Predictive distractor context facilitates attentional selection of high, but not intermediate and low, salience targets

ERPs and alpha oscillations track the encoding and maintenance of object-based representations in visual working memory

When memorizing an integrated object such as a Kanizsa figure, the completion of parts into a coherent whole is attained by grouping processes which render a whole-object representation in visual working memory (VWM). The present study measured event-related potentials (ERPs) and oscillatory amplitudes to track these processes of encoding and representing multiple features of an object in VWM. To this end, a change detection task was performed, which required observers to memorize both the orientations and colors of six "pacman" items while inducing configurations of the pacmen that systematically varied in terms of their grouping strength. The results revealed an effect of object configuration in VWM despite physically constant visual input: change detection for both orientation and color features was more accurate with increased grouping strength. At the electrophysiological level, the lateralized ERPs and alpha activity mirrored this behavioral pattern. Perception of the orientation features gave rise to the encoding of a grouped object as reflected by the amplitudes of the Ppc. The grouped object structure, in turn, modulated attention to both orientation and color features as indicated by the enhanced N1pc and N2pc. Finally, during item retention, the representation of individual objects and the concurrent allocation of attention to these memorized objects were modulated by grouping, as reflected by variations in the CDA amplitude and a concurrent lateralized alpha suppression, respectively. These results indicate that memorizing multiple features of grouped, to-be-integrated objects involves multiple, sequential stages of processing, providing support for a hierarchical model of object representations in VWM.

When experience with scenes foils attentional orienting: ERP evidence against flexible target-context mapping in visual search

Visual search is speeded when a target is repeatedly presented in an invariant scene context of nontargets (contextual cueing), demonstrating observers' capability for using statistical long-term memory (LTM) to make predictions about upcoming sensory events, thus improving attentional orienting. In the current study, we investigated whether expectations arising from individual, learned environmental structures can encompass multiple target locations. We recorded event-related potentials (ERPs) while participants performed a contextual cueing search task with repeated and non-repeated spatial item configurations. Notably, a given search display could be associated with either a single target location (standard contextual cueing) or two possible target locations. Our result showed that LTM-guided attention was always limited to only one target position in single- but also in the dual-target displays, as evidenced by expedited reaction times (RTs) and enhanced N1pc and N2pc deflections contralateral to one ("dominant") target of up to two repeating target locations. This contrasts with the processing of non-learned ("minor") target positions (in dual-target displays), which revealed slowed RTs alongside an initial N1pc "misguidance" signal that then vanished in the subsequent N2pc. This RT slowing was accompanied by enhanced N200 and N400 waveforms over fronto-central electrodes, suggesting that control mechanisms regulate the competition between dominant and minor targets. Our study thus reveals a dissociation in processing dominant versus minor targets: While LTM templates guide attention to dominant targets, minor targets necessitate control processes to overcome the automatic bias towards previously learned, dominant target locations.

Contralateral delay activity, but not alpha lateralization, indexes prioritization of information for working memory storage

Working memory is inherently limited, which makes it important to select and maintain only task-relevant information and to protect it from distraction. Previous research has suggested the contralateral delay activity (CDA) and lateralized alpha oscillations as neural candidates for such a prioritization process. While most of this work focused on distraction during encoding, we examined the effect of external distraction presented during memory maintenance. Participants memorized the orientations of three lateralized objects. After an initial distraction-free maintenance interval, distractors appeared in the same location as the targets or in the opposite hemifield. This distraction was followed by another distraction-free interval. Our results show that CDA amplitudes were stronger in the interval before compared with the interval after the distraction (i.e., CDA amplitudes were stronger in response to targets compared with distractors). This amplitude reduction in response to distractors was more pronounced in participants with higher memory accuracy, indicating prioritization and maintenance of relevant over irrelevant information. In contrast, alpha lateralization did not change from the interval before distraction compared with the interval after distraction, and we found no correlation between alpha lateralization and memory accuracy. These results suggest that alpha lateralization plays no direct role in either selective maintenance of task-relevant information or inhibition of distractors. Instead, alpha lateralization reflects the current allocation of spatial attention to the most salient information regardless of task-relevance. In contrast, CDA indicates flexible allocation of working memory resources depending on task-relevance.

Priming of probabilistic attentional templates

Attentional priming has a dominating influence on vision, speeding visual search, releasing items from crowding, reducing masking effects, and during free-choice, primed targets are chosen over unprimed ones. Many accounts postulate that templates stored in working memory control what we attend to and mediate the priming. But what is the nature of these templates (or representations)? Analyses of real-world visual scenes suggest that tuning templates to exact color or luminance values would be impractical since those can vary greatly because of changes in environmental circumstances and perceptual interpretation. Tuning templates to a range of the most probable values would be more efficient. Recent evidence does indeed suggest that the visual system represents such probability, gradually encoding statistical variation in the environment through repeated exposure to input statistics. This is consistent with evidence from neurophysiology and theoretical neuroscience as well as computational evidence of probabilistic representations in visual perception. I argue that such probabilistic representations are the unit of attentional priming and that priming of, say, a repeated single-color value simply involves priming of a distribution with no variance. This "priming of probability" view can be modelled within a Bayesian framework where priming provides contextual priors. Priming can therefore be thought of as learning of the underlying probability density function of the target or distractor sets in a given continuous task.

Temporal integration of feature probability distributions

Humans are surprisingly good at learning the statistical characteristics of their visual environment. Recent studies have revealed that not only can the visual system learn repeated features of visual search distractors, but also their actual probability distributions. Search times were determined by the frequency of distractor features over consecutive search trials. The search displays applied in these studies involved many exemplars of distractors on each trial and while there is clear evidence that feature distributions can be learned from large distractor sets, it is less clear if distributions are well learned for single targets presented on each trial. Here, we investigated potential learning of probability distributions of single targets during visual search. Over blocks of trials, observers searched for an oddly colored target that was drawn from either a Gaussian or a uniform distribution. Search times for the different target colors were clearly influenced by the probability of that feature within trial blocks. The same search targets, coming from the extremes of the two distributions were found significantly slower during the blocks where the targets were drawn from a Gaussian distribution than from a uniform distribution indicating that observers were sensitive to the target probability determined by the distribution shape. In Experiment 2, we replicated the effect using binned distributions and revealed the limitations of encoding complex target distributions. Our results demonstrate detailed internal representations of target feature distributions and that the visual system integrates probability distributions of target colors over surprisingly long trial sequences.

Tracking Neural Markers of Template Formation and Implementation in Attentional Inhibition under Different Distractor Consistency

Performing visual search tasks requires optimal attention deployment to promote targets and inhibit distractors. Rejection templates based on the feature of the distractor can be built to constrain the search process. We measured electroencephalography (EEG) of human participants of both sexes when they performed a visual search task in conditions where the distractor cues were constant within a block (fixed cueing) or changed on a trial-by-trial basis (varied cueing). In the fixed-cueing condition, sustained decoding of the cued colors could be achieved during the retention interval, and participants with higher decoding accuracy showed larger suppression benefits of the distractor cueing in the search period. In the varied-cueing condition, the cued color could only be transiently decoded after its onset, and higher decoding accuracy was observed from the participants who demonstrated lower suppression benefit. The differential neural representations of the to-be-ignored color in the two cueing conditions as well as their reverse associations with behavioral performance implied that rejection templates were formed in the fixed-cueing condition but not in the varied-cueing condition. Additionally, we observed stronger posterior alpha lateralization and midfrontal theta/beta power during the retention interval of the varied-cueing condition, indicating the cognitive costs in template formation caused by the trialwise change of distractor colors. Together, our findings revealed the neural markers associated with the critical roles of distractor consistency in linking template formation to successful inhibition.SIGNIFICANCE STATEMENT How do we strategically build a rejection template based on distractor features to filter out matched items when performing visual search tasks? Previous studies have suggested that the consistency of the to-be-ignored feature may play a significant role in this process. We recorded scalp EEGs when human participants searched for a target among distractors. Capitalized on multivariate decoding technique and time-frequency analysis, we revealed the neural markers of the rejection template under different distractor consistencies. Being able to track these processes in visual search could help us to understand the connection between template formation and successful distractor inhibition. Our findings may also benefit future EEG-based interventions on individuals with deficits in attentional control.

Contextual cueing in co-active visual search: Joint action allows acquisition of task-irrelevant context

Repeatedly presenting a target within a stable search array facilitates visual search, an effect termed contextual cueing. Previous solo-performance studies have shown that successful acquisition of contextual memories requires explicit allocation of attentional resources to the task-relevant repeated contexts. By contrast, repeated but task-irrelevant contexts could not be learned when presented together with repeated task-relevant contexts due to a blocking effect. Here we investigated if such blocking of context learning could be diminished in a social context, when the task-irrelevant context is task-relevant for a co-actor in a joint action search mode. We adopted the contextual cueing paradigm and extended this to the co-active search mode. Participants learned a context-cued subset of the search displays (color-defined) in the training phase, and their search performance was tested in the transfer phase, where previously irrelevant and relevant subsets were swapped. The experiments were conducted either in a solo search mode (Experiments 1 and 3) or in a co-active search mode (Experiment 2). Consistent with the classical contextual cueing studies, contextual cueing was observed in the training phase of all three experiments. Importantly, however, in the "swapped" test session, a significant contextual cueing effect was manifested only in the co-active search mode, not in the solo search mode. Our findings suggest that social context may widen the scope of attention, thus facilitating the acquisition of task-irrelevant contexts.

EEG evidence for enhanced attentional performance during moderate-intensity exercise

Research on attentional control within real-world contexts has become substantially more feasible and thus frequent over the past decade. However, relatively little is known regarding how these processes may be influenced by common naturalistic behaviors such as engaging in physical activity, which is thought to modulate the availability of neurometabolic resources. Here, we used an event-related potential (ERP) approach to determine whether various intensities of aerobic exercise might affect the concurrent performance of attentional control mechanisms. Participants performed an additional-singleton visual search task across three levels of aerobic activity while seated on a stationary bicycle: at rest, during moderate-intensity exercise, and during vigorous-intensity exercise. In addition to behavioral measures, attentional processing was assessed via lateralized ERPs referencing target selection (PCN) and distractor suppression (P_D ) mechanisms. Whereas engaging in exercise resulted in speeded response times overall, moderate-intensity exercise was found to uniquely eliminate the expression of distractor interference by the PCN while also giving rise to an unanticipated distractor-elicited Ppc. These findings demonstrate workload-specific and object-selective influences of aerobic exercise on attentional processing, providing insights not only for approaching attention in real-world contexts but also for understanding how attentional resources are used overall.

Extrafoveal Processing in Categorical Search for Geometric Shapes: General Tendencies and Individual Variations

The paper addresses the capabilities and limitations of extrafoveal processing during a categorical visual search. Previous research has established that a target could be identified from the very first or without any saccade, suggesting that extrafoveal perception is necessarily involved. However, the limits in complexity defining the processed information are still not clear. We performed four experiments with a gradual increase of stimuli complexity to determine the role of extrafoveal processing in searching for the categorically defined geometric shape. The series of experiments demonstrated a significant role of extrafoveal processing while searching for simple two-dimensional shapes and its gradual decrease in a condition with more complicated three-dimensional shapes. The factors of objects' spatial orientation and distractor homogeneity significantly influenced both reaction time and the number of saccades required to identify a categorically defined target. An analysis of the individual p-value distributions revealed pronounced individual differences in using extrafoveal analysis and allowed examination of the performance of each particular participant. The condition with the forced prohibition of eye movements enabled us to investigate the efficacy of covert attention in the condition with complicated shapes. Our results indicate that both foveal and extrafoveal processing are simultaneously involved during a categorical search, and the specificity of their interaction is determined by the spatial orientation of objects, type of distractors, the prohibition to use overt attention, and individual characteristics of the participants.

Gaze dynamics of feature-based distractor inhibition under prior-knowledge and expectations

Prior information about distractor facilitates selective attention to task-relevant items and helps the optimization of oculomotor planning. In the present study, we capitalized on gaze-position decoding to examine the dynamics of attentional deployment in a feature-based attentional task that involved two groups of dots (target/distractor dots) moving toward different directions. In Experiment 1, participants were provided with target cues indicating the moving direction of target dots. The results showed that participants were biased toward the cued direction and tracked the target dots throughout the task period. In Experiment 2 and Experiment 3, participants were provided with cues that informed the moving direction of distractor dots. When the distractor cue varied on a trial-by-trial basis (Experiment 2), participants continuously monitored the distractor's direction. However, when the to-be-ignored distractor direction remained constant (Experiment 3), participants would strategically bias their attention to the distractor's direction before the cue onset to reduce the cost of redeployment of attention between trials and reactively suppress further attraction evoked by distractors during the stimulus-on stage. This functional dissociation reflected the distinct influence that expectation produced on ocular control. Taken together, these results suggest that monitoring the distractor's feature is a prerequisite for feature-based attentional inhibition, and this process is facilitated by the predictability of the distractor's feature.

Automatic Guidance (and Misguidance) of Visuospatial Attention by Acquired Scene Memory: Evidence From an N1pc Polarity Reversal

Visual search is facilitated when the target is repeatedly encountered at a fixed position within an invariant (vs. randomly variable) distractor layout—that is, when the layout is learned and guides attention to the target, a phenomenon known as contextual cuing. Subsequently changing the target location within a learned layout abolishes contextual cuing, which is difficult to relearn. Here, we used lateralized event-related electroencephalogram (EEG) potentials to explore memory-based attentional guidance (N = 16). The results revealed reliable contextual cuing during initial learning and an associated EEG-amplitude increase for repeated layouts in attention-related components, starting with an early posterior negativity (N1pc, 80–180 ms). When the target was relocated to the opposite hemifield following learning, contextual cuing was effectively abolished, and the N1pc was reversed in polarity (indicative of persistent misguidance of attention to the original target location). Thus, once learned, repeated layouts trigger attentional-priority signals from memory that proactively interfere with contextual relearning after target relocation.

Neural differences between chromatic- and luminance-driven attentional salience in visual search

Previous electroencephalographic research on attentional salience did not fully capture the complexities of low-level vision, which relies on both cone-opponent chromatic and cone-additive luminance mechanisms. We systematically varied color and luminance contrast using a visual search task for a higher contrast target to assess the degree to which the salience-computing attentional mechanisms are constrained by low-level visual inputs. In our first experiment, stimuli were defined by contrast that isolated chromatic or luminance mechanisms. In our second experiment, targets were defined by contrasts that isolated or combined achromatic and chromatic mechanisms. In both experiments, event-related potential waveforms contralateral and ipsilateral to the target were qualitatively different for chromatic- compared to luminance-defined stimuli. The same was true of the difference waves computed from these waveforms, with isoluminant stimuli eliciting a mid-latency posterior contralateral negativity (PCN) component and achromatic stimuli eliciting a complex of multiple components, including an early posterior contralateral positivity followed by a late-latency PCN. Combining color with luminance resulted in waveform and difference wave patterns equivalent to those of achromatic stimuli. When large levels of chromaticity contrast were added to targets with small levels of luminance contrast, PCN latency was speeded. In conclusion, the mechanisms underlying attentional salience are constrained by the low-level inputs they receive. Furthermore, speeded PCN latencies for stimuli that combine color and luminance signals compared to stimuli that contain luminance alone demonstrate that color and luminance channels are integrated during pre-attentive visual processing, before top-down allocation of attention is triggered.

Kanizsa-figure object completion gates selection in the attentional blink

Previous work has demonstrated that perceptual grouping modulates the selectivity of attention across space. By contrast, how grouping influences the allocation of attention over time is much less clear. This study investigated this issue, using an attentional blink (AB) paradigm to test how grouping influences the initial selection and the subsequent short-term memory consolidation of a target. On a given trial, two red Kanizsa-type targets (T1 and T2) with varying grouping strength were embedded in a rapid serial visual presentation stream of irrelevant distractors. Our results showed the typical AB finding: impaired identification of T2 when presented close in time following T1. Moreover, the AB was modulated by the T2 grouping-independently of the T1 structure-with stronger grouping leading to a decreased AB and overall higher performance. Conversely, a reversed pattern, namely an increased AB with increasing grouping strength was observed when the Kanizsa figure was not task-relevant. Together, these findings suggest that the grouping benefit emerges at early perceptual stages, automatically drawing attentional resources, thereby leading to either sustained benefits or transient costs-depending on the task-relevance of the grouped object. This indicates that grouping modulates processing of objects in time.

Electrophysiological evidence for temporal dynamics associated with attentional processing in the zoom lens paradigm

Background

Visuospatial processing requires wide distribution or narrow focusing of attention to certain regions in space. This mechanism is described by the zoom lens model and predicts an inverse correlation between the efficiency of processing and the size of the attentional scope. Little is known, however, about the exact timing of the effects of attentional scaling on visual searching and whether or not additional processing phases are involved in this process.

Method

Electroencephalographic recordings were made while participants performed a visual search task under different attentional scaling conditions. Two concentric circles of different sizes, presented to the participants at the center of a screen modulated the attentional scopes, and search arrays were distributed in the space areas indicated by these concentric circles. To ensure consistent eccentricity of the search arrays across different conditions, we limited our studies to the neural responses evoked by the search arrays distributed in the overlapping region of different attentional scopes.

Results

Consistent with the prediction of the zoom lens model, our behavioral data showed that reaction times for target discrimination of search arrays decreased and the associated error rates also significantly decreased, with narrowing the attentional scope. Results of the event-related potential analysis showed that the target-elicited amplitude of lateral occipital N1, rather than posterior P1, which reflects the earliest visuospatial attentional processing, was sensitive to changes in the scaling of visuospatial attention, indicating that the modulation of the effect of changes in the spatial scale of attention on visual processing occurred after the delay period of P1. The N1 generator exhibited higher activity as the attentional scope narrowed, reflecting more intensive processing resources within the attentional focus. In contrast to N1, the amplitude of N2pc increased with the expansion of the attentional focus, suggesting that observers might further redistribute attentional resources according to the increased task difficulty.

Conclusion

These findings provide electrophysiological evidence that the neural activity of the N1 generator is the earliest marker of the zoom lens effect of visual spatial attention. Furthermore, evidence from N2pc shows that there is also a redistribution of attentional resources after the action of the zoom lens mechanism, which allows for better perform of the search task in the context of low attentional resolution. On the basis of the timing of P1, N1, and N2pc, our findings provide compelling evidence that visuospatial attention processing in the zoom lens paradigm involves multi-stage dynamic processing.

Event-related Electroencephalographic Lateralizations Mark Individual Differences in Spatial and Nonspatial Visual Selection

Selective attention controls the distribution of our visual system's limited processing resources to stimuli in the visual field. Two independent parameters of visual selection can be quantified by modeling an individual's performance in a partial-report task based on the computational theory of visual attention (TVA): (i) top-down control α, the relative attentional weighting of relevant over irrelevant stimuli, and (ii) spatial bias w_λ, the relative attentional weighting of stimuli in the left versus right hemifield. In this study, we found that visual event-related electroencephalographic lateralizations marked interindividual differences in these two functions. First, individuals with better top-down control showed higher amplitudes of the posterior contralateral negativity than individuals with poorer top-down control. Second, differences in spatial bias were reflected in asymmetries in earlier visual event-related lateralizations depending on the hemifield position of targets; specifically, individuals showed a positivity contralateral to targets presented in their prioritized hemifield and a negativity contralateral to targets presented in their nonprioritized hemifield. Thus, our findings demonstrate that two functionally different aspects of attentional weighting quantified in the respective TVA parameters are reflected in two different neurophysiological measures: The observer-dependent spatial bias influences selection by a bottom-up processing advantage of stimuli appearing in the prioritized hemifield. By contrast, task-related target selection governed by top-down control involves active enhancement of target, and/or suppression of distractor, processing. These results confirm basic assumptions of the TVA framework, complement the functional interpretation of event-related lateralization components in selective attention studies, and are of relevance for the development of neurocognitive attentional assessment procedures.

Attending to multiple objects relies on both feature- and dimension-based control mechanisms: Evidence from human electrophysiology

Numerous everyday search tasks require humans to attentionally select and temporally store more than one object present in the visual environment. Recently, several enumeration studies sought to isolate the mechanisms underlying multiple object processing by means of electrophysiological measures, which led to a more fine-grained picture as to which processing stages are modulated by object numerosity. One critical limitation that most of these studies share is that they used stimulus designs in which multiple targets were exclusively defined by the same feature value. Accordingly, it remains an open issue whether these findings generalize to search scenarios in which multiple targets are physically not identical. To systematically address this issue, we introduced three target context conditions in which multiple targets were defined randomly by (1) the same feature (sF), (2) different features within the same dimension (dFsD), or (3) different features across dimensions (dD). Our findings revealed that participants' ability to enumerate multiple targets was remarkably influenced by inter-target relationships, with fastest responses for sF trials, slowest responses for dD trials, and responses of intermediate speed for dFsD trials. Our electrophysiological analyses disclosed that one source of this response slowing was feature-based and originated from the stage of attentional selection (as indexed by PCN waves), whereas another source was dimension-based and associated with working memory processes (as indexed by P3b waves). Overall, our results point to a significant role of physical inter-target relationships in multiple object processing-a factor that has been largely neglected in most studies on enumeration.

Different Measures of Structural Similarity Tap Different Aspects of Visual Object Processing

The structural similarity of objects has been an important variable in explaining why some objects are easier to categorize at a superordinate level than to individuate, and also why some patients with brain injury have more difficulties in recognizing natural (structurally similar) objects than artifacts (structurally distinct objects). In spite of its merits as an explanatory variable, structural similarity is not a unitary construct, and it has been operationalized in different ways. Furthermore, even though measures of structural similarity have been successful in explaining task and category-effects, this has been based more on implication than on direct empirical demonstrations. Here, the direct influence of two different measures of structural similarity, contour overlap and within-item structural diversity, on object individuation (object decision) and superordinate categorization performance is examined. Both measures can account for performance differences across objects, but in different conditions. It is argued that this reflects differences between the measures in whether they tap: (i) global or local shape characteristics, and (ii) between- or within-category structural similarity.

Attentional weights in vision as products of spatial and nonspatial components

The relationship between visual attentional selection of items in particular spatial locations and selection by nonspatial criteria was investigated in a partial report experiment with report of letters (as many as possible) from brief postmasked exposures of circular arrays of letters and digits. The data were fitted by mathematical models based on Bundesen's (Psychological Review, 97, 523-547, 1990) theory of visual attention (TVA). Both attentional weights of targets (letters) and attentional weights of distractors (digits) showed strong variations across the eight possible target locations, but for each of the ten participants, the ratio of the weight of a distractor at a given location to the weight of a target at the same location was approximately constant. The results were accommodated by revising the weight equation of TVA such that the attentional weight of an object equals a product of a spatial weight component (weight due to being at a particular location) and a nonspatial weight component (weight due to having particular features other than locations), the two components scaling the effects of each other multiplicatively.

Item-based selection is in good shape in visual compound search: A view from electrophysiology

We argue that although the framework put forward by Hulleman & Olivers (H&O) can successfully explain much of visual search behaviour, it appears limited to tasks without precise target identification demands. In particular, we contend that the unit of selection may be larger than a single item in standard detection tasks, whereas the unit may mandatorily be item-based in compound tasks.

Gestalt Perceptual Organization of Visual Stimuli Captures Attention Automatically: Electrophysiological Evidence

The visual system leverages organizational regularities of perceptual elements to create meaningful representations of the world. One clear example of such function, which has been formalized in the Gestalt psychology principles, is the perceptual grouping of simple visual elements (e.g., lines and arcs) into unitary objects (e.g., forms and shapes). The present study sought to characterize automatic attentional capture and related cognitive processing of Gestalt-like visual stimuli at the psychophysiological level by using event-related potentials (ERPs). We measured ERPs during a simple visual reaction time task with bilateral presentations of physically matched elements with or without a Gestalt organization. Results showed that Gestalt (vs. non-Gestalt) stimuli are characterized by a larger N2pc together with enhanced ERP amplitudes of non-lateralized components (N1, N2, P3) starting around 150 ms post-stimulus onset. Thus, we conclude that Gestalt stimuli capture attention automatically and entail characteristic psychophysiological signatures at both early and late processing stages. Highlights We studied the neural signatures of the automatic processes of visual attention elicited by Gestalt stimuli. We found that a reliable early correlate of attentional capture turned out to be the N2pc component. Perceptual and cognitive processing of Gestalt stimuli is associated with larger N1, N2, and P3.

The effect of modulating top-down attention deployment on the N2pc/PCN

The N2pc (PCN) component of the event-related potential (ERP) waveform provides a useful tool for directly assessing the locus of spatial attention in visual search. It is still unclear whether the amplitude of the N2pc/PCN relates to the deployment of attentional resources. A key issue is the lack of evidence that top-down allocation of attention affects the N2pc/PCN amplitude. Previous findings could be explained if manipulating different expectancy strategies changes participants' search mode, causing them to redefine the target's features. In this study, we explored the relationship between N2pc/PCN amplitude and top-down attention allocation by manipulating the discriminative difficulty (differences in the response-defining feature) but leaving the search difficulty (target's saliency) unchanged. Using the same sets of stimuli, in a blocked condition, participants showed the expected higher amplitude of N2pc/PCN in the hardest condition, compared to easier discrimination conditions. Importantly, there was no difference in the N2pc/PCN when the exact same stimulus sets were presented in a randomly interleaved mixed set. At a behavioral level, in both conditions performance was significantly slower for the hardest condition. This finding indicates that the N2pc/PCN component is modulated by the predictability of discriminative difficulty, which reflects the modulation of top-down attentional deployment.

Searching for targets in visual working memory: investigating a dimensional feature bundle (DFB) model

The human visual working memory (WM) system enables us to store a limited amount of task-relevant visual information temporally in mind. One actively debated issue in cognitive neuroscience centers around the question of how this WM information is maintained. The currently dominant views advocated by prominent WM models hold that the units of memory are configured either as independent feature representations, integrated bound objects, or a combination of both. Here, we approached this issue by measuring lateralized brain electrical activity during a retro-cue paradigm, in order to track people's ability to access WM representations as a function of the dimensional relation between WM items and task settings. Both factors were revealed to selectively influence WM access: whereas cross relative to intradimensional WM targets gave rise to enhanced contralateral delay activity (CDA) amplitudes, localization relative to identification task demands yielded speeded CDA and manual response times. As these dimension-based findings are not reconcilable with contemporary feature- and/or object-based accounts, an alternative view that is based on the hierarchical feature-bundle model is proposed. We argue that WM units may consist of three hierarchically structured levels of representations, with an intermediate dimensionally organized level that mediates between top-level object and lower-level feature representations.