Can salient stimuli really be suppressed?

Evidence that proactive distractor suppression does not require attentional resources

Does the suppression of irrelevant visual features require attentional resources? McDonald et al. (2023, Psychonomic Bulletin & Review, 30, 224-234) proposed that suppression processes are unavailable while a person is busy performing another task. They reported the absence of the PD (believed to index suppression) when two tasks were presented close together in time. We looked for converging evidence using established behavior measures of suppression. Following McDonald et al., our participants performed a rapid serial visual presentation (RSVP) task followed by a search task. For the RSVP task, participants determined whether the target digit 4 or 6 appeared within a string of other digits. The search display appeared at a lag of 2 or 8 digits after the RSVP target. Participants searched for a yellow target circle amongst nine background circles, which included a uniquely colored distractor for some trials. The main question was whether distractor suppression would occur at Lag 2, while attentional resources were still processing the RSVP target. Suppression was assessed using the capture-probe paradigm. On 30% of trials, probe letters appeared inside the colored circles and participants reported those letters. Probe recall accuracy was lower at locations with distractor colors than those with neutral colors (the baseline), suggesting proactive suppression. Critically, this difference in probe recall accuracy was similar at Lag 2 and Lag 8, suggesting that the ability to proactively suppress distractors remains intact while dual-tasking. We argue that although reactive suppression likely requires attentional resources, proactive suppression-an implicit process-does not.

Relative saliency affects attentional capture and suppression of color and face singleton distractors: evidence from event-related potential studies

Prior research has yet to fully elucidate the impact of varying relative saliency between target and distractor on attentional capture and suppression, along with their underlying neural mechanisms, especially when social (e.g. face) and perceptual (e.g. color) information interchangeably serve as singleton targets or distractors, competing for attention in a search array. Here, we employed an additional singleton paradigm to investigate the effects of relative saliency on attentional capture (as assessed by N2pc) and suppression (as assessed by PD) of color or face singleton distractors in a visual search task by recording event-related potentials. We found that face singleton distractors with higher relative saliency induced stronger attentional processing. Furthermore, enhancing the physical salience of colors using a bold color ring could enhance attentional processing toward color singleton distractors. Reducing the physical salience of facial stimuli by blurring weakened attentional processing toward face singleton distractors; however, blurring enhanced attentional processing toward color singleton distractors because of the change in relative saliency. In conclusion, the attentional processes of singleton distractors are affected by their relative saliency to singleton targets, with higher relative saliency of singleton distractors resulting in stronger attentional capture and suppression; faces, however, exhibit some specificity in attentional capture and suppression due to high social saliency.

On preventing capture: Does greater salience cause greater suppression?

It has been proposed that salient objects have high potential to disrupt target performance, and so people learn to proactively suppress them, thereby preventing these salient distractors from capturing attention in the future. Consistent with this hypothesis, Gaspar et al. (Proceedings of the National Academy of Sciences, 113(13), 3693-3698, 2016) reported that the PD (believed to index suppression) was larger for high-salient color distractors than for low-salient color distractors. The present study looked for converging evidence that salience triggers suppression using established behavior measures of suppression. Following Gaspar et al., our participants searched for a yellow target circle among nine background circles, which sometimes included one circle with a unique color. The distractor was either high or low in salience with respect to the background circles. The question was whether the high-salient color would be proactively suppressed more strongly than the low-salient color. This was assessed using the capture-probe paradigm. On 33% of trials, probe letters appeared inside colored circles and participants were to report those letters. If high-salient colors are more strongly suppressed, then probe recall accuracy should be lower at locations with the high-salient color than those with the low-salient color. Experiment 1 found no such effect. A similar finding was observed in Experiment 2 after addressing possible floor effects. These findings suggest that proactive suppression is not caused by salience. We propose that the PD reflects not only proactive suppression but also reactive suppression.

The Distractor Positivity Component and the Inhibition of Distracting Stimuli

There has been a long-lasting debate about whether salient stimuli, such as uniquely colored objects, have the ability to automatically distract us. To resolve this debate, it has been suggested that salient stimuli do attract attention but that they can be suppressed to prevent distraction. Some research supporting this viewpoint has focused on a newly discovered ERP component called the distractor positivity (PD), which is thought to measure an inhibitory attentional process. This collaborative review summarizes previous research relying on this component with a specific emphasis on how the PD has been used to understand the ability to ignore distracting stimuli. In particular, we outline how the PD component has been used to gain theoretical insights about how search strategy and learning can influence distraction. We also review alternative accounts of the cognitive processes indexed by the PD component. Ultimately, we conclude that the PD component is a useful tool for understanding inhibitory processes related to distraction and may prove to be useful in other areas of study related to cognitive control.

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.

Differential effects of intra-modal and cross-modal reward value on perception: ERP evidence

In natural environments objects comprise multiple features from the same or different sensory modalities but it is not known how perception of an object is affected by the value associations of its constituent parts. The present study compares intra- and cross-modal value-driven effects on behavioral and electrophysiological correlates of perception. Human participants first learned the reward associations of visual and auditory cues. Subsequently, they performed a visual discrimination task in the presence of previously rewarded, task-irrelevant visual or auditory cues (intra- and cross-modal cues, respectively). During the conditioning phase, when reward associations were learned and reward cues were the target of the task, high value stimuli of both modalities enhanced the electrophysiological correlates of sensory processing in posterior electrodes. During the post-conditioning phase, when reward delivery was halted and previously rewarded stimuli were task-irrelevant, cross-modal value significantly enhanced the behavioral measures of visual sensitivity, whereas intra-modal value produced only an insignificant decrement. Analysis of the simultaneously recorded event-related potentials (ERPs) of posterior electrodes revealed similar findings. We found an early (90-120 ms) suppression of ERPs evoked by high-value, intra-modal stimuli. Cross-modal stimuli led to a later value-driven modulation, with an enhancement of response positivity for high- compared to low-value stimuli starting at the N1 window (180-250 ms) and extending to the P3 (300-600 ms) responses. These results indicate that sensory processing of a compound stimulus comprising a visual target and task-irrelevant visual or auditory cues is modulated by the reward value of both sensory modalities, but such modulations rely on distinct underlying mechanisms.

Does attentional suppression occur at the level of perception or decision-making? Evidence from Gaspelin et al.'s (2015) probe letter task

Visual attention is often inadvertently captured by salient stimuli. It was suggested that it is possible to prevent attentional capture in some search tasks by suppressing salient stimuli below baseline. Evidence for attentional suppression comes from a probe task that was interleaved with the main search task. In the probe task of Gaspelin et al. (Psychol Sci 26(11):1740-1750, 2015. https://doi.org/10.1177/0956797615597913 ), letters were shown on the stimuli of the search display and participants had to identify as many letters as possible. Performance was found to be worse for letters shown on the distractor compared to non-salient non-target stimuli, suggesting that distractor processing was suppressed below baseline. However, it is unclear whether suppression occurred at the level of perception or decision-making because participants may have reported letters on the distractor less frequently than letters on nontargets. This decision-level bias may have degraded performance for letters on distractor compared to nontarget stimuli without changing perception. After replicating the original findings, we conducted two experiments where we avoided report bias by cueing only a single letter for report. We found that the difference between distractor and nontarget stimuli was strongly reduced, suggesting that decision-level processes contribute to attentional suppression. In contrast, the difference between target and non-target stimuli was unchanged, suggesting that it reflected perceptual-level enhancement of the target stimuli.

Feature-blind attentional suppression of salient distractors

A recent paper has reported, for the first time, that people are capable of suppressing salient singleton distractors of unknown color if the search task requires them to search for the most prevalent of several shapes in the display. We identify here several potential limitations of the earlier findings. In particular, in the reported experiments, the likelihood of a salient distractor was higher than what is typically studied, the distractor object was similar in shape to the relevant objects, only two colors were studied, the distractor was consistently a fixed shape, and the distractor was always a unique shape different from the search targets. Each of these limitations leaves open some questions about the generality of the findings. We address each of the concerns here, and show, in five experiments, that the ability to suppress distractors of unknown color is a robust finding that is not compromised by the potential limitations identified. When searching for the most prevalent of several shapes in a display, people can indeed suppress capture by otherwise-salient color singleton distractors even when their color is not known in advance (i.e., in a feature-blind manner), facilitating efficient search. The experiments confirm the ability to suppress visual elements based on second-order (e.g., a unique color) or global salience information, and not merely based on first-order (e.g., a specific color) information.

Can enhancement and suppression concurrently guide attention? An assessment at the individual level

Background: Although people can pay attention to targets while ignoring distractors, previous research suggests that target enhancement and distractor suppression work separately and independently. Here, we sought to replicate previous findings and re-establish their independence. Methods: We employed an internet-based psychological experiment. We presented participants with a visual search task in which they searched for a specified shape with or without a singleton. We replicated the singleton-presence benefit in search performance, but this effect was limited to cases where the target color was fixed across all trials. In a randomly intermixed probe task (30% of all trials), the participants searched for a letter among colored probes; we used this task to assess how far attention was separately allocated toward the target or distractor dimensions. Results: We found a negative correlation between target enhancement and distractor suppression, indicating that the participants who paid closer attention to target features ignored distractor features less effectively and vice versa. Averaged data showed no benefit from target color or cost from distractor color, possibly because of the substantial differences in strategy across participants. Conclusions: These results suggest that target enhancement and distractor suppression guide attention in mutually dependent ways and that the relative contribution of these components depends on the participants’ search strategy.

Eye movements reveal the contributions of early and late processes of enhancement and suppression to the guidance of visual search

In visual search attention can be directed towards items matching top-down goals, but this must compete with factors such as salience that can capture attention. However, under some circumstances it appears that attention can avoid known distractor features. Chang and Egeth (Psychological Science, 30 (12), 1724–1732, 2019) found that such inhibitory effects reflect a combination of distractor-feature suppression and target-feature enhancement. In the present study (N = 48), we extend these findings by revealing that suppression and enhancement effects guide overt attention. On search trials (75% of trials) participants searched for a diamond shape among several other shapes. On half of the search trials all objects were the same colour (e.g., green) and on the other half of the search trials one of the non-target shapes appeared in a different colour (e.g., red). On interleaved probe trials (25% of trials), subjects were presented with four ovals. One of the ovals was in either the colour of the target or the colour of the distractor from the search trials. The other three ovals were on neutral colours. Critically, we found that attention was overtly captured by target colours and avoided distractor colours when they were viewed in a background of neutral colours. In addition, we provided a time course of attentional control. Within visual search tasks we observed inhibition aiding early attentional effects, indexed by the time it took gaze to first reach the target, as well as later decision-making processes indexed by the time for a decision to be made once the target as found.

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.

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.

On preventing attention capture: Is singleton suppression actually singleton suppression?

It is commonly assumed that salient singletons generate an "attend-to-me signal" which causes suppression to develop over time, eventually preventing capture. Despite this assumption and the name "singleton suppression," a causal link between salience and suppression has not yet been clearly established. We point out the plausibility of a simple alternative mechanism: distractors might be suppressed because they are distractors rather than targets, even when non-salient. To look for evidence of salience-based suppression, we had participants search for a target shape among distractors, which sometimes included irrelevant-colored distractors. The critical manipulation was whether the irrelevant-colored distractor was salient (a color singleton) or non-salient (three non-target colored shapes; a triplet). On 30% of trials, probe letters were presented briefly inside each shape and participants were to report those letters. Probe recall below baseline indicates suppression. Experiment 1 showed that suppression was not triggered any more strongly by salient distractors (singletons) than by non-salient distractors (triplets). Experiment 2 showed that strong suppression effects developed rapidly even in the absence of salient singletons. These findings raise the thus far neglected question of whether salience plays any role in suppression.

Across-trial spatial suppression in visual search

In order to focus on objects of interest, humans must be able to avoid distraction by salient stimuli that are not relevant to the task at hand. Many recent studies have shown that through statistical learning we are able to suppress the location that is most likely to contain a salient distractor. Here we demonstrate a remarkable flexibility in attentional suppression. Participants had to search for a shape singleton while a color distractor singleton was present. Unbeknown to the participant, the color distractor was presented according to a consistent pattern across trials. Our findings show that participants learn this distractor sequence as they proactively suppressed the anticipated location of the distractor on the next trial. Critically, none of the participants were aware of these hidden sequences. We conclude that the spatial priority map is highly flexible, operating at a subconscious level preparing the attentional system for what will happen next.

A finer-grained search reveals no evidence of the attentional capture by to-be-ignored features

The contingent capture account of involuntary attention claims that it is guided by top-down factors, such as volitional goals or task instructions. The contrasting rapid disengagement account holds that the contingent capture account relies on the spatial precueing paradigm, which is vulnerable to the elimination of the cue-validity effect through rapid attentional disengagement. In the present study, five experiments were conducted to examine whether a spatial cue presented in a target-defining or distractor-defining color that predicted the location of a subsequently presented target at the chance level involuntarily captures attention by measuring the cue-validity effect. Additionally, to examine the influence of cue-target compatibility as an alternative indicator of attentional capture, an object identical to or different from the target object was presented at the cued location in the cue display in all experiments. The results showed that the cue-validity effect and the cue-target compatibility effect were present only when the target-color cue was presented. The object of the target display presented at the location cued by the target color was recognized even on invalid trials. By contrast, the distractor color cue did not show any indication of attentional capture or postattentive inhibition. These results imply that preattentive selection and postattentive inhibition depend on top-down attentional control setting. Furthermore, the absence of a cue-validity effect with a distractor feature is not due to the inhibition of the cued location after attentional disengagement.