Classic Visual Search Effects in an Additional Singleton Task: An Open Dataset

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.

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.

Oculomotor suppression of abrupt onsets versus color singletons

There is considerable evidence that salient items can be suppressed in order to prevent attentional capture. However, this evidence has relied almost exclusively on paradigms using color singletons as salient distractors. It is therefore unclear whether other kinds of salient stimuli, such as abrupt onsets, can also be suppressed. Using an additional singleton paradigm optimized for detecting oculomotor suppression, we directly compared color singletons with abrupt onsets. Participants searched for a target shape (e.g., green diamond) and attempted to ignore salient distractors that were either abrupt onsets or color singletons. First eye movements were used to assess whether salient distractors captured attention or were instead suppressed. Initial experiments using a type of abrupt onset from classic attentional capture studies (four white dots) revealed that abrupt onsets strongly captured attention whereas color singletons were suppressed. After controlling for important differences between the onsets and color singletons - such as luminance and color - abrupt-onset capture was reduced but not eliminated. We ultimately conclude that abrupt onsets are not suppressed like color singletons.

Covert attention is attracted to prior target locations: Evidence from the probe paradigm

There is growing evidence that visual attention can be guided by selection history. One example of this is intertrial location priming, whereby attention is attracted to the target location from the previous trial. Most previous demonstrations of location priming have relied on manual response time effects whereby search is speeded when the target location repeats from the previous trial. However, these latency-based effects have recently been challenged as being due to response facilitation that occurs after the target has been found, rather than an attentional bias toward the previous target location. To resolve this, the current study used a probe paradigm to assess whether covert attention is biased to the previous-trial target location. On search trials, participants searched for a specific target shape amongst distractor shapes and made a speeded response to the location of a dot inside the target. On probe trials, letters briefly appeared at each search location and after a delay, participants were asked to report as many letters as possible. Probe report accuracy was used to assess the likelihood that a given location was attended. Three experiments indicated that probe report accuracy was greatly improved for letters at the previous-trial target location compared with baseline levels. Importantly, this occurred even when strong attentional guidance to the target was encouraged and even when a nontarget stimulus appeared at the primed location. Altogether, the results suggest that covert attention is strongly attracted to the previous target location during visual search.

Testing the underlying processes leading to learned distractor rejection: Learned oculomotor avoidance

Target templates stored in visual memory guide visual attention toward behaviorally relevant target objects. Visual attention also is guided away from nontarget distractors by longer-term learning, a phenomenon termed "learned distractor rejection." Template guidance and learned distractor rejection can occur simultaneously to further increase search efficiency. However, the underlying processes guiding learned distractor rejection are unknown. In two visual search experiments employing eye-tracking, we tested between two plausible processes: proactive versus reactive attentional control. Participants searched through two-color, spatially unsegregated displays. Participants could guide attention by both target templates and consistent nontarget distractors. We observed fewer distractor fixations (including the first eye movement) and shorter distractor dwell times. The data supported a single mechanism of learned distractor rejection, whereby observers adopted a learned, proactive attentional control setting to avoid distraction whenever possible. Further, when distraction occurred, observers rapidly recovered. We term this proactive mechanism "learned oculomotor avoidance." The current study informs theories of visual attention by demonstrating the underlying processes leading to learned distractor suppression during strong target guidance.

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.