Attentional capture by visual singletons is mediated by top-down task set: new evidence from the N2pc component

To investigate whether attentional capture by salient visual stimuli is mediated by current task sets, we measured the N2pc component as a marker of the spatial locus of visual attention during visual search. In each trial, a singleton stimulus that could either be a target (color task: red circle; shape task: green diamond) or a nontarget (blue circle or green square) was presented among uniform distractors (green circles). As predicted by the view that attentional capture is contingent on task set, the N2pc was strongly affected by task instructions. It was maximal for targets, attenuated but still reliably present for nontarget singletons defined in the target dimension (even when these were accompanied by an irrelevant-dimension singleton), and small or absent for equally salient irrelevant-dimension singletons. Results demonstrate that attentional capture is not a purely bottom-up phenomenon, but is strongly determined by top-down task set.     

Deficits in feature-based control of attention in a patient with a right fronto-temporal lesion

Three experiments examined feature-based top-down control of search in a patient with right frontal-temporal lobe brain damage (YW), in comparison with normal control participants. In Experiment 1, YW showed normal search functions for single-feature targets. However, he showed abnormal search functions relative to normal control participants when a singleton distractor irrelevant to the target-defining feature was presented. YW showed longer RTs when a target was presented in the contralesional visual field and a singleton distractor was in the ipsilesional visual field (Experiment 1), or when both were presented in the contralesional visual field (Experiment 2). These results suggest that YW's difficulty is due to a failure of selectivity for targets based on top-down control of target-defining features. Experiment 3 compared singleton detection tasks and feature detection tasks. YW showed no deficits on the singleton detection tasks, where no knowledge of target-defining features was required. However, on the feature detection tasks, YW showed long RTs when targets were presented in the contralesional visual field where knowledge of target-defining features was required. We concluded that YW's difficulty in visual search with singleton distractors was due to deficits in feature-based control. We discuss the role of fronto-temporal regions in feature-based control of attention.     

Attentional capture by a color singleton is stronger at spatially relevant than irrelevant locations: Evidence from an ERP study

Top-down spatial attention can modulate contingent attentional capture, but the underlying mechanism is still not clear. Using variants of spatial cueing paradigms, our previous event-related potential study showed that peripheral color singleton cues with task-relevant features captured attention (indexed by cue-elicited N2pc) even when the targets appeared at central locations, but the magnitude of attentional capture was smaller than when the targets appeared at same peripheral locations. One reasonable explanation is that the modulation effect is due to spatial relevance of cues. However, several other confounding factors might also explain the modulation effect, such as task difficulty, spread of attentional window, and inside/outside relation between cue and attentional window. In the present study, we rearranged the relative locations between cues and targets to control these factors and to further examine whether inequivalence of attentional capture across attended and unattended locations was a common phenomenon. In two experiments, color singleton cues elicited apparent N2pc components when participants were searching for targets possessing the same color, which replicated typical findings of contingent attentional capture. More importantly, the modulation effect of top-down spatial attention on the cue-elicited N2pc still existed even when the factors of task difficulty (Experiment 1), spread of attentional window (Experiment 1), or inside/outside relation between cue and attentional window (Experiment 2) were controlled. These results consistently demonstrate that attentional capture by a color singleton is stronger at spatially relevant locations than at spatially irrelevant locations, suggesting an important role of spatial relevance on contingent attentional capture.     

Electrophysiological correlates of attentional selection in tactile search tasks: The impact of singleton distractors on target selection

Recent event-related potential (ERP) studies have suggested that the N140cc component reflects target selection mechanisms in tactile search tasks in which the target is presented simultaneously with homogeneous distractors. To investigate if and how the attentional selection of the tactile target is affected by the presence of a singleton distractor, we presented a four-item search array (two stimuli to the middle and index fingers of the left and right hand) which included a singleton distractor in addition to the target and two other homogenous distractors. Participants had to localize the target (top or bottom finger on either hand), while ignoring all distractors. Behavioral results revealed that the target localization was hindered by the presence of the singleton distractor as demonstrated by distractor-presence costs for both speed and accuracy. ERP results confirmed that attention was directed to the singleton distractor when this was the only singleton item in the array as suggested by the presence of a distractor-related N140cc component on distractor only trials. Furthermore, when target and singleton distractor were presented to opposite hands (contralateral distractor trials) the target-related N140cc amplitude was reduced as compared to target only trials, suggesting reduced attentional resources to the target. However, when target and singleton distractor were presented to the same hand (ipsilateral distractor trials), the N140cc amplitude was comparable to that observed on target only trials. These findings suggest that the N140cc reflects the attentional selection of the target side rather than the competition between stimuli presented to the same hand.     

Prior knowledge of category size impacts visual search

Prior research has shown that category search can be similar to one‐item search (as measured by the N2pc ERP marker of attentional selection) for highly familiar, smaller categories (e.g., letters and numbers) because the finite set of items in a category can be grouped into one unit to guide search. Other studies have shown that larger, more broadly defined categories (e.g., healthy food) also can elicit N2pc components during category search, but the amplitude of these components is typically attenuated. Two experiments investigated whether the perceived size of a familiar category impacts category and exemplar search. We presented participants with 16 familiar company logos: 8 from a smaller category (social media companies) and 8 from a larger category (entertainment/recreation manufacturing companies). The ERP results from Experiment 1 revealed that, in a two‐item search array, search was more efficient for the smaller category of logos compared to the larger category. In a four‐item search array (Experiment 2), where two of the four items were placeholders, search was largely similar between the category types, but there was more attentional capture by nontarget members from the same category as the target for smaller rather than larger categories. These results support a growing literature on how prior knowledge of categories affects attentional selection and capture during visual search. We discuss the implications of these findings in relation to assessing cognitive abilities across the lifespan, given that prior knowledge typically increases with age.     

Guidance of visual attention from working memory contents depends on stimulus attributes

Recent work shows that items maintained in working memory could guide the orienting of attention in visual search, demonstrating an interesting interaction between working memory and attention. For such guidance effect, some studies emphasize the importance of search task type: for fixed-target search, task-irrelevant memory items stay inside the attentional focus and guides attention; for varied-target search, these items stay outside attentional focus and do not guide attention. With two experiments, we showed that stimulus attribute of items held in working memory can play an important role in attentional guidance. The first behavioral experiment duplicated a previous fixed-target search paradigm demonstrating robust guidance effect but failed to find such effect when the stimuli was simply changed from colored shapes to complex artificial shapes. The second event-related potential experiment duplicated a previous varied-target search paradigm that used complex shapes but did not observe any guidance effect. A clear guidance effect was found when we changed the stimuli to colored shapes. The results suggest that attentional guidance from working memory depends on stimulus attributes of the items held in working memory. When effective attribute is used, task-irrelevant items that stay outside the attentional focus are still able to guide attention.     

Brain electrical correlates of dimensional weighting: an ERP study

In visual search, there is a reaction time (RT) cost for targets on a given trial if the previous target was defined in a different dimension. According to the "dimension-weighting" account (Müller, Heller, & Ziegler, 1995), limited attentional weight needs to be shifted to the new dimension, resulting in slower RTs. The present study aimed at identifying brain electrical correlates associated with the weight shift. Analyses of ERPs revealed several components to reflect dimension changes whether the task was to detect the target or to identify its defining dimension. N2 amplitudes were more negative whenever the dimension changed. The P3 exhibited latency differences that mirrored RTs in both tasks, but the amplitudes showed no direct relation to stimulus- or response-related processes. Finally, slow-wave amplitudes were enhanced for dimension changes. Taken together, the results provide support for relatively early, perceptual processes underlying dimension change costs.     

Deficits in feature-based control of attention in a patient with a right fronto-temporal lesion

Three experiments examined feature-based top-down control of search in a patient with right frontal-temporal lobe brain damage (YW), in comparison with normal control participants. In Experiment 1, YW showed normal search functions for single-feature targets. However, he showed abnormal search functions relative to normal control participants when a singleton distractor irrelevant to the target-defining feature was presented. YW showed longer RTs when a target was presented in the contralesional visual field and a singleton distractor was in the ipsilesional visual field (Experiment 1), or when both were presented in the contralesional visual field (Experiment 2). These results suggest that YW's difficulty is due to a failure of selectivity for targets based on top-down control of target-defining features. Experiment 3 compared singleton detection tasks and feature detection tasks. YW showed no deficits on the singleton detection tasks, where no knowledge of target-defining features was required. However, on the feature detection tasks, YW showed long RTs when targets were presented in the contralesional visual field where knowledge of target-defining features was required. We concluded that YW's difficulty in visual search with singleton distractors was due to deficits in feature-based control. We discuss the role of fronto-temporal regions in feature-based control of attention.     

The distractor positivity (Pd) signals lowering of attentional priority: Evidence from event‐related potentials and individual differences

We investigated the effects of task demands and individual differences on the allocation of attention. Using the same stimuli, participants indicated the orientation of a line contained in a shape singleton (identification task) or the presence of singletons (detection task). Shape singletons in the identification task elicited a contralateral negativity (N2pc) whereas shape singletons in the detection task elicited a contralateral positivity (Pd). We suggest that the reduction of attentional priority of a salient stimulus, reflected by the Pd, occurred more rapidly with the less demanding detection task. Further, fewer distractible participants showed a larger N2pc to lateral color distractors than highly distractible participants. We suggest that highly distractible participants developed compensatory mechanisms to suppress distracting stimuli.     

A bilateral N2pc (N2pcb) component is elicited by search targets displayed on the vertical midline

The study of visually elicited event-related potentials (ERPs) detected at posterior recording sites during visual search has enormously advanced our knowledge about how and when visuo-spatial attention locks onto one or more laterally presented target objects. The N2pc component to lateral targets has been pivotal to further our understanding of the mechanisms and time course of target selection in visual search. However, the N2pc cannot track visuo-spatial attention deployment to targets displayed along the vertical midline. Here, we introduce a new ERP marker (N2pcb component) that is elicited during the selection of such midline targets. In line with retinal and callosal projections from striate to ventral extrastriate cortex, this component reflects an enhanced negativity elicited by midline targets over both posterior hemispheres. By comparing the attentional selection of lateral and midline targets in a singleton search condition and a feature search condition, we show that the N2pcb is triggered at the same time as the N2pc to lateral targets, and shows the same onset latency difference between singleton and feature search. We conclude that the N2pcb and N2pc components reflect the same attentional target selection processes in visual search.     

The role of top‐down spatial attention in contingent attentional capture

It is well known that attentional capture by an irrelevant salient item is contingent on top‐down feature selection, but whether attentional capture may be modulated by top‐down spatial attention remains unclear. Here, we combined behavioral and ERP measurements to investigate the contribution of top‐down spatial attention to attentional capture under modified spatial cueing paradigms. Each target stimulus was preceded by a peripheral circular cue array containing a spatially uninformative color singleton cue. We varied target sets but kept the cue array unchanged among different experimental conditions. When participants’ task was to search for a colored letter in the target array that shared the same peripheral locations with the cue array, attentional capture by the peripheral color cue was reflected by both a behavioral spatial cueing effect and a cue‐elicited N2pc component. When target arrays were presented more centrally, both the behavioral and N2pc effects were attenuated but still significant. The attenuated cue‐elicited N2pc was found even when participants focused their attention on the fixed central location to identify a colored letter among an RSVP letter stream. By contrast, when participants were asked to identify an outlined or larger target, neither the behavioral spatial cueing effect nor the cue‐elicited N2pc was observed, regardless of whether the target and cue arrays shared same locations or not. These results add to the evidence that attentional capture by salient stimuli is contingent upon feature‐based task sets, and further indicate that top‐down spatial attention is important but may not be necessary for contingent attentional capture.     

Facilitation and interference components in the joint Simon task

Two experiments were conducted to assess whether the joint Simon effect is composed of facilitation and interference and whether facilitation is increased by a joint spatially compatible practice performed before performing the joint Simon task. In both experiments, participants were required to perform a Simon task along another person. Trials could be corresponding, non-corresponding, and neutral. In Experiment 1, participants performed only the Simon task. In Experiment 2, participants first practiced on a joint spatial compatibility task with a compatible mapping and, after a 5-min delay, transferred to a joint Simon task. Results indicated that the joint Simon effect consisted primarily of interference, which was significantly increased by a spatially compatible practice performed jointly. These results allow us to better define in what ways the presence of the other influences performance, in showing that when participants perform a task along with another individual, they display a disadvantage (i.e., slower RTs) when they have to respond to stimuli appearing on the other agent's side.     

Quantitative modeling of visual attention processes in patients with Parkinson's disease: effects of stimulus integrality on selective attention and dimensional integration.

Parkinson's disease (PD) patients and normal controls (NCs) were administered a series of visual attention tasks. The dimensional integration task required integration of information from 2 stimulus dimensions. The selective attention task required selective attention to 1 stimulus dimension while ignoring the other stimulus dimension. Both integral- and separable-dimension stimuli were examined. A series of quantitative models of attentional processing was applied to each participant's data. The results suggest that (a) PD patients were not impaired in integrating information from 2 stimulus dimensions, (b) PD patients were impaired in selective attention, (c) selective attention deficits in PD patients were not due to perceptual interference, and (d) PD patients were affected by manipulations of stimulus integrality and separability in much the same way as were NCs.     

Influence of attentional setting and priming on the inter-trial facilitation effect on pop-out

In visual search situations, it is known that not only bottom-up factors relative to the presented stimulus display but also top-down factors, which include foreknowledge about the target or observers' strategies for the task, have an influence on search performance. The present study examines how top-down attentional control settings affect the visual search process by using the within-dimension facilitation (WDF) and the inter-trial facilitation (ITF) effects. In Experiment 1 we showed the WDF effect using foreknowledge about the target-defining dimension within one block. In Experiment 2 we found the ITF effect using foreknowledge about the target-defining dimension in every trial. These results suggest that search performance is improved both top-down attentional setting using foreknowledge and inter-trial priming by repetition of target-defining dimensions, and they are interactive. Furthermore, the influence of the priming effect strongly appeared when observers had an attentional setting every trial. These results show that the magnitude of bottom-up inter-trial priming effects depends on the top-down attentional setting.     

The dynamics of cardiac defense: From attention to action

The attentional and motivational significance of cardiac defense is examined in two studies. In Study 1, cardiac defense was evoked by an intense acoustic stimulus in the context of either a visual search or a memory search task using letters as stimuli. Results showed a potentiation of the long latency acceleration of cardiac defense in the visual search task. In Study 2, participants performed the same visual search task using pleasant, neutral, and unpleasant pictures as stimuli. Results showed a further potentiation of the long latency acceleration of cardiac defense when the visual search task was performed with unpleasant, compared to pleasant or neutral pictures. These results indicate that cardiac defense has both attentional and motivational contributions, where the attentional significance is related to increased sensory processing, whereas the motivational significance is associated with preparation for active defense.     

Emergence of the benefits and costs of grouping for visual search

The present study investigated how grouping related items leads to the emergence of benefits (facilitation when related items are search targets) and costs (interference when related items are distractors) in visual search. Participants integrated different views (related items) of a novel Lego object via (a) assembling the object, (b) disassembling the object, or (c) sitting quietly without explicit instructions. An omnibus ANOVA revealed that neural responses (N2pc ERP) for attentional selection increased between pretest to posttest regardless of the training condition when a specific target view appeared (benefit) and when a nontarget view from the same object as the target view appeared (cost). Bonferroni‐corrected planned comparisons revealed that assembling the object (but not disassembling the object or no training) had a significant impact from pretest to posttest, although the ANOVA did not reveal any interaction effects, suggesting that the effects might not differ across training conditions. This study is one of the first to demonstrate the emergence of the costs and benefits of grouping novel targets on visual search efficiency.     

Detection of deception: Event‐related potential markers of attention and cognitive control during intentional false responses

Successful deception requires the coordination of multiple mental processes, such as attention, conflict monitoring, and the regulation of emotion. We employed a simple classification task, assessing ERPs to further investigate the attentional and cognitive control components of (instructed) deception. In Experiment 1, 20 participants repeatedly categorized visually presented names of five animals and five plants. Prior to the experiment, however, each participant covertly selected one animal and one plant for deliberate misclassification. For these critical items, we observed significantly increased response times (RTs), error rates, and amplitudes of three ERP components: anterior P3a indicating the processing of task relevance, medial‐frontal negativity reflecting conflict monitoring, and posterior P3b indicating sustained visual attention. In a blind identification of the individual critical words based on a priori defined criteria, an algorithm using two behavioral and two ERP measures combined showed a sensitivity of 0.73 and a specificity of 0.95, thus performing far above chance (0.2/0.2). Experiment 2 used five clothing and five furniture names and successfully replicated the findings of Experiment 1 in 25 new participants. For detection of the critical words, the algorithm from Experiment 1 was reused with only slight adjustments of the ERP time windows. This resulted in a very high detection performance (sensitivity 0.88, specificity 0.94) and significantly outperformed an algorithm based on RT alone. Thus, at least under controlled laboratory conditions, a highly accurate detection of instructed lies via the attentional and cognitive control components is feasible, and benefits strongly from combined behavioral and ERP measures.     

N2pc and attentional capture by colour and orientation-singletons in pure and mixed visual search tasks

The capture of attention by singleton stimuli in visual search is a matter of contention. Some authors propose that singletons capture attention in a bottom–up fashion if they are salient. Others propose that capture is contingent upon whether or not the stimuli share task-relevant attributes with the target. This study assessed N2pc elicited by colour and orientation singletons in a mixed task (the singleton defined as target changed block-to-block), and a pure task (the target was the same across the whole task). Both singletons elicited N2pc when acting as targets; when acting as non-targets, orientation singletons elicited N2pc only in the mixed task. The results suggest that the singletons were not salient enough to engage attention in a purely bottom–up fashion. Elicitation of N2pc by non-targets in the mixed task should be attributed to top–down processes associated with the current task. Stimuli that act as targets in part of the blocks become not completely irrelevant when non-targets.     

The contextual effect in object detection from picture

Participants detected a target from search stimuli which contained either eight (Experiment 1) or four (Experiment 2) living or nonliving common objects. Objects were represented either as pictures or as words. The target and other stimuli were from a same category, either living or nonliving, in the congruent condition, and were from a different category in the incongruent condition. The results of Experiment 1 showed that picture targets were detected more quickly in the incongruent than in the congruent condition while word targets were detected more quickly in the congruent than in the incongruent condition. These results indicate an interference effect of visual context in picture detection and a facilitation effect of semantic context in word detection. However, Experiment 2 using four stimuli did not show the same contextual effects as Experiment 1, probably because the task became easy.     

Contingent capture can occur at specific feature values: Behavioral and electrophysiological evidence

The notion that attentional top–down control can be tuned to a stimulus feature is widely accepted. Although previous studies suggested that the stimulus-driven attentional capture could be contingent on top–down attentional control settings, it was uncertain whether contingent capture can occur at a specific feature value. Three experiments were conducted to address this issue using both behavioral and ERPs measures. Participants were required to respond to one color singleton in the search display (target) but refrain from responding to the search display containing another color singleton (nontarget). When target and nontarget belonged to different color categories (Experiment 1), only the target-color cue and within category irrelevant-color cue elicited the significant cue validity effect (i.e. RTs were shorter when the target was presented at the same location as the preceding cue rather than at a different location); they also lead to a robust N2pc effect, indicative of attention-capture. In addition, these two cue types had similar attention-capturing capacity. However, when target and nontarget belonged to the same color category (Experiments 2 and 3), only the target-color cue elicited the significant cue validity effect and the robust N2pc effect. The same within category irrelevant-color cue no longer elicited the cue validity effect, and the N2pc effect was also attenuated. Present findings suggest that contingent capture can occur at a specific feature value.