Auditory change detection and visual selective attention: association between MMN and N2pc

While the auditory and visual systems each provide distinct information to our brain, they also work together to process and prioritize input to address ever-changing conditions. Previous studies highlighted the trade-off between auditory change detection and visual selective attention; however, the relationship between them is still unclear. Here, we recorded electroencephalography signals from 106 healthy adults in three experiments. Our findings revealed a positive correlation at the population level between the amplitudes of event-related potential indices associated with auditory change detection (mismatch negativity) and visual selective attention (posterior contralateral N2) when elicited in separate tasks. This correlation persisted even when participants performed a visual task while disregarding simultaneous auditory stimuli. Interestingly, as visual attention demand increased, participants whose posterior contralateral N2 amplitude increased the most exhibited the largest reduction in mismatch negativity, suggesting a within-subject trade-off between the two processes. Taken together, our results suggest an intimate relationship and potential shared mechanism between auditory change detection and visual selective attention. We liken this to a total capacity limit that varies between individuals, which could drive correlated individual differences in auditory change detection and visual selective attention, and also within-subject competition between the two, with task-based modulation of visual attention causing within-participant decrease in auditory change detection sensitivity.

The Distractor Positivity Component and the Inhibition of Distracting Stimuli

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

Spatial and chromatic properties of numerosity estimation in isolation and context

Numerosity estimation around the subitizing range is facilitated by a shape-template matching process and shape-coding mechanisms are selective to visual features such as colour and luminance contrast polarity. Objects in natural scenes are often embedded within other objects or textured surfaces. Numerosity estimation is improved when objects are grouped into small clusters of the same colour, a phenomenon termed groupitizing, which is thought to leverage on the subitizing system. Here we investigate whether numerosity mechanisms around the subitizing range are selective to colour, luminance contrast polarity and orientation, and how spatial organisation of context and target elements modulates target numerosity estimation. Stimuli consisted of a small number (3-to-6) of target elements presented either in isolation or embedded within context elements. To examine selectivity to colour, luminance polarity and orientation, we compared target-only conditions in which all elements were either the same or different along one of these feature dimensions. We found comparable performance in the same and different feature conditions, revealing that subitizing mechanism do not depend on ‘on-off’ luminance-polarity, colour or orientation channel interactions. We also measured the effect of varying spatial organisation of (i) context, by arranging the elements either in a grid, mirror-symmetric, translation-symmetric or random; (ii) target, by placing the elements either mirror-symmetric, on the vertices of simple shapes or random. Our results indicate higher accuracy and lower RTs in the grid compared to all other context types, with mirror symmetric, translation and random arrangements having comparable effects on target numerosity. We also found improved performance with shape-target followed by symmetric and random target arrangements in the absence and presence of context. These findings indicate that numerosity mechanisms around the subitizing range are not selective to colour, luminance polarity and orientation, and that symmetric, translation and random contexts organisations inhibit target-numerosity encoding stronger than regular/grid context.

Visual field inhomogeneous in brain-computer interfaces based on rapid serial visual presentation

OBJECTIVE

Visual attention is not homogeneous across the visual field, while how to mine the effective EEG characteristics that are sensitive to the inhomogeneous of visual attention and further explore applications such as the performance of brain-computer interface (BCI) are still distressing explorative scientists.

APPROACH

Images were encoded into a rapid serial visual presentation (RSVP) paradigm, and were presented in three visuospatial patterns (central, left/right, upper/lower) at the stimulation frequencies of 10Hz, 15Hz and 20Hz. The comparisons among different visual fields were conducted in the dimensions of subjective behavioral and EEG characteristics. Furthermore, the effective features (e.g. SSVEP, N2pc and P300) that sensitive to visual-field asymmetry were also explored.

RESULTS

The visual fields had significant influences on the performance of RSVP target detection, in which the performance of central was better than that of peripheral visual field, the performance of horizontal meridian was better than that of vertical meridian, the performance of left visual field was better than that of right visual field, and the performance of upper visual field was better than that of lower visual field. Furthermore, stimuli of different visual fields had significant effects on the spatial distributions of EEG, in which N2pc and P300 showed left-right asymmetry in occipital and frontal regions, respectively. In addition, the evidences of SSVEP characteristics indicated that there was obvious overlap of visual fields on the horizontal meridian, but not on the vertical meridian.

SIGNIFICANCE

The conclusions of this study provide insights into the relationship between visual field inhomogeneous and EEG characteristics. In addition, this study has the potential to achieve precise positioning of the target's spatial orientation in RSVP-BCIs.

Eye movements are not mandatorily preceded by the N2pc component

Researchers typically distinguish between two mechanisms of attentional selection in vision: overt and covert attention. A commonplace assumption is that overt eye movements are automatically preceded by shifts of covert attention during visual search. Although the N2pc component is a putative index of covert attentional orienting, little is currently known about its relationship with overt eye movements. This is because most previous studies of the N2pc component prohibit overt eye movements. The current study assessed this relationship by concurrently measuring covert attention (via the N2pc) and overt eye movements (via eye tracking). Participants searched displays for a lateralized target stimulus and were allowed to generate overt eye movements during the search. We then assessed whether overt eye movements were preceded by the N2pc component. The results indicated that saccades were preceded by an N2pc component, but only when participants were required to carefully inspect the target stimulus before initiating the eye movement. When participants were allowed to make naturalistic eye movements in service of visual search, there was no evidence of an N2pc component before eye movements. These findings suggest that the N2pc component does not always precede overt eye movements during visual search. Implications for understanding the relationship between covert and overt attention are discussed.

Spatially Specific Attention Mechanisms Are Sensitive to Competition during Visual Search

Extensive studies have focused on selection mechanisms during visual search. One important influence on these mechanisms is the perceptual characteristics of the stimuli. We investigated the impact of perceptual similarity between targets and nontargets (T-N similarity) in a visual search task using EEG. Participants searched for a predefined target letter among five nontargets. The T-N similarity was manipulated with three levels: high, middle, and low. We tested for the influences of T-N similarity on an ERP (e.g., N2pc) and alpha oscillations. We observed a significant N2pc effect across all levels of similarity. The N2pc amplitude was reduced and occurred later for high similarity relative to low and middle similarities. We also showed that the N2pc amplitude was inversely correlated with the RTs across all similarities. Importantly, we found a significant alpha phase adjustment about the same time as the N2pc for high similarity; by contrast, no such effect was observed for middle and low similarities. Finally, we showed a positive correlation between the phase-locking value and the N2pc—the stronger the alpha phase-locking value, the larger the N2pc, when the T-N similarity was high. In conclusion, our results provide novel evidence for multiple competitive mechanisms during visual search.

The success of the representation maintenance affects the memory-guided search processing: an ERP study

Previous evidence showed that working memory (WM) contents can bias visual selection. However, less is known about how the WM effects change when the WM representation is not held successfully. Here, we investigated this problem using event-related potentials. Subjects maintained a color in WM while performing a search task. The color cue contained the target (valid) or the distractor (invalid). Subjects could either remember the color accurately (correct WM) or not (incorrect WM). An N2-posterior contralateral component and a sustained posterior contralateral negativity (SPCN) were recorded in the valid and incorrect WM condition, while only an attenuated SPCN was elicited in the valid and correct WM condition. No reliable lateralized components were found for the invalid trials. These findings suggest that the WM effects on visual search are affected by the resource interchange between WM and search processes.

Reduced visual attention in heterogeneous textures is reflected in occipital alpha and theta band activity

Increasing context heterogeneity has been found to reduce attention deployment towards an embedded target item. Heterogeneity in visual search tasks is typically induced by segmenting the background into several perceptual groups. In the present study, however, context heterogeneity was induced by varying whole-field heterogeneity, i.e., the degree of distractor variability within the entire context. This allowed us to (i) more gradually vary context heterogeneity, and (ii) investigate attention deployment on a whole-field scale. Results showed that both search performance and amplitude of the N2pc (lateralized ERP; posterior contralateral negativity in the N2 range) monotonically decreased with increasing context heterogeneity, which confirmed that there was less efficient attention deployment for more heterogeneous contexts. The amplitude of the bilateral N2 exhibited a U-shaped function, suggesting global perception for the lowest and highest levels of heterogeneity, but local processing for intermediate heterogeneity levels. Independent component analyses revealed an occipital ERP-contributing effective source cluster that may reflect stimulus representations on a saliency map. With increasing heterogeneity, these sources exhibited more theta band activity for distractors and less theta band activity for targets. Alpha band activity of a second component cluster varied with heterogeneity level, and low-theta/delta activity of a third source cluster distinguished target presence versus absence. In sum, our results suggest that independent brain sources contributed to the differential processing of heterogeneous versus homogeneous contexts.

Simultanagnosia and object individuation

Simultanagnosic patients have difficulty in perceiving multiple objects when presented simultaneously. In this review article, I discuss how neuropsychological research on simultanagnosia has been inspirational for two interconnected lines of research related to the core mechanisms by which the visual system processes cluttered scenes. First, I review previous studies on enumeration tasks indicating that, despite their inability to identify multiple objects, simultanagnosic patients can enumerate up to 2-3 elements as efficiently as healthy individuals (the so-called "subitizing" phenomenon). This intriguing observation is one of the first results to support the existence of an "object individuation" mechanism that can spatially tag a limited set of objects simultaneously, and resonates with recent research on the brain dynamics of enumeration in healthy individuals. Second, I further develop the implications of the dissociation between object identification and object enumeration in simultanagnosia specifically for the distinction between object identification and individuation. The latter distinction has been the subject of recent neuroimaging research that has provided fine-grained information on the spatial as well as temporal aspects of object individuation and recognition. The lessons learned from neuropsychological research on exact enumeration in simultanagnosia can be generalized to the normal functioning of the human mind, and have provided insightful clues for cognitive neuroscience.

Neural signatures of adaptive post-error adjustments in visual search

Errors in speeded choice tasks can lead to post-error adjustments both on the behavioral and on the neural level. There is an ongoing debate whether such adjustments result from adaptive processes that serve to optimize performance or whether they reflect interference from error monitoring or attentional orientation. The present study aimed at identifying adaptive adjustments in a two-stage visual search task, in which participants had to select and subsequently identify a target stimulus presented to the left or right visual hemifield. Target selection and identification can be measured by two distinct event-related potentials, the N2pc and the SPCN. Using a decoder analysis based on multivariate pattern analysis, we were able to isolate the processing stages related to error sources and post-error adjustments. Whereas errors were linked to deviations in the N2pc and the SPCN, only for the N2pc we identified a post-error adjustment, which exhibits key features of source-specific adaptivity. While errors were associated with an increased N2pc, post-error adjustments consisted in an N2pc decrease. We interpret this as an adaptive adjustment of target selection to prevent errors due to disproportionate processing of the task-irrelevant target location. Our study thus provides evidence for adaptive post-error adjustments in visual search.

Active suppression of salient-but-irrelevant stimuli does not underlie resistance to visual interference

In visual search for a shape target, interference from salient-but-irrelevant color singletons can be resisted in feature search mode, but not in singleton detection mode. In singleton detection mode, we observed a contralateral positivity (P_D) after 260-340ms, suggesting that the salient distractor was suppressed. Because RTs in singleton detection mode increased when a distractor was present, we conclude that active suppression of distractors takes time. In feature search mode, no increase in RTs and no P_D to the distractor was observed, showing that resistance to interference was not accomplished by suppression. Rather, the smaller N2pc to the target in feature search than in singleton detection mode suggests that enhancement of target features avoided interference. Thus, the strong top-down set in feature search mode eliminated the need to suppress the early attend-to-me signal (corresponding to the Ppc, from 160 to 210ms) that was generated by salient stimuli independently of search mode.

Individual differences in perceptual abilities predict target visibility during masking

Many studies have shown that the visual system can implicitly process a single stimulus under conditions of low visibility. However, it remains unknown whether this ability extends when viewing conditions become more difficult, and whether differences in early perceptual abilities modulate masking sensitivity. To address these issues, participants enumerated a variable number of target elements among distracters in two electroencephalography experiments. Either one (Experiment 1) or all targets (Experiment 2) were masked through object-substitution. Results showed that an event-related potential measure of selective individuation, the N2pc component, was modulated by target numerosity in both masked and unmasked trials, suggesting that multiple object individuation can operate in conditions of limited visibility. However, this effect was present mainly for participants with low masking effects, who overall showed more pronounced N2pc modulations as a function of target numerosity. Finally, oscillatory activity analyses revealed that early segmentation mechanisms, as reflected by lateralized gamma synchronization, were more active in participants with low sensitivity to masking, suggesting that individual variation in early perceptual functions is associated with susceptibility to masking such that more efficient segmentation and individuation mechanisms reduce the effects of masking. These findings cast doubt on the claim that effectively masked stimuli can be individuated.

Failure of temporal selectivity: Electrophysiological evidence for (mis)selection of distractors during the attentional blink

The attentional blink (AB) refers to the impairment in accurate report of a second target (T2) when presented shortly after a first target (T1) in a rapid serial visual presentation of distractors. The goal of the present study was to determine whether the AB is caused by a failure in early selection processes, which leads to the selection and consolidation of the wrong item in working memory, by measuring the frequency-related P3 to T2 and to T2 + 1. During the AB, an attenuation of the P3 to T2 was observed, as well as an increase in the amplitude of the P3 to T2 + 1. Whereas the P3 to T2 was observed only when T2 was correctly reported, the P3 to T2 + 1 was observed only in trials where T2 was incorrectly reported, and its amplitude was correlated to individual differences in misselection rate. These results support the claim that failure of temporal selection underlies the AB.

Electrophysiological evidence for failures of item individuation in crowded visual displays

Visual perception is strongly impaired when peripheral targets are surrounded by nearby distractors, a phenomenon known as visual crowding. One common behavioral signature of visual crowding is an increased tendency for observers to mistakenly report the features of nearby distractors instead of the target item. Here, our goal was to distinguish between two possible explanations of such substitution errors. On the one hand, crowding may have its effects after the deployment of attention toward-and individuation of-targets and flankers, such that multiple individuated perceptual representations compete to guide the behavioral response. On the other hand, crowding may prevent the individuation of closely spaced stimuli, thereby reducing the number of apprehended items. We attempted to distinguish these alternatives using the N2pc, an ERP that has been shown to track the deployment of spatial attention and index the number of individuated items within a hemifield. N2pc amplitude increased monotonically with set size in uncrowded displays, but this set size effect was abolished in crowded visual displays. Moreover, these crowding-induced declines in N2pc amplitude predicted individual differences in the rate of substitution errors. Thus, crowding-induced confusions between targets and distractors may be a consequence of failures to individuate target and distractor stimuli during early stages of visual selection.

The attentional blink freezes spatial attention allocation to targets, not distractors: evidence from human electrophysiology

Previous work found a significant reduction of the amplitude of the N2pc ERP component during the attentional blink in response to lateral visual targets, suggesting that the allocation of attention to visual targets is impaired during the attentional blink. Recent theorizing on the processes reflected by the N2pc suggests the possibility of distinct sets of neural mechanisms underlying its generation, one responsible for target activation, and one for distractor inhibition. To disentangle whether either or both of these mechanisms are impaired during the attentional blink, an RSVP sequence of circles, equidistant from fixation was used. The first target frame (T1) contained the same repeated target colour circle and target whereas the second target frame (T2) contained a distractor colour singleton as well as a target colour singleton. Only the target or only the distractor was presented at a lateral position; the other singleton was presented on the vertical midline so as not to elicit any event-related lateralization. Impaired T2 report accuracy at a short stimulus-onset asynchrony (SOA) was accompanied by a significant delay of the N2pc to lateral T2 targets when compared to a long SOA condition. No such delay was found when the lateralized stimulus was a distractor, suggesting that the attentional blink impacts attention allocation to targets, not distractors. We also observed a lateralized component earlier than the N2pc, a posterior contralateral positivity (Ppc) that did not depend on T1-T2 SOA and that was elicited by both lateral targets and distractors. We conclude that, contrary to N2pc, the Ppc likely reflects activity of bottom-up mechanisms responding unselectively to asymmetrical visual displays.

Bias for the Left Visual Field in Rapid Serial Visual Presentation: Effects of Additional Salient Cues Suggest a Critical Role of Attention

Everyday experience suggests that people are equally aware of stimuli in both hemifields. However, when two streams of stimuli are rapidly presented left and right, the second target (T2) is better identified in the left hemifield than in the right hemifield. This left visual field (LVF) advantage may result from differences between hemifields in attracting attention. Therefore, we introduced a visual cue shortly before T2 onset to draw attention to one stream. Thus, to identify T2, attention was correctly positioned with valid cues but had to be redirected to the other stream with invalid ones. If the LVF advantage is caused by differences between hemifields in attracting attention, invalid cues should increase, and valid cues should reduce the LVF advantage as compared with neutral cues. This prediction was confirmed. ERP analysis revealed that cues evoked an early posterior negativity, confirming that attention was attracted by the cue. This negativity was earlier with cues in the LVF, which suggests that responses to salient events are faster in the right hemisphere than in the left hemisphere. Valid cues speeded up, and invalid cues delayed T2-evoked N2pc; in addition, valid cues enlarged T2-evoked P3. After N2pc, right-side T2 evoked more sustained contralateral negativity than left T2, least long-lasting after valid cues. Difficulties in identifying invalidly cued right T2 were reflected in prematurely ending P3 waveforms. Overall, these data provide evidence that the LVF advantage is because of different abilities of the hemispheres in shifting attention to relevant events in their contralateral hemifield.

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.

Specific fear modulates attentional selectivity during visual search: electrophysiological insights from the N2pc

In the present study, we used high-density EEG during a visual search task to investigate the dynamics of spatial attention to fear-relevant targets and background stimuli in small animal phobia during a visual search task. Twenty-five spider fearful (22 females) and 25 healthy nonfearful participants (19 females) were measured, while searching for discrepant objects in visual arrays. Compared to nonfearful participants, spider fearful individuals showed a more enhanced posterior N2pc to spider (vs. butterfly) targets in an array of flowers. Furthermore, spider fearful participants showed enhanced hypervigilance for all presented stimuli compared to controls as reflected by enhanced N1 amplitudes (160-200 ms). Our findings provide neural evidence for early, enhanced selective spatial attention for fear-relevant stimuli.

Perceptual grouping and visual enumeration

We used lateralized Event-Related Potential (ERP) measures - the N2pc and CDA/SPCN components - to assess the role of grouping by target similarity during enumeration. Participants saw a variable number (0, 1, 2 or 3) of same- or differently-colored targets presented among homogeneous distracters, and performed an enumeration task. Results showed that the N2pc, but not the CDA, was larger for multiple targets of identical color relative to targets of different colors. The findings are interpreted in terms of the effects of grouping on early versus late stages of multiple object processing. Within this framework, they reveal that grouping has an effect on early individuation mechanisms, while later processing mechanisms are less prone to such an influence.

Temporal variability of the N2pc during efficient and inefficient visual search

Efficient and inefficient visual search are characterized by the difference in the time required to find the target. Efficient "popout" search time is relatively unaffected by increases in the number of search items, whereas inefficient "non-popout" search time requires more time increases in duration. Electrophysiological investigations of the neural correlates of visual search have revealed a component of the event-related potential (ERP) known as the N2pc. The N2pc is thought to reflect the orienting of attention to the target during visual search. If the N2pc is more closely associated in time with the moment of target selection than the onset of the search display, it may be predicted that the N2pc would be delayed or more temporally variable during inefficient compared to efficient visual search. In the present study, we contrasted efficient "popout" search with inefficient "non-popout" search to investigate the hypothesis that the N2pc is temporally associated with the moment of search completion, and this would be reflected as a delayed or attenuated N2pc during non-popout search compared to popout. In Experiment 1, we observed a robust N2pc during popout search, but not during non-popout search. However, sorting trials by the N2pc latency recovered the N2pc during non-popout search. In Experiment 2 we replicated this observation and demonstrated that popout and non-popout search reflected differences in the temporal variability of the N2pc latency. Further investigation using time-spectral analysis suggests that evoked posterior-contralateral theta (4-6 Hz) underlies the N2pc during both popout and non-popout search.

The face is more than its parts--brain dynamics of enhanced spatial attention to schematic threat

A rapid response to environmental threat is crucial for survival and requires an appropriate attention allocation toward its location. Visual search paradigms have provided evidence for the enhanced capture of attention by threatening faces. In two EEG experiments, we sought to determine whether the detection of threat requires complete faces or salient features underlying the facial expression. Measuring the N2pc component as an electrophysiological indicator of attentional selection we investigated participants searching for either a complete discrepant schematic threatening or friendly face within an array of neutral faces, or single features (eyebrows and eyes vs. eyebrows) of threatening and friendly faces. Threatening faces were detected faster compared to friendly faces. In accordance, threatening angry targets showed a more pronounced occipital N2pc between 200 and 300 ms than friendly facial targets. Moreover, threatening configurations, were detected more rapidly than friendly-related features when the facial configuration contained eyebrows and eyes. No differences were observed when only a single feature (eyebrows) had to be detected. Threatening-related and friendly-related features did not show any differences in the N2pc across all configuration conditions. Taken together, the findings provide direct electrophysiological support for rapid prioritized attention to facial threat, an advantage that seems not to be driven by low level visual features.

Temporal brain dynamics of multiple object processing: the flexibility of individuation

The ability to process concurrently multiple visual objects is fundamental for a coherent perception of the world. A core component of this ability is the simultaneous individuation of multiple objects. Many studies have addressed the mechanism of object individuation but it remains unknown whether the visual system mandatorily individuates all relevant elements in the visual field, or whether object indexing depends on task demands. We used a neural measure of visual selection, the N2pc component, to evaluate the flexibility of multiple object individuation. In three ERP experiments, participants saw a variable number of target elements among homogenous distracters and performed either an enumeration task (Experiment 1) or a detection task, reporting whether at least one (Experiment 2) or a specified number of target elements (Experiment 3) was present. While in the enumeration task the N2pc response increased as a function of the number of targets, no such modulation was found in Experiment 2, indicating that individuation of multiple targets is not mandatory. However, a modulation of the N2pc similar to the enumeration task was visible in Experiment 3, further highlighting that object individuation is a flexible mechanism that binds indexes to object properties and locations as needed for further object processing.

Cooperative and opposing effects of strategic and involuntary attention

To assess whether working memory contents can effectively bias visual selection even when they do not represent the current target in the attention task, we recorded the ERP activity from participants performing both a memory task and, in the retention period, a visual search task. In this task, a distracter matching the memory content could be presented on the same side (congruent trials) or on the opposite side (incongruent trials) relative to the target location (Experiment 1 and Experiment 2). On some trials, only the matching distracter (but no target) was presented (catch trials, Experiment 2). Results showed that the N2pc component was modulated by the presence and location of a matching distracter. We interpret these results as evidence that the involuntary control exerted by the irrelevant memory contents coexists with the strategic mechanism related to the search target, influencing attention selection with roughly equal power. In Experiment 3, we found that the modulation of the N2pc is not strictly related to the active maintenance of the memory-target features but can also be elicited by repetition priming. Overall, these findings suggest that, together with the physical properties of the stimuli presented in the visual field, irrelevant memory contents represent a powerful class of factors that lead to involuntary attentional control.

Neural representation of feature synergy

Interactive non-linear cooperation of different feature dimensions, feature synergy, has been studied in psychophysics, but the neural mechanism is unknown. The present study investigated the neural representation of feature synergy of two second-order visual features by combining electroencephalography (EEG) with the signal detection theory (SDT). Two kinds of a 27-by-27 array of Gabor patches were presented in a random order; a reference stimulus which has no segregated region, and a target stimulus whose inner region differed in spatial frequency, orientation, or both from the surround. Subjects performed a Yes-No discrimination of whether the inner region was different from the surround, while EEG signals were recorded from 62 locations. When the SDT measure showed feature synergy, EEG activity showed a long-lasting enhancement starting at 130 ms around the inferior temporal region. In contrast, no EEG modulation was observed when feature synergy was not present. Thus, our combined approach demonstrates that non-linear cooperation between different features is represented by neural activity starting at 130 ms post-stimulus in the ventral visual stream.

Capture versus suppression of attention by salient singletons: electrophysiological evidence for an automatic attend-to-me signal

There is considerable controversy about whether salient singletons capture attention in a bottom-up fashion, irrespective of top-down control settings. One possibility is that salient singletons always generate an attention capture signal, but this signal can be actively suppressed to avoid capture. In the present study, we investigated this issue by using event-related potential recordings, focusing on N2pc (N2-posterior-contralateral; a measure of attentional deployment) and Pd (distractor positivity; a measure of attentional suppression). Participants searched for a specific letter within one of two regions, and irrelevant color singletons were sometimes present. We found that the irrelevant singletons did not elicit N2pc but instead elicited Pd; this occurred equally within the attended and unattended regions. These findings suggest that salient singletons may automatically produce an attend-to-me signal, irrespective of top-down control settings, but this signal can be overridden by an active suppression process to prevent the actual capture of attention.