Category‐based inhibition of focused attention across consecutive trials

The distractor previewing effect (DPE) refers to the behavioral phenomenon that search times increase for oddball targets containing features recently associated with the absence of a target. Previous work using a color‐oddball search task showed that the DPE covaried with the N2pc component of the event‐related potential (an index of attention allocation) but not with other components, suggesting that the DPE reflects shifts in attentional sets. We sought to determine whether the previous results could generalize to a category‐oddball search task. Results showed that the DPE co‐occurred with N2pc effects in about 60% of the participants, and the DPE occurred with no N2pc effects in the rest of the participants. These results support a domain‐general, attention‐based account of the DPE, but also suggest that the attention‐based DPE account requires some modifications.     

Evidence for an attentional component of inhibition of return in visual search

Inhibition of return (IOR) is typically described as an inhibitory bias against returning attention to a recently attended location as a means of promoting efficient visual search. Most studies examining IOR, however, either do not use visual search paradigms or do not effectively isolate attentional processes, making it difficult to conclusively link IOR to a bias in attention. Here, we recorded ERPs during a simple visual search task designed to isolate the attentional component of IOR to examine whether an inhibitory bias of attention is observed and, if so, how it influences visual search behavior. Across successive visual search displays, we found evidence of both a broad, hemisphere‐wide inhibitory bias of attention along with a focal, target location‐specific facilitation. When the target appeared in the same visual hemifield in successive searches, responses were slower and the N2pc component was reduced, reflecting a bias of attention away from the previously attended side of space. When the target occurred at the same location in successive searches, responses were facilitated and the P1 component was enhanced, likely reflecting spatial priming of the target. These two effects are combined in the response times, leading to a reduction in the IOR effect for repeated target locations. Using ERPs, however, these two opposing effects can be isolated in time, demonstrating that the inhibitory biasing of attention still occurs even when response‐time slowing is ameliorated by spatial priming.     

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.     

On the time course of bottom‐up and top‐down processes in selective visual attention: An EEG study

According to recent models on visual attention, both the salience of signals (bottom‐up) and the intention to search for particular stimuli (top‐down) are determinants for attentional selection. We investigated these mechanisms by varying the top‐down set of participants that had to detect either luminance or orientation changes of two symmetrically located bars. Irrelevant changes impaired target detection when they were presented spatially separated to the relevant change. Initial attentional selection was represented in posterior N1 asymmetries and was determined by both the relative salience of orientation changes and a subsequent intentional bias towards relevant stimuli. Only when salient orientation changes interfered with luminance target selection in the N1 time window did an N2pc occur. Thus, the selection of relevant information proceeds in a network whose activation is induced by a dynamic interplay of bottom‐up and top‐down processes.     

Rewarded visual items capture attention only in heterogeneous contexts

Reward is known to affect visual search performance. Rewarding targets can increase search performance, whereas rewarding distractors can decrease search performance. We used subcomponents of the N2pc in the event‐related EEG, the N_T (target negativity) and N_D/P_D (distractor negativity/positivity), in a visual search task to disentangle target and distractor processing related to reward. The visual search task comprised homogeneous and heterogeneous contexts in which a target and a colored distractor were embedded. After each correct trial, participants were given a monetary reward that depended on the color of the distractor. We found longer response times for displays with high‐reward distractors compared to displays with low‐reward distractors, indicating reward‐induced interference, however, only for heterogeneous contexts. The N_T component, indicative of attention deployment to the target, showed that target selection was impaired by high‐reward distractors, regardless of the context homogeneity. Processing of distractors was not affected by reward in homogeneous contexts. In heterogeneous contexts, however, high‐reward distractors were more likely to capture attention (N_D) and required more effort to be suppressed (P_D) than low‐reward distractors. In sum the results showed that, despite the fact that target selection is impaired by high‐reward distractors in both homogeneous and heterogeneous background contexts, high‐reward distractors capture attention only in scenarios that foster attentional capture.     

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.     

Top‐down control of audiovisual search by bimodal search templates

To test whether the attentional selection of targets defined by a combination of visual and auditory features is guided in a modality‐specific fashion or by control processes that are integrated across modalities, we measured attentional capture by visual stimuli during unimodal visual and audiovisual search. Search arrays were preceded by spatially uninformative visual singleton cues that matched the current target‐defining visual feature. Participants searched for targets defined by a visual feature, or by a combination of visual and auditory features (e.g., red targets accompanied by high‐pitch tones). Spatial cueing effects indicative of attentional capture were reduced during audiovisual search, and cue‐triggered N2pc components were attenuated and delayed. This reduction of cue‐induced attentional capture effects during audiovisual search provides new evidence for the multimodal control of selective attention.     

Contextual cueing effects despite spatially cued target locations

Reaction times (RT) to targets are faster in repeated displays relative to novel ones when the spatial arrangement of the distracting items predicts the target location (contextual cueing). It is assumed that visual–spatial attention is guided more efficiently to the target resulting in reduced RTs. In the present experiment, contextual cueing even occurred when the target location was previously peripherally cued. Electrophysiologically, repeated displays elicited an enhanced N2pc component in both conditions and resulted in an earlier onset of the stimulus‐locked lateralized readiness potential (s‐LRP) in the cued condition and in an enhanced P3 in the uncued condition relative to novel displays. These results indicate that attentional guidance is less important than previously assumed but that other cognitive processes, such as attentional selection (N2pc) and response‐related processes (s‐LRP, P3) are facilitated by context familiarity.     

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 N2cc component as an electrophysiological marker of space‐based and feature‐based attentional target selection processes in touch

An electrophysiological correlate of attentional target selection processes in touch (N2cc component) has recently been discovered in lateralized tactile working memory experiments. This tactile N2cc emerges at the same time as the visual N2pc component but has a different modality‐specific topography over central somatosensory areas. Here, we investigated links between N2cc components and the space‐based versus feature‐based attentional selection of task‐relevant tactile stimuli. On each trial, a pair of tactile items was presented simultaneously to one finger on the left and right hand. Target stimuli were defined by their location (e.g., left index finger; Spatial Attention Task), by a nonspatial feature (continuous vs. pulsed; Feature‐Based Attention Task), or by a combination of spatial and nonspatial features (Conjunction Task). Reliable N2cc components were observed in all three tasks. They emerged considerably earlier in the Spatial Attention Task than in the Feature‐Based Attention Task, suggesting that space‐based selection mechanisms in touch operate faster than feature‐guided mechanisms. The temporal pattern of N2cc components observed in the Conjunction Task revealed that space‐based and feature‐based attention both contributed to target selection, which was initially driven primarily by spatial location. Overall, these findings establish the N2cc component as a new electrophysiological marker of the selective attentional processing of task‐relevant stimuli in touch.     

The effect of category learning on visual attention and visual representation

Subordinate‐level category learning recruits neural resources associated with perceptual expertise, including the N250 component of the ERP, a posterolateral negative wave maximal between 230 and 330 ms. The N250 is a relatively late visual ERP and could plausibly be driven by attention to the features of categorized objects. Indeed, it has a latency and scalp distribution similar to the selection negativity (SN), an ERP component long known to be sensitive to attentional selection of target features. To clarify sensitivity of the N250 to attention and to more generally investigate the effect of category learning on attentional modulation of learned features, we independently manipulated subordinate‐level category learning and target detection in a speeded paradigm designed to optimally elicit the SN and accompanying frontal selection positivity (FSP). Participants first practiced categorizing a set of artificial animal stimuli and then performed a speeded target detection task on trained and untrained stimuli while ERPs were recorded. SN and FSP were roughly linearly related to the number of target features in the stimulus. Trained stimuli elicited a significantly larger N250 than untrained stimuli. The SN and N250 effects were additive, with all levels of target similarity equally affected by training, and had different time courses. Training had little effect on the FSP. The results suggest that (a) the N250 and SN have different sources, and (b) at the very least, the learning‐induced N250 indexes a different attentional subprocess from the target‐induced SN and could be driven by a different cognitive process altogether.     

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.     

Target selection for visually guided reaching in macaque

We examined target selection for visually guided reaching in monkeys using a visual search task in which an odd-colored target was presented with distractors. The colors of the target and distractors were randomly switched in each trial between red and green, and the number of distractors was varied. Previous studies of saccades and attention have shown that target selection in this task is easier when a greater number of homogenous distractors is present. We found that monkeys made fewer reaches to distractors and that reaches to the target were completed more quickly when a greater number of homogenous distractors was present. When the target was presented in a sparse array of distractors, reaches had longer movement durations and greater trajectory curvature. Reaching errors were directed more often to a distractor adjacent to the target, suggesting a spatially coarse-to-fine progression during target selection. Reaches were also influenced by the properties of trials in the recent past. When the colors of the target and distractors remained the same from trial to trial rather than switching, reaches were completed more quickly and accurately, indicating that color priming across trials facilitates target selection. Moreover, when difficult search trials were randomly intermixed with easier trials without distractors, reach latencies were influenced by the difficulty of previous trials, indicating that motor initiation strategies are gradually adjusted based on accumulated experience. Overall, these results are consistent with reaching results in humans, indicating that the monkey provides a sound model for understanding the neural underpinnings of reach target selection.     

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.     

Selection history alters attentional filter settings persistently and beyond top‐down control

Visual selective attention is known to be guided by stimulus‐based (bottom‐up) and goal‐oriented (top‐down) control mechanisms. Recent work has pointed out that selection history (i.e., the bias to prioritize items that have been previously attended) can result in a learning experience that also has a substantial impact on subsequent attention guidance. The present study examined to what extent goal‐oriented top‐down control mechanisms interact with an observer's individual selection history in guiding attention. Selection history was manipulated in a categorization task in a between‐subjects design, where participants learned that either color or shape was the response‐relevant dimension. The impact of this experience was assessed in a compound visual search task with an additional color distractor. Top‐down preparation for each search trial was enabled by a pretrial task cue (Experiment 1) or a fixed, predictable trial sequence (Experiment 2). Reaction times and ERPs served as indicators of attention deployment. Results showed that attention was captured by the color distractor when participants had learned that color predicted the correct response in the categorization learning task, suggesting that a bias for predictive stimulus features had developed. The possibility to prepare for the search task reduced the bias, but could not entirely overrule this selection history effect. In Experiment 3, both tasks were performed in separate sessions, and the bias still persisted. These results indicate that selection history considerably shapes selective attention and continues to do so persistently even when the task allowed for high top‐down control.     

Electrophysiological evidence of feature-based inhibition of focused attention across consecutive trials

Monitoring the visual environment for targets has evolutionary and ecological significance. Exploration of the visual scene can be hypothesized to use "saliency maps" representing the priorities allocated to different locations and features, which change dynamically based on previous experience. This is supported by the observation that sequential effects influence the speed and accuracy of visual search tasks. Yet, the mechanisms underlying these effects are not well understood. Here we used electrophysiological methods to identify some of the mechanisms underlying a specific sequential effect—the distractor previewing effect (DPE). We found that the DPE is correlated with electrophysiological indices of attention allocation but not with sensory or motor components, suggesting that the DPE is related to attentional phenomena, rather than perceptual or motor levels of processing.     

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.     

Guidance of spatial attention during associative learning: Contributions of predictability and intention to learn

Expectations of an event can facilitate its neural processing. One of the ways we build expectations is through associative learning. Interestingly, the learning of contingencies between events can also occur without intention. Here, we study feature‐based attention during associative learning, by asking how a learned association between a cue and a target outcome impacts the attention allocated to this outcome. Moreover, we investigate attention in learning depending on the intention to learn the association. We used an associative learning paradigm where we manipulated outcome predictability and intention to learn an association within streams of cue‐target outcome visual stimuli, while stimulus characteristics and probability were held constant. In order to measure the event‐related component N2pc, widely recognized to reflect allocation of spatial attention, every outcome was embedded among distractors. Importantly, the location of the target outcome could not be anticipated. We found that predictable target outcomes showed an increased spatial attention as indexed by a greater N2pc component. A later component, the P300, was sensitive to the intention to learn the association between the cue and the target outcome. The current study confirms the remarkable ability of the brain to extract and update predictive information, in accordance with a predictive‐coding model of brain function. Associative learning can guide a visual search and shape covert attentional selection in our rich environments.     

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.     

Electrophysiological correlates of early attentional feature selection and distractor filtering

Using electrophysiology, the attentional functions of target selection and distractor filtering were investigated during visual search. Observers searched for multiple tilted line segments amidst vertical distractors. In different conditions, observers were either looking for a specific line orientation (“feature-based” selection), or for any tilted line (“salience-based”). The search array could contain both left- and rightward tilted lines simultaneously (requiring spatial filtering) or only one line type (no filtering). The amplitude of the P1 event-related potential component was reduced during feature-based selection, compared to salience-based selection. The N1 showed a similar effect, at least when filtering was required. Amplitudes were also somewhat reduced when competing nontarget stimuli required filtering. Interactions between selection and filtering became stronger on the N2a and P3. When both feature-based selection and filtering were required, N2a amplitude was highest, and P3 amplitude was lowest. The results support an early locus of feature-based attentional selection in multi-item search.