Electrophysiological evidence of low salience distractor interference during visual search

Visual search displays often include distractors of lesser salience in addition to a target and one or more salient distractors. We investigated low salience distractor effects on the N2pc, an ERP component indexing the deployment of attention, and the Ptc, a component purported to reflect attentional disengagement. We hypothesized that salient distractors pull the attentional focus away from the target, which could lead to increased attentional processing of low salience distractors close to the target and salient distractor. Participants looked for a colored inverted T during a visual search task while ignoring an L of the same color at a fixed distance on an imaginary circle around fixation. There were four conditions: no additional gray (low salience) distractors, two additional gray distractors between color items, two additional gray distractors just outside the area delimited by the colored items, and four additional gray distractors inside and outside the attended region. The gray distractors impacted N2pc and Ptc amplitude and latency, indicating an effect of gray distractors on attentional processing. Also, additional gray distractors led to increasingly more deviation of the N2pc and Ptc waveforms from the baseline offered by the condition with no additional gray distractors. When we increased the difficulty to individuate the target, we observed more displacement of lateralized activity from the N2pc to the Ptc time window. We argue that distractor‐related modulations likely result from increased variance in the latency of attentional engagement activity to the target instead of distractor inhibition or attentional disengagement.     

Competitive interaction degrades target selection: An ERP study

Localized attentional interference (LAI) occurs when attending to a visual object degrades processing of nearby objects. Competitive interaction accounts of LAI explain the phenomenon as the result of competition among objects for representation in extrastriate cortex. Here, we examined the N2pc component of the event-related potential (ERP) as a likely neural correlate of LAI. In Experiment 1 , participants responded to the orientation of a target while ignoring a nearby decoy. At small target–decoy separations, N2pc amplitude was attenuated whereas the amplitude of a later, positive component (Ptc) was potentiated. Experiment 2 ruled out sensory explanations of these effects. The N2pc results are consistent with the idea that spatially mediated competition for representation in extrastriate cortex degrades target selection. Moreover, the Ptc may reflect a bias signal needed to resolve the competition at smaller target–decoy separations.     

The allocation of attention in displays with simultaneously presented singletons

In an ERP experiment, we investigated whether a ‘permanent’ salient distractor changes the deployment of attention to target and nontarget singletons. Observers searched for a color target in a search array that mainly consisted of black vertical lines, but also always contained a line in a task-irrelevant color. Together with this distractor, a target or nontarget singleton was presented. Nontargets could be salient on the task-relevant dimension (color), or on a neutral dimension (line orientation). N2pc amplitude was maximal for targets, no N2pc was elicited by color nontargets, and orientation nontargets elicited an inverse N2pc. Targets and color nontargets elicited larger N2 amplitude than orientation singletons. P3 amplitude was high for relevant and low for irrelevant singletons. Targets also elicited higher reaction times and more errors. Attention seemed thus driven by the target feature, and by its feature dimension, even when constant distraction on that dimension had to be suppressed.     

Electrophysiological correlates of stimulus processing in change blindness

Change blindness—the inability to detect salient changes when a distractor event occurs simultaneously—has been repeatedly used to investigate the neural correlates of awareness. The fact that the N2pc, which is basically assigned to attention processing, has been observed only for detected changes in such tasks lead to the assumption that this component may also reflect awareness. In contrast to previous electrophysiological studies, we used mudsplashes (experiment 1) or a very short blank (experiment 2) to induce change blindness so that the change was not occluded. A change, regardless of its detection, elicited a reliable N2pc. Successful change detection, however, was reflected in an enhanced amplitude of the N2pc component. Thus, the N2pc cannot be taken as a direct correlate of awareness but rather as a marker for a process that is necessary but not sufficient for awareness. Taking into account the generation of the N2pc in extrastriate visual areas, this finding fits nicely with the recent discussion about reentrant processing as a basis for visual awareness.     

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.     

Attentional capture during visual search is attenuated by target predictability: Evidence from the N2pc,Pd, and topographic segmentation

Attentional capture by salient distractors has been confirmed by the occurrence of an N2pc to the salient distractor. To clarify some failures to replicate this finding, we varied target predictability to induce different search modes. In the unpredictable target condition, the target shape varied randomly from trial to trial, favoring singleton detection mode. In the predictable target condition, the target shape remained the same in a block of trials, favoring feature search mode. With unpredictable targets, we observed an N2pc toward the salient color distractor, confirming attentional capture in singleton search mode. With predictable targets, there was no N2pc to the salient distractor, but a distractor positivity (Pd), suggesting distractor suppression. Also, differences emerged in the topographic segmentation of N2pc and Pd. Further, the amplitude of the N2pc toward the target was larger with predictable than with unpredictable targets.     

Toward the influence of temporal attention on the selection of targets in a visual search task: An ERP study

We used ERPs to investigate whether temporal attention interacts with spatial attention and feature‐based attention to enhance visual processing. We presented a visual search display containing one singleton stimulus among a set of homogenous distractors. Participants were asked to respond only to target singletons of a particular color and shape that were presented in an attended spatial position. We manipulated temporal attention by presenting a warning signal before each search display and varying the foreperiod (FP) between the warning signal and the search display in a blocked manner. We observed distinctive ERP effects of both spatial and temporal attention. The amplitudes for the N2pc, SPCN, and P3 were enhanced by spatial attention indicating a processing benefit of relevant stimulus features at the attended side. Temporal attention accelerated stimulus processing; this was indexed by an earlier onset of the N2pc component and a reduction in reaction times to targets. Most importantly, temporal attention did not interact with spatial attention or stimulus features to influence visual processing. Taken together, the results suggest that temporal attention fosters visual perceptual processing in a visual search task independently from spatial attention and feature‐based attention; this provides support for the nonspecific enhancement hypothesis of temporal attention.     

Colour-specific differences in attentional deployment for equiluminant pop-out colours: Evidence from lateralised potentials

We investigated how target colour affected behavioural and electrophysiological results in a visual search task. Perceptual and attentional mechanisms were tracked using the N2pc component of the event-related potential and other lateralised components. Four colours (red, green, blue, or yellow) were calibrated for each participant for luminance through heterochromatic flicker photometry and equated to the luminance of grey distracters. Each visual display contained 10 circles, 1 colored and 9 grey, each of which contained an oriented line segment. The task required deploying attention to the colored circle, which was either in the left or right visual hemifield. Three lateralised ERP components relative to the side of the lateral coloured circle were examined: a posterior contralateral positivity (Ppc) prior to N2pc, the N2pc, reflecting the deployment of visual spatial attention, and a temporal and contralateral positivity (Ptc) following N2pc. Red or blue stimuli, as compared to green or yellow, had an earlier N2pc. Both the Ppc and Ptc had higher amplitudes to red stimuli, suggesting particular selectivity for red. The results suggest that attention may be deployed to red and blue more quickly than to other colours and suggests special caution when designing ERP experiments involving stimuli in different colours, even when all colours are equiluminant.     

Understanding how visual attention locks on to a location: Toward a computational model of the N2pc component

Competing theories regarding the neural mechanism underlying the N2pc suggest that it reflects target enhancement or distractor suppression, but we suggest a different theory. In our study, participants viewed a bilateral dual‐stream rapid serial visual presentation (RSVP). Each trial had either one or two targets (T1, T2) presented either in the same or different streams. We found that the N2pc elicited by T1 was not affected by an immediately following T2 at the same location. However, if T2 was on the opposite hemifield, a second N2pc was elicited. These results suggest that the N2pc reflects neural processes involved in localizing a target to initiate the deployment of attention, rather than the sustained enhancement of processing at that location or distractor inhibition. We have created a computational model of a convergent gradient‐filter model of attention that reproduces the observed event‐related potential traces and makes five testable predictions to be addressed in future work.     

Attentional selection and identification of visual objects are reflected by distinct electrophysiological responses

Lateralised ERP responses were measured over posterior visual brain regions in response to visual search arrays that contained one colour singleton. In the localisation task, responses were determined by the visual hemifield where this singleton was presented. In the discrimination task, they were determined by the singletons’ shape. While an N2pc component was elicited in an identical fashion in both tasks, a subsequent sustained contralateral negativity was consistently present at posterior sites in the discrimination task only. This dissociation demonstrates that these two activations reflect distinct visual processing stages. We suggest that while the N2pc reflects the ability of the visual system both to identify and localise a relevant stimulus in the scene, the late sustained activity reflects the subsequent in-depth analysis and identification of these stimuli.     

Perceptual learning induces active suppression of physically nonsalient shapes

Visual attention can be attracted by salient‐but‐irrelevant features, a phenomenon called attentional capture. Accompanying attentional capture, a top‐down inhibitory mechanism is usually enacted to suppress the attentional shift. Our recent study showed that a physically nonsalient shape, which may be considered as a conjunction of basic features within the form domain, can also provoke a robust capture of attention as a consequence of perceptual learning, supporting an important role of prior experience in attentional deployment. It remains unclear, however, whether prior experience can also induce attentional suppression to conjunctions of features. Here, we examine whether and in which condition the brain would initiate an active suppression process to a physically nonsalient shape which has acquired an ability to capture attention after perceptual learning. We show that detectability of a shape after perceptual learning may be a key factor determining whether the shape would be actively suppressed or not. After extensive training as a target in visual search, a physically nonsalient shape could elicit an N2pc component when it was a distractor in a visual search task or a peripheral irrelevant stimulus in a central focused attention task, indicating a capture of attention induced by perceptual learning. Following the N2pc component, a Pd component would be elicited by the trained shape only if its detectability is relatively high. These findings suggest that an active suppression process could be applied not only to salient features but also to physically nonsalient shapes. A physically nonsalient shape could improve its salience through perceptual learning and would be actively suppressed when its learned salience reaches a certain level.     

Early and late selection modulate competition for representation: Evidence from the N2pc in a multiple frame procedure

In this study, both bottom‐up and top‐down influences on processes of visual attention deployment are tested by examining the interaction between the similarity of a salient distractor to a salient target and the physical distance separating attended items in the visual field. Distractor‐target similarity was varied both on a semantic and a physical level. Additionally, across experiments, the similarity of the salient items could vary in form, color, or both. Three experiments identified interactive relationships between these factors on the amplitude and the latency of N2pc, an ERP associated to the deployment of visuospatial attention. Results show the influence of both early form‐based selection and late identification‐based selection processes on the N2pc. Furthermore, selection processes appear to be dependent on the spatial proximity of the attended items, suggesting that some processes may operate on a local spatial scale.     

A psychological refractory period in access to visual short-term memory and the deployment of visual-spatial attention: multitasking processing deficits revealed by event-related potentials

In this psychological refractory period (PRP) experiment, a tone (T1) was presented, followed by a visual target (T2) embedded in a bilateral display, and a speeded response was required for each target. The T1-T2 stimulus onset asynchrony (SOA) was 300, 650, or 1000 ms. Mean response time to T2 increased as SOA was reduced, replicating the well-known PRP effect. Importantly, the N2pc component of the event-related potential was progressively attenuated as SOA was reduced, and the onset latency of the sustained posterior contralateral negativity (SPCN) that follows the N2pc was progressively lengthened. Conditional analysis based on Task1 difficulty corroborated the analyses based on effects of SOA. The results suggest that central processing leading to the PRP effect interferes with the deployment of visual-spatial attention (as indexed by the N2pc) and delays encoding into visual short-term memory (as indexed by the SPCN onset latency).     

The “red‐alert” effect in visual search: Evidence from human electrophysiology

Participants had to determine the orientation of a segment inside a target color circle among other gray distractor circles. The target circle was either red or green and was accompanied in the display by a distractor in the other color. To dissociate event‐related potentials of target and distractor processing, one of them was on the vertical meridian and the other in a lateral position. In Experiment 1, the target color was indicated on a per‐trial basis and, in Experiment 2, on a per‐block basis. The results revealed the N2pc elicited by red targets had an earlier latency relative to the N2pc elicited by green targets. Contralateral responses of positive polarity linked to distractor inhibition were found only with red lateral distractors. The results suggest that the choice of colors to distinguish targets from distractors may play a role in visual search performance and in the functional characterization of event‐related lateralizations.     

Attentional processing of multiple targets and distractors

We assessed the functioning of attention when multiple relevant objects are intermingled with multiple distractors, measuring two electrophysiological subcomponents of the N2pc that have been associated, respectively, with target selection and distractor suppression: the target negativity (Nt) and the distractor positivity (Pd). To this aim, we orthogonally manipulated the number of targets and distractors in an enumeration task. The Nt was modulated by target, but not distractor numerosity, suggesting that an increase in target numerosity leads to an increase in attentional resources needed to form individual representations of the targets. In contrast, the number of distractors did not differentially alter the Pd. We hypothesize that distractors sharing similar visual features can be processed (and possibly suppressed) as a set, without the need for individuation.     

The N2pc component and its links to attention shifts and spatially selective visual processing

The N2pc component has recently become a popular tool in attention research. To investigate whether this component exclusively reflects attentional target selection or also prior stages in attentional processing (covert orienting, target-unspecific spatial attention), a spatial cuing procedure was combined with a visual search task. In some blocks, informative cues indicated the side of upcoming singleton targets that were present on most trials among uniform distractors. In other blocks, cues were spatially uninformative, and no preparatory shifts of attention were possible. The N2pc in response to targets was unaffected by this manipulation, showing that this component is not associated with attention shifts. Following informative cues, an attenuated N2pc was elicited by uniform nontarget arrays, suggesting that the N2pc may also reflect spatially specific processing of stimulus features at task-relevant locations prior to target selection.     

Spatial attention freezes during the attention blink

A variant of the rapid serial visual presentation paradigm was used to display sequentially two lateral sequences of stimuli, one to the left and one to the right of fixation, embedding two pairs of target stimuli, T1 and T2. T1 was composed of a pair of alphanumeric characters, and subjects had either to ignore T1 or to encode T1 for a delayed response. T2 was a lateral square of a prespecified color. The square had a small gap in one side, and the task for this stimulus was to report which side had the gap. When subjects were required to ignore T1, the T2-locked ERP produced a clear N2pc, that is, a greater negativity at electrode sites contralateral to the position occupied by T2. This N2pc was followed by a sustained posterior contralateral negativity (SPCN). When subjects were required to monitor T1 in addition to T2, both the N2pc and the SPCN components amplitude depended on the difficulty of the task associated with T1. If T1 was composed of digits that had to be encoded for a delayed same/different judgment, both the N2pc and the SPCN components were entirely suppressed. Although attenuated, such components were present when T1 was composed of a pair of symbols that subjects could disregard. The results suggest that a set of mechanisms subserving the allocation of attention in the spatial domain, resulting in the N2pc, suffer significant interference from concurrent cognitive operations required to encode information into visual short-term memory.     

Event‐related potentials reveal the effect of prior knowledge on competition for representation and attentional capture

Objects compete for representation in our limited capacity visual system. We examined how this competition is influenced by top‐down knowledge using event‐related potentials. Competition was manipulated by presenting visual search arrays in which the target or distractor was the only color singleton compared to displays in which both singletons were presented. Experiments 1 and 2 manipulated whether the observer knew the color of the target in advance. Experiment 3 ruled out low‐level sensory explanations. Results show that, under conditions of competition, the distractor does not elicit an N2pc when the target color is known. However, the N2pc elicited by the target is reduced in the presence of a distractor. These findings suggest that top‐down knowledge can prevent the capture of attention by distracting information, but this prior knowledge does not eliminate the competitive influence of the distractor on the target.     

An ERP study of visual spatial attention and letter target detection for isoluminant and nonisoluminant stimuli

The event-related potential (ERP) effects of visual spatial attention and letter target detection for stimuli presented against a (nonisoluminant) dark background or against an isoluminant grey background were investigated. The goal was to study how the perceptual variable of luminance would influence early ERP reflections of selective attention. Such effects could further substantiate the claim that selective attention operates at the level of early perceptual processing and could provide evidence regarding the role of different visual routes in selective attention. Isoluminance increased the peak latency of the early ERP deflections (NP80, PI, and NI) by 40–50 ms. The ERP effects of spatial attention. consisting of PI and NI amplitude enhancements, were similarly delayed by isoluminance. supporting the idea that early selective processing is strongly dependent on bottom-up perceptual processing. P300 latency and reaction time were delayed by 70–75 ms, the additional delay probably reflecting that isoluminance affected decision processes in addition to perceptual processes. Isoluminance left the scalp topographies of the early ERP deflections largely unaffected, although a slight shift of the NI topography in the isoluminant condition toward more inferior lateral posterior regions of the scalp could have reflected an increased contribution from ventral (occipitotemporal) brain areas. Relative to nontarget letters, targels presented at both attended and unattended spatial positions elicited an early contralaterally dominant lateral occipitotemporal negativity (N2pc). This ERP component is proposed to reflect an early, partly automatic process of template matching, consistent with indications from spatiotemporal dipole modelling that the N2pc was generated in inferior occipitotemporal brain regions.     

Does focused endogenous attention prevent attentional capture in pop‐out visual search?

To investigate whether salient visual singletons capture attention when they appear outside the current endogenous attentional focus, we measured the N2pc component as a marker of attentional capture in a visual search task where target or nontarget singletons were presented at locations previously cued as task-relevant, or in the uncued irrelevant hemifield. In two experiments, targets were either defined by color or by a combination of color and shape. The N2pc was elicited both for attended singletons and for singletons on the uncued side, demonstrating that focused endogenous attention cannot prevent attentional capture by salient unattended visual events. However, N2pc amplitudes were larger for attended and unattended singletons that shared features with the current target, suggesting that top-down task sets modulate the capacity of visual singletons to capture attention both within and outside the current attentional focus.