton distractors, as indexed by the PD component. This suppression occurred even though a computational model of saliency confirmed that the color singleton was highly salient. Altogether, this supports the signal suppression hypothesis and is inconsistent with stimulus-driven models of attentional capture.