objects were defined by a known color (e.g., Luck & Hillyard, 1994; Kiss et al., 2009; Grubert & Eimer, 2013), and demonstrates that target objects can be selected rapidly when this selection process is guided by a color-specific attentional template. In contrast, no N2pc component was present during the first 260 ms after search display onset for horizontal variable-color targets, in line with our prediction that participants would adopt the instructed serial selection strategy, with attention being directed first to the fixed-color target before being allocated to the variable-color target. The absence of an early N2pc to horizontal variable-color targets that were accompanied by a fixed-color target on the vertical meridian demonstrates that during the initial phase of attentional processing, attention was exclusively focused on the fixed-color target before it was allocated to the variable-color target. If serial movements of attention to new locations in the visual field generally require about 250-300 ms, as suggested by some previous behavioral studies (e.g., e.g., Reeves & Sperling, 1986; Sperling & Weichselgartner, 1995), this should have been reflected by a much longer delay of the N2pc to variable-color targets relative to fixed-color targets, with the variable-color target N2pc only emerging at post-stimulus latencies beyond 400 ms. However, this was clearly not the case. The N2pc to variable-color targets was triggered within 260 ms after search display onset, that is, only approximately 60 ms after the onset of the N2pc to fixed-color targets. This observation suggests that serial movements of focal attention during priority-driven search can be initiated remarkably rapidly, at speeds that are in line with the estimates of attentional shift times that were derived from the search slopes observed in visual search experiments (e.g., Wolfe, 1998a, 1998b).
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On no-competition trials, reliable N2pc components were elicited not only by targets, but also by colour-matching or shape-matching nontarget objects, demonstrating that these objects were able to attract attention. The fact that target-absent RTs were slower and error rates higher when displays contained a partially matching nontarget object relative to distractor-only displays (Figure 2) provides further evidence that these objects could not be completely ignored. The presence of N2pc components to partially target- matching objects demonstrates that some aspects of attentional selection in visual search operate in a feature-based fashion, in the sense that they are controlled independently by guidance signals from different feature dimensions. N2pc components elicited by partially matching nontargets on no-competition trials were smaller than the N2pc to target objects. In fact, the sum of the two N2pc components to colour-matching and shape-matching nontargets was virtually identical to the N2pc component to targets during the early phase of spatially selective attentional processing until around 250 ms post-stimulus (Figure 3, bottom panel). This finding strongly suggests that the spatial bias produced by signals from colour and shape modules affects the initial allocation of spatial attention in the visual field independently and in an additive fashion. Importantly, from about 250 ms after search display onset, the target N2pc became larger than the summed N2pc components to partially matching nontargets. This emergence of a superadditive target N2pc highlights the point in time when spatially selective attentional processing is no longer guided exclusively by signals from independent feature-specific channels, and attentional control begins to be integrated across feature dimensions. In other words, it marks the transition from the initial feature-based guidance of spatial attention to a second stage where the allocation of focal attention is controlled in an object-based fashion.
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conjunction of two features within the same dimension (colour). Previous behavioural experiments have found that this type of search is generally very difficult (e.g., Wolfe et al., 1990; Wing & Allport, 1972). However, other studies have suggested that colour/colour conjunction search can sometimes operate in a relatively efficient fashion (e.g., Carrasco et al., 1998; Linnell & Humphreys, 2001, 2002). We employed the N2pc component as a temporal marker of the deployment of attention to colour/colour conjunction targets and to objects that matched one target-defining colour. Experiment 1 showed that attention was initially allocated in parallel and independently to all objects with target-matching colours. At a later stage that was marked by the emergence of a superadditive N2pc component to target objects, attentional selection processes no longer operated in an independent feature-specific fashion, suggesting that at this stage, information about the presence of multiple target- defining attributes of the same object was combined across features. Experiment 2 ruled out an alternative interpretation of the superadditive target N2pc in terms of a selective withdrawal of attention from partially target-matching distractor objects.
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Figure 1B shows ERPs measured for 12 participants at lateral posterior electrodes (PO7/PO8) contralateral and ipsi- lateral to the side where the first target was presented. ERPs are shown separately for trials where the two targets appeared on the same side and trials where they appeared on opposite sides, for all four SOA conditions. Time is plotted relative to the onset of the first display. The N2pc component first emerged contralateral to the first target (T1), with a mean latency of 194 ms after stimulus onset (averaged across all task condi- tions). As predicted, the N2pc reversed polarity (arrows in Fig- ure 1B) when the second target (T2) was presented on the opposite side. This N2pc polarity reversal was closely linked to the arrival time of the second target: it was triggered earlier when the SOA between the two displays was short and emerged later for longer SOAs. This can be seen in N2pc differ- ence waveforms obtained by subtracting ipsilateral from contralateral ERPs on trials with same-side and opposite- side targets (Figure 1C). Arrows in Figure 1C mark the point in time when these N2pc waveforms started to differ in each SOA condition. This critical moment when the arrival of the second target began to affect the allocation of attention in the visual field is shown most clearly in Figure 1D, which plots the difference between the same-side and opposite-side N2pc waveforms in Figure 1C for all four SOA conditions. To deter- mine the onset of the N2pc differences shown in Figure 1D, we used a jackknife-based procedure with a 50% maximum amplitude criterion [11, 12]. The filled circles in Figure 1D mark the onset latency estimates for each SOA condition. *Correspondence: firstname.lastname@example.org
The central new finding of the current study was that target-colour items in the second display only elicited N2pc component when they were task-relevant, but not in D1 blocks when they had to be ignored and were presented after the target item had already been encountered, despite the fact that the second display appeared only 100 ms after the first display. The observation that task-irrelevant target-colour items in the second display failed to trigger N2pc components in both experiments strongly suggests that these items no longer rapidly attracted attention to their location once the search goal for the current trial was achieved. This could suggest that attentional templates can be deactivated very rapidly, within 100 ms after the selection of the current target. However, this conclusion is not in line with other findings of the present study. In both experiments, target-colour objects in the second display elicited longer-latency SPCN components in D1 blocks. Because the SPCN is elicited contralateral to the visual field where these objects appeared, its presence demonstrates that the location of these objects was being registered, resulting in a spatially selective modulation of visual processing that emerged around 300-350 ms after the onset of the second display. If search templates had been switched off entirely immediately after the target had been found, template-matching objects in the second display should no longer have been able to trigger a contralateral ERP component such as the SPCN. As SPCN components are usually interpreted as a marker of the activation of representations in visual working memory (e.g., Mazza et al., 2007; Jolicoeur et al., 2008), their presence in response to irrelevant target-colour objects in D1 blocks suggests that even though these objects failed to capture attention, they were still encoded into working memory. The fact that, in Experiment 1, the second object still exerted congruency effects on the response to the first target further corroborates this view.
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Mental representations of target features (attentional templates) control the selection of candidate target objects in visual search. The question where templates are maintained remains controversial. We employed the N2pc component as an electrophysiological marker of template-guided target selection to investigate whether and under which conditions templates are held in visual working memory (vWM). In two experiments, participants memorized one or four shapes (low versus high vWM load) before either being tested on their memory or performing a visual search task. When targets were defined by one of two possible colours (e.g., red or green), target N2pcs were delayed with high vWM load. This suggests that the maintenance of multiple shapes in vWM interfered with the activation of colour-specific search templates, supporting the hypothesis that these templates are held in vWM. This was the case despite participants always searching for the same two target colours. In contrast, the speed of target selection in a task where a single target colour remained relevant throughout was unaffected by concurrent load, indicating that a constant search template for a single feature may be maintained outside vWM in a different store. Additionally, early visual N1 components to search and memory test displays were attenuated under high load, suggesting a competition between external and internal attention. The size of this attenuation predicted individual vWM performance. These results provide new electrophysiological evidence for impairment of top-down attentional control mechanisms by high vWM load, demonstrating that vWM is involved in the guidance of attentional target selection during search.
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search display. The N2pc component is typically elicited at poststimulus latencies of 200 to 350 msec at poste- rior electrodes contralateral to the side of a task-relevant visual event, such as a target in a visual search task, and is assumed to reflect the attentional selection of task-relevant events and inhibition of irrelevant distrac- tors (cf., Woodman & Luck, 1999; Eimer, 1996; Luck & Hillyard, 1994). N2pc amplitudes reflect the difference in ERP activity between electrodes contralateral and ipsilateral to a target, and thus provide a direct mea- sure of the relative distribution of attention in the visual field. The current study focused on the N2pc because this component provides a unique online marker of the selective attentional processing of targets versus distrac- tors: Large N2pc amplitudes indicate fully focused at- tention and effective distractor inhibition, whereas small and delayed N2pc components are linked to a more dif- fuse attentional state.
quantified these differences in terms of display set size effects (Watson & Humphreys, 1997) or target-distractor interference effects (Donk & Theeuwes, 2001) have remained inconclusive with respect to which stages of attentional selectivity are affected by the presence versus absence of preview displays. Here, we used ERPs to track the attentional processing of full search displays on preview versus no-preview trials in real time. To do this, we compared ERPs elicited at posterior electrodes contralateral and ipsilateral to the currently task-relevant hemifield. Previous ERP studies of visual search have identified two successive contralateral components that are linked to the selective attentional processing of candidate target objects. The rapid allocation of attention to such objects is reflected by the N2pc component. The N2pc is an enhanced contralateral negativity over posterior visual areas that emerges approximately 200 ms after search display onset (e.g., Eimer, 1996; Luck & Hillyard, 1994; Eimer & Kiss, 2008), and is generated within extrastriate ventral visual cortex (e.g., Hopf et al., 2000). This
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This possibility that during multiple-colour search, distractor-colour cues remain able to attract attention can be tested with event-related brain potential (ERP) measures, which offer temporally precise markers of attentional selection processes. To assess the time course of attentional capture by target-matching and nonmatching colour cues when a multiple-colour task set is active, the N2pc component is a particularly useful tool, because this component is an established electrophysiological marker of the spatially selective attentional processing of candidate target objects in extrastriate visual areas. The N2pc is an enhanced negativity that is triggered at posterior scalp electrodes contralateral to targets that are presented among distractor objects in visual search arrays. This component typically emerges between 180 ms and 200 ms after stimulus onset of visual arrays that contain a candidate target item, and is assumed to reflect the attentional selection of task-set matching objects (Luck & Hillyard, 1994; Eimer, 1996; Woodman & Luck, 1999; Mazza, Turatto, Umiltà, & Eimer, 2007). Previous ERP studies of task-set contingent attentional capture have demonstrated that the N2pc can be used to measure currently active top- down attentional control settings. During search for a specific target feature, task-set matching colour singleton cues (e.g., red singleton cues during search for red targets) triggered an N2pc, but nonmatching cues did not (e.g., red singleton cues during search for blue targets or small targets; e.g., Eimer & Kiss, 2008; Lien, Ruthruff, Goodin, & Remington, 2008; Leblanc, Prime, & Jolicoeur, 2008; Eimer, Kiss, Press, & Sauter, 2009). The presence of an N2pc to target-matching cues shows that these cues capture attention at a relatively early stage of visual-perceptual processing, while the absence of an N2pc to nonmatching cues indicates that their features fail to capture attention because they are not part of the currently active task set.
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During visual search, target representations (attentional templates) control the allocation of attention to template-matching objects. The activation of new attentional templates can be prompted by verbal or pictorial target specifications. We measured the N2pc component of the event-related potential (ERP) as a temporal marker of attentional target selection to determine the role of color signals in search templates for real-world search target objects that are set up in response to word or picture cues. On each trial run, a word cue (e.g., “apple”) was followed by three search displays that contained the cued target object among three distractors. The selection of the first target was based on the word cue only, while selection of the two subsequent targets could be controlled by templates set up after the first visual presentation of the target (picture cue). In different trial runs, search displays either contained objects in their natural colors or monochromatic objects. These two display types were presented in different blocks (Experiment 1) or in random order within each block (Experiment 2). RTs were faster and target N2pc components emerged earlier for the 2 nd and 3 rd display of each trial run relative to the 1 st display, demonstrating that pictures are more effective than word cues in guiding search. N2pc components were triggered more rapidly for targets in the 2 nd and 3 rd display in trial runs with colored displays. This demonstrates that when visual target attributes are fully specified by picture cues, the additional presence of color signals in target templates facilitates the speed with which attention is allocated to template-matching objects. No such selection benefits for colored targets were found when search templates were set up in response to word cues. Experiment 2 showed that color templates activated by word cues can even impair the attentional selection of non-colored targets. Results provide new insights into the status of color during the guidance of visual search for real-world target objects. Color is a powerful guiding feature when the precise visual properties of these objects are known, but seems to be less important when search targets are specified by word cues.
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PO8, PO9, PO10, and Oz. A 500-Hz sampling rate with a 40 Hz low-pass filter was used, with no other offline filters applied. Channels were referenced online to a left-earlobe electrode, and re-referenced offline to an average of both earlobes. Trials with eye-movements (exceeding ±30 µV in the HEOG channels during the interval from 200 ms before to 200 ms after the onset of a search display), eye blinks (exceeding ±60 µV at Fpz) and movement- related artifacts (exceeding ±80 µV at all other channels) were rejected. Following artefact rejection, ERPs were computed for trials where a search display was presented after the retention period. These trials were segmented into 600 ms epochs (from 100 ms before to 500 ms after the onset of the search display). Averaged ERP waveforms from trials with correct responses were computed for search displays where a target bar appeared in the left or right visual field, separately for low and high WM load blocks. N2pc components were quantified on the basis of ERPs obtained at posterior electrode sites PO7 and PO8 contralateral and ipsilateral to the visual field of the target, where the N2pc component is maximal. To compute and compare target N2pc onset latencies in low and high WM load blocks, a jackknife-based analysis method was employed (see Miller, Patterson, & Ulrich, 1998, for details) for contralateral minus ipsilateral difference waveforms. N2pc onsets were calculated on the basis of an absolute onset criterion of -0.5 µV within the entire 500 ms interval following search display onset. To compute N2pc mean amplitude values, previous N2pc studies of visual search have typically used a pre-defined 200-300 ms post-stimulus window. In contrast to these previous studies, the current experiment employed a dual task design where visual search was combined with a concurrent WM task. Because of the possibility that the additional WM task might affect the timing of N2pc components to search targets, we did not employ a pre-defined N2pc mean amplitude window, but instead based this window on the outcome of the N2pc onset analyses described above. ERP mean amplitudes at PO7 and PO8 were quantified within a 250-350 ms interval following search display onset (reflecting the estimated onset of target N2pcs in low WM load blocks), and between 350-500 ms (reflecting the estimated onset of the contralateral negativity in high WM load blocks).
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he present study aimed to re-examine the discrete model of object processing in VWM proposed by Shen et al. (2007) and to investigate the relationship between VWM resources and object processing using the event-related brain potential (ERP) component N2pc. The N2pc is an electrophysiological marker for the allocation of VWM resources that is usually elicited at posterior electrodes contralateral to the visually presented task-relevant stimuli between 180 and 300 ms after target appearance (Kiss, Van Velzen, & Eimer, 2008). The N2pc is an enhanced negativity component that indexes the amount of attention deployed to a stimulus; its latency corresponds to the point in time of attentional deployment (Brisson, Robitaille, & Jolicoeur, 2007; Eimer & Kiss, 2008).
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attention shift. In this case, priority-driven attention was “pulled” by a particular target- defining feature. There is another type of priority-driven attention where target locations for an attention shift are defined by visual signals at a different location, such as spatially informative cues. In this case, attention is “pushed” towards a new location by the visual properties of a cue at a currently attended position. A classic example is the spatial cueing paradigm developed by Posner and colleagues (e.g., Posner, Snyder, & Davidson, 1980; Posner, 1980), where informative arrow cues signal the location where expected target objects are likely to appear. Behavioural studies have suggested that attention shifts triggered by arrow cues are relatively slow, and take about 250-300 ms to be completed (e.g., Müller & Rabbitt, 1989; Cheal & Lyon, 1991; see also Müller, Teder-Sälejärvi, & Hillyard, 1998, for electrophysiological evidence). Although such cued attention shifts are usually described as “endogenous”, they are controlled by visual attributes of the cue (e.g., arrow direction). For this reason, and analogous to template-guided shifts of attention, attention shifts in response to spatial cues are also priority-driven. The aim of Experiment 2 was to employ N2pc components to determine the speed of such priority-driven shifts of attention that are triggered by spatial cues, and to compare it to the speed of fully voluntary attention shifts. Procedures were similar to Experiment 1. T1 benchmark objects were now defined by colour, and there were two shift tasks (see Figure 1). In the priority-driven task, the response-relevant T2 object was signalled by the arrow cue at the benchmark location. In the voluntary task, T2 was the object that was located clockwise or anticlockwise relative to the colour-defined benchmark.
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interval between cue and search displays at posterior electrodes contralateral to the side of the currently task-relevant items in the cue displays. In line with the findings of Carlisle et al. (2011), CDA components were expected to emerge in variable-colour blocks, and to increase in amplitude as a function of colour load. If attentional templates are no longer held in working memory when target features remain unchanged across blocks of trials, no CDA components should be found at all in constant-colour blocks. To assess our main research question regarding the speed and efficiency with which focal attention was deployed to target objects in the subsequent search displays, we measured N2pc components elicited in response to these targets. If multiple colour-specific attentional templates can be activated simultaneously when target colours remain constant but not when these colours vary across trials, target N2pc components should differ between constant-colour and variable-colour blocks during multiple- colour search (Two and Three Colour tasks). N2pc components should be elicited by target objects in constant-colour blocks, but should be strongly delayed or perhaps even entirely absent in variable-colour blocks. Furthermore, the costs of increasing colour load from one to two (Experiment 1) or from two to three (Experiment 2) should affect target N2pc amplitudes and latencies more strongly in variable-colour relative to constant-colour blocks.
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the second phase, attentional control starts to be affected by spatial-configural information, resulting in an attentional bias towards objects with shape components in the correct spatial arrangement. This configural guidance of attentional selectivity could be based on integrated target object templates that represent overall target shape rather than individual shape parts, or on separate templates for each target shape that also represent the assigned task-relevant locations of these shapes (e.g., circle in lower visual field). The fact that attentional biases towards objects with the relevant spatial arrangement of shape components emerged beyond 230 ms post-stimulus not only when target objects were composed of spatially aligned component shapes (in Experiment 1), but also when these shapes were spatially separated (in Experiment 2) suggests that this type of attentional control is more likely to be based on separate templates for specific shape/location conjunctions. Recent electrophysiological studies (Adamo, Pun, & Ferber, 2010; Berggren, Jenkins, McCants, & Eimer, in press) have suggested that it is difficult to employ multiple templates for different target features at particular positions in order to restrict feature-based attentional guidance to specific locations. However, these experiments have only investigated task sets for specific colour/location combinations, and it therefore remains possible that such templates might be available in the shape domain.
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The findings from the One Colour task confirmed the observations from our previous study (Eimer & Grubert, 2014). N2pc components were elicited by the first and second target in each trial, and the temporal separation of these two N2pc components closely matched the objective time interval between the two successive search displays. The onset difference of the N2pc to H1 and H2 targets was 110 ms in the SOA100 condition and 11 ms in the SOA10 conditions. These findings demonstrate that the allocation of attention to new target objects can be triggered extremely rapidly, even when attention had been directed to another object just a few milliseconds earlier. The fact that the N2pc components to H1 and H2 targets in the SOA10 condition overlapped in time, and were identical in amplitude (see Figure 2) provides strong evidence for the parallel allocation of attention to multiple target objects. If attention had to be de-allocated from its previous position before it could be directed to a new target location, as implied by strictly serial models of attentional object selection, the N2pc to H1 targets should have been very small and short-lived in the SOA10 condition, and should show only minimal temporal overlap with the N2pc to H2 targets. There was no evidence for this in the One Colour task, which strongly suggests that attention was allocated in parallel and independently to the two successive target objects in this task. The fact that the onset latency difference between the N2pc components to H1 and H2 targets in the SOA10 condition (11 ms) matched the objective interval between these two targets suggests that both were selected independently, and the two parallel selection processes followed their own distinct time course.
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Compatibility of two components is determined by three requirements firstly present the same operational interface to its environment, secondly component should be able to read and write the same data and conform to the semantics in all interactions in which they are engaged. The replacement component’s provided features should obviously be at least the same as those of the current one, otherwise its clients will not be able to successfully communicate with it. Component compatibility defines two levels. Strict compatibility requires that replacement component should be a subtype of the current one. Relaxed level compatibility referred as contextual compatibility because it takes into account two important aspects of the environment in which the current component is deployed. One aspect is which of the current component’s provided features are actually used by other components in the given application configuration. Secondly requirements of the current component are not considered. 
Each of the components featured in there is obtained by deformation of one reference component chosen among a set of few reference components. Each reference component is provided with some basis functions (reduced basis functions) that represent the behavior of the set of all the PDE solutions on such subdomains. The restriction of the solution to (1) to every component is then sought as a linear combination of those basis functions mapped onto the component from the associated reference component.
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situation for reusable software consumers: there are many more accessible reuse candidates. Consequently, many organizations are spending much time in reusable component selection since the choice of the appropriate components has a major impact on the project and resulting product. The component selection process is not defined so each project finds its own approach to it. Here a method has been introduce which supports the search, evolution and selection of reusable software and provides specific techniques for defining the evolution criteria, comparing the cost and benefits of alternatives and consolidating the evolution and selection and benefits of alternatives and consolidating the evolution results in decision making.
contrast to multivariate BDM analyses, the N2pc components shown in Fig. 6C are still computed on the basis of contralateral/ipsilateral differences in response to left versus right targets, and therefore cannot discriminate neural responses to target stimuli on the same side. Similarly, the N2pc is not able to fully discriminate between targets at different elevations, as for example the Upper and Bottom field presentations (see Fig. 6C). To further underline this point, we assessed whether target locations can be successfully discriminated with BDM even when they are in the same visual quadrant. Figure 7 shows four additional BDM classifications, one for each quadrant. The comparisons were conducted for targets in the top right (positions 1 versus 2), bottom right (positions 3 versus 4), bottom left (positions 5 versus 6), and top left quadrants (positions 7 versus 8). As shown in Fig. 7, all of these within-quadrant classifications were performed with above-chance accuracy for EEG data recorded between 200–300 ms post-stimulus (two-tailed cluster p-values for the first significant cluster after trial onset were p = 0.022 for top right, p = 0.001 for top left, and p < 10 −3 for bottom right and bottom left). Accuracies were
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