Salience-based integration of redundant signals in visual pop-out search: evidence from behavioral and electrophysiological measures.

Behavioral and electrophysiological evidence is presented suggesting that, in visual search for feature singleton targets, multidimensional signals are integrated at a preselective stage of processing. Observers searched for a target that was consistently defined by the same features, but differed from the variable context either nonredundantly by one or redundantly by two dimensionally different features. The behavioral results showed reaction time redundancy gains and evidence of coactive processing, and the electrophysiological analyses revealed the latency of the N2pc component of the event-related potential (ERP) to be expedited by redundant relative to nonredundant displays, while the response-related lateralized readiness potential (LRP) remained unaffected. These findings suggest that target signal integration in singleton search paradigms occurs pre-attentively, that is, prior to focal-attentional target selection, with observers basing their responses on the detection of featureless saliency signals, even under conditions in which the target features remain constant and are known in advance. These results have implications for theories assuming top-down influences in feature detection.

Perceptual basis of redundancy gains in visual pop-out search.

The redundant-signals effect (RSE) refers to a speed-up of RT when the response is triggered by two, rather than just one, response-relevant target elements. Although there is agreement that in the visual modality RSEs observed with dimensionally redundant signals originating from the same location are generated by coactive processing architectures, there has been a debate as to the exact stage(s)--preattentive versus postselective--of processing at which coactivation arises. To determine the origin(s) of redundancy gains in visual pop-out search, the present study combined mental chronometry with electrophysiological markers that reflect purely preattentive perceptual (posterior-contralateral negativity [PCN]), preattentive and postselective perceptual plus response selection-related (stimulus-locked lateralized readiness potential [LRP]), or purely response production-related processes (response-locked LRP). As expected, there was an RSE on target detection RTs, with evidence for coactivation. At the electrophysiological level, this pattern was mirrored by an RSE in PCN latencies, whereas stimulus-locked LRP latencies showed no RSE over and above the PCN effect. Also, there was no RSE on the response-locked LRPs. This pattern demonstrates a major contribution of preattentive perceptual processing stages to the RSE in visual pop-out search, consistent with parallel-coactive coding of target signals in multiple visual dimensions [Müller, H. J., Heller, D., & Ziegler, J. Visual search for singleton feature targets within and across feature dimensions.

Redundancy gains in pop-out visual search are determined by top-down task set: behavioral and electrophysiological evidence.

We combined behavioral and electrophysiological measures to find out whether redundancy gain effects in pop-out visual search are exclusively determined by bottom-up salience or are modulated by top-down task search goals. Search arrays contained feature singletons that could be defined in a single dimension (color or shape) or redundantly in both dimensions. In the baseline condition, both color and shape were task-relevant, and behavioral redundancy gain effects were accompanied by an earlier onset of the N2pc component for redundant as compared to single-dimension targets. This demonstrates that redundancy gains are generated at an early visual-perceptual level of processing. In the color target and shape target conditions, only one dimension was task-relevant, while the other could be ignored. In these two conditions, behavioral and electrophysiological redundancy gains were eliminated. We conclude that redundant-signals effects in pop-out visual search are not driven by bottom-up salience but are instead strongly dependent on top-down task set.

Display probability modulates attentional capture by onset distractors.

Attention can be stimulus-driven and bottom-up or goal-driven and top-down. Bottom-up attention and, particularly, attentional capture are often thought to be strongly automatic, i.e., not modulable. For example, in visual search, it has been shown that salient distractors strongly attract attention even though observers were instructed to ignore them. However, it was also shown that the strength of distraction can be modulated by the display probabilities of the distractors. Hence, bottom-up attention seems not to be completely automatic. In these studies, the distractors were salient by color differences to the other items in the display. Such color distractors, however, do not necessarily trigger bottom-up attention. Here, we presented onset distractors, that is, distractors displayed after the onset of the other search items, which are thought to strongly elicit bottom-up attention and to capture eye movements. Varying the display probabilities of the onset distractors strongly modulated attentional capture. We suggest that modulation was due to statistical learning. This study adds further evidence that bottom-up processes are not completely automatic.

Attentional capture by salient color singleton distractors is modulated by top-down dimensional set.

Three experiments examined whether salient color singleton distractors automatically interfere with the detection singleton form targets in visual search (e.g., J. Theeuwes, 1992), or whether the degree of interference is top-down modulable. In Experiments 1 and 2, observers started with a pure block of trials, which contained either never a distractor or always a distractor (0% or 100% distractors)--varying the opportunity to learn distractor suppression. In the subsequent trial blocks, the proportion of distractors was systematically varied (within-subjects factor in Experiment 1, between-subjects factor in Experiment 2)--varying the incentive to use distractor suppression. In Experiment 3, observers started with 100% distractors in the first block and were presented with "rare" color or luminance distractors, in addition to "frequent" color distractors, in the second block. The results revealed distractor interference to vary as a function of both the initial experience with distractors and the incentive to suppress them: the interference was larger without relevant practice and with a lesser incentive to apply suppression (Experiments 1-3). This set of findings suggests that distractor interference is top-down modulable.

Dimension- and space-based intertrial effects in visual pop-out search: modulation by task demands for focal-attentional processing.

Two experiments compared reaction times (RTs) in visual search for singleton feature targets defined, variably across trials, in either the color or the orientation dimension. Experiment 1 required observers to simply discern target presence versus absence (simple-detection task); Experiment 2 required them to respond to a detection-irrelevant form attribute of the target (compound-search task). Experiment 1 revealed a marked dimensional intertrial effect of 34 ms for an target defined in a changed versus a repeated dimension, and an intertrial target distance effect, with an 4-ms increase in RTs (per unit of distance) as the separation of the current relative to the preceding target increased. Conversely, in Experiment 2, the dimension change effect was markedly reduced (11 ms), while the intertrial target distance effect was markedly increased (11 ms per unit of distance). The results suggest that dimension change/repetition effects are modulated by the amount of attentional focusing required by the task, with space-based attention altering the integration of dimension-specific feature contrast signals at the level of the overall-saliency map.

A (fascinating) litmus test for human retino- vs. non-retinotopic processing.

In human vision, the optics of the eye map neighboring points of the environment onto neighboring photoreceptors in the retina. This retinotopic encoding principle is preserved in the early visual areas. Under normal viewing conditions, due to the motion of objects and to eye movements, the retinotopic representation of the environment undergoes fast and drastic shifts. Yet, perceptually our environment appears stable suggesting the existence of non-retinotopic representations in addition to the well-known retinotopic ones. Here, we present a simple psychophysical test to determine whether a given visual process is accomplished in retino- or non-retinotopic coordinates. As examples, we show that visual search and motion perception can occur within a non-retinotopic frame of reference. These findings suggest that more mechanisms than previously thought operate non-retinotopically. Whether this is true for a given visual process can easily be found out with our "litmus test."

The redundant-signals paradigm and preattentive visual processing.

Physiological and cognitive models of vision agree that the pre-attentive processing of visual stimuli is organized in a parallel and segregated fashion. However, several incompatible models have been proposed for the subsequent processing stages. They differ in their assumptions about architecture (serial, parallel, or coactive/integrative), stopping-rules (self-terminating, or exhaustive), spatial specificity of saliency signal coding (signal pooling across locations, or spatially distinct processing), and dependency of target detection on the prior allocation of attention (pre-attentive, or post-selective). We review how studies employing the redundant-signals paradigm in visual pop-out search contribute to discerning between the different assumptions. We find strong support for the notion of a saliency map, into which feature contrast signals are pooled, and especially the dimension weighting account (1) receives further support: Instead of a priming mechanism that could increase weights for several dimensions independently, evidence favors a weighting mechanism that effectively limits the total weight available for allocation to the various dimensions through competitive interactions, whereby increasing the weight for one dimension goes along with decreasing the weights for other dimensions.

Expectancies modulate attentional capture by salient color singletons.

In singleton feature search for a form-defined target, the presentation of a task-irrelevant, but salient singleton color distractor is known to interfere with target detection [Theeuwes, J. (1991). Cross-dimensional perceptual selectivity. Perception & Psychophysics, 50, 184-193; Theeuwes, J. (1992). Perceptual selectivity for color and form. Perception & Psychophysics, 51, 599-606]. The present study was designed to re-examine this effect, by presenting observers with a singleton form target (on each trial) that could be accompanied by a salient) singleton color distractor, with the proportion of distractor to no-distractor trials systematically varying across blocks of trials. In addition to RTs, eye movements were recorded in order to examine the mechanisms underlying the distractor interference effect. The results showed that singleton distractors did interfere with target detection only when they were presented on a relatively small (but not on a large) proportion of trials. Overall, the findings suggest that cross-dimensional interference is a covert attention effect, arising from the competition of the target with the distractor for attentional selection [Kumada, T., & Humphreys, G. W. (2002). Cross-dimensional interference and cross-trial inhibition. Perception & Psychophysics, 64, 493-503], with the strength of the competition being modulated by observers' (top-down) incentive to suppress the distractor dimension.

Cross-trial priming of element positions in visual pop-out search is dependent on stimulus arrangement.

Two experiments examined cross-trial positional priming (V. Maljkovic & K. Nakayama, 1994, 1996, 2000) in visual pop-out search. Experiment 1 used regularly arranged target and distractor displays, as in previous studies. Reaction times were expedited when the target appeared at a previous target location (facilitation relative to neutral baseline) and slowed when the target appeared at a previous distractor location (inhibition). In contrast to facilitation, inhibition emerged only after extended practice. Experiment 2 revealed reduced facilitatory and no inhibitory priming when the elements' spatial arrangement was made irregular, indicating that positional--in particular, inhibitory--priming critically depends on the configuration of the display elements across sequences of trials. These results are discussed with respect to the role of the context for cross-trial priming in visual pop-out search.

Visual search for singleton feature targets across dimensions: Stimulus- and expectancy-driven effects in dimensional weighting.

Four pop-out search experiments investigated whether dimension-based visual attention is top-down modulable. Observers searched for singleton feature targets defined, variably across trials, by a color or an orientation difference to nontargets. Observers were precued to the most probable target-defining dimension (e.g., by the word color) or feature (red) on a given trial. Results revealed expedited reaction times (RTs) for valid-dimension targets relative to neutral-cue conditions, and slowed RTs for invalid-dimension targets. Cue information as to precise target feature yielded some extra effect only for color targets. The dimensional cuing significantly reduced, but did not abolish, the dimension-specific influence of the previous target on detection of the current target (same-dimension RT < different-dimension RT). These findings confirm that top-down dimensional set modulates stimulus-driven dimension processes in the detection of pop-out signals. ((c) 2003 APA, all rights reserved)

Visual search for dimensionally redundant pop-out targets: parallel-coactive processing of dimensions is location specific.

Three visual search experiments investigated redundancy gains for single and dual odd-one-out feature targets that differed from distractors in orientation, color, or both. In Experiment 1, redundant-target displays contained (a) a single target defined in 2 dimensions, (b) dual targets each defined in a different dimension, or (c) dual targets both defined in the same dimension. The redundancy gains, relative to single nonredundant targets, decreased from the first condition on, with violations of J. Miller's (1982) race model inequality (RMI) manifested only in the first 2 conditions. Experiment 2 systematically varied the spatial separation between dual targets each defined in a different dimension. Violations of the RMI were evident only when the 2 targets occupied nearby locations. Experiment 3 provided evidence of RMI violations by dimensionally redundant targets at both precued (likely) and noncued (unlikely) display locations. Taken together, these results suggest that there is coactivation of a common mechanism by target signals in different dimensions (not by signals in the same dimension), that the coactivation effects are spatially specific, and that the coactivated mechanisms are located at a preattentive, perceptual stage of processing.

Visual search for singleton targets redundantly defined in two feature dimensions: Coactive processing of color-motion targets?

In 2 visual search experiments, the role of feature contrast/saliency signals in generating detection responses to singleton feature targets in visual search was investigated using the redundant-target paradigm. Experiment 1 showed that coactive integration of dimensional signals is not restricted to targets defined on the color and orientation dimensions; rather, targets involving any of the combinations of color, orientation, and motion, are integrated coactively, as evidenced by violations of Miller's (1982) race model inequality. Experiment 2 replicated the findings of Experiment 1 for color-motion targets, with the target items' luminance adjusted, individually for each observer, to that of the distractors. The evidence for coactive processing of motion (saliency) with color and, respectively, orientation (saliency) signals suggests that, at variance with a recent suggestion by Li (2002; Koene & Zhaoping, 2007), signal integration in feature search tasks occurs at a stage following initial feature coding in primary visual cortex (V1), even though feature contrast computations in V1 may well contribute to saliency coding. In sum, the results suggest that detection responses were based on an integrated, overall-saliency representation indicating the presence of an odd-one-out item in the display, consistent with the dimension-weighting account of visual search (Müller et al., 1995, 2003).

From features to dimensions: cognitive and motor development in pop-out search in children and young adults.

In an experiment involving a total of 124 participants, divided into eight age groups (6-, 8-, 10-, 12-, 14-, 16-, 18-, and 20-year-olds) the development of the processing components underlying visual search for pop-out targets was tracked. Participants indicated the presence or absence of color or orientation feature singleton targets. Observers also solved a detection task, in which they responded to the onset of search arrays. There were two main results. First, analyses of inter-trial effects revealed differences in the search strategies of the 6-year-old participants compared to older age groups. Participants older than 8 years based target detection on feature-less dimensional salience signals (indicated by cross-trial RT costs in target dimension change relative to repetition trials), the 6-year-olds accessed the target feature to make a target present or absent decision (cross-trial RT costs in target feature change relative to feature repetition trials). The result agrees with predictions derived from the Dimension Weighting account and previous investigations of inter-trial effects in adult observers (Müller et al., 1995; Found and Müller, 1996). The results are also in line with theories of cognitive development suggesting that the ability to abstract specific visual features into feature categories is developed after the age of 7 years. Second, overall search RTs decreased with increasing age in a decelerated fashion. RT differences between consecutive age groups can be explained by sensory-motor maturation up to the age of 10 years (as indicated by RTs in the onset detection task). Expedited RTs in older age groups (10-, vs. 12-year-olds; 14- vs. 16-year-olds), but also in the 6- vs. 8-year-olds, are due to the development of search-related (cognitive) processes. Overall, the results suggest that the level of adult performance in visual search for pop-out targets is achieved by the age of 16.

Coarse guidance by numerosity in visual search.

In five experiments, we examined whether the number of items can guide visual focal attention. Observers searched for the target area with the largest (or smallest) number of dots (squares in Experiment 4 and "checkerboards" in Experiment 5) among distractor areas with a smaller (or larger) number of dots. Results of Experiments 1 and 2 show that search efficiency is determined by target to distractor dot ratios. In searches where target items contained more dots than did distractor items, ratios over 1.5:1 yielded efficient search. Searches for targets where target items contained fewer dots than distractor items were harder. Here, ratios needed to be lower than 1:2 to yield efficient search. When the areas of the dots and of the squares containing them were fixed, as they were in Experiments 1 and 2, dot density and total dot area increased as dot number increased. Experiment 3 removed the density and area cues by allowing dot size and total dot area to vary. This produced a marked decline in search performance. Efficient search now required ratios of above 3:1 or below 1:3. By using more realistic and isoluminant stimuli, Experiments 4 and 5 show that guidance by numerosity is fragile. As is found with other features that guide focal attention (e.g., color, orientation, size), the numerosity differences that are able to guide attention by bottom-up signals are much coarser than the differences that can be detected in attended stimuli.

Happy with a difference, unhappy with an identity: observers' mood determines processing depth in visual search.

Visual search for feature targets was employed to investigate whether the mechanisms underlying visual selective attention are modulated by observers' mood. The effects of induced mood on overall mean reaction times and on changes and repetitions of target-defining features and dimensions across consecutive trials were measured. The results showed that reaction times were significantly slower in the negative than in the positive and neutral mood groups. Furthermore, the results demonstrated that the processing stage that is activated to select visual information in a feature search task is modulated by the observer's mood. In participants with positive or neutral moods, dimension-specific, but no feature-specific, intertrial transition effects were found, suggesting that these observers based their responses on a salience signal coding the most conspicuous display location. Conversely, intertrial effects in observers in a negative mood were feature-specific in nature, suggesting that these participants accessed the feature identity level before responding.

Dynamic weighting of feature dimensions in visual search: behavioral and psychophysiological evidence.

Dimension-based accounts of visual search and selection have significantly contributed to the understanding of the cognitive mechanisms of attention. Extensions of the original approach assuming the existence of dimension-based feature contrast saliency signals that govern the allocation of focal attention have recently been employed to explain the spatial and temporal dynamics of the relative strengths of saliency representations. Here we review behavioral and neurophysiological findings providing evidence for the dynamic trial-by-trial weighting of feature dimensions in a variety of visual search tasks. The examination of the effects of feature and dimension-based inter-trial transitions in feature detection tasks shows that search performance is affected by the change of target-defining dimensions, but not features. The use of the redundant-signals paradigm shows that feature contrast saliency signals are integrated at a pre-selective processing stage. The comparison of feature detection and compound search tasks suggests that the relative significance of dimension-dependent and dimension-independent saliency representations is task-contingent. Empirical findings that explain reduced dimension-based effects in compound search tasks are discussed. Psychophysiological evidence is presented that confirms the assumption that the locus of the effects of feature dimension changes is perceptual pre-selective rather than post-selective response-based. Behavioral and psychophysiological results are considered within in the framework of the dimension weighting account of selective visual attention.

Dimension intertrial and cueing effects in localization: support for pre-attentively weighted one-route models of saliency.

There are several alternative accounts of dimensional intertrial and cueing effects in singleton feature search tasks. Some accounts assume that these effects arise at post-selective processing stages; dual-route accounts assume them to be perceptual in nature, but coming into play only in non-spatial tasks (e.g., detection but not localization). By contrast, the Dimension Weighting Account (DWA) assumes dimensional effects to arise at pre-attentive processing stages of spatial as well as non-spatial tasks. The data available are ambiguous, permitting no clear-cut choice among these accounts. Therefore, the present study examined for early effects of dimensional weighting in a spatial task, the presence of which is only predicted by the DWA and not by post-selective or dual-route accounts. Salience is known to saturate for high feature contrast and long presentation times. Consequently, with lower bottom-up salience that still permits efficient search, dimensional weights would produce a greater modulation--if present at all. Thus, we examined localization accuracy under brief-presentation conditions in Experiment 1, and localization speed under conditions of low versus high feature contrast in Experiment 2. Both experiments revealed significant dimension intertrial and cueing effects. This strongly argues against dual-route accounts and strengthens evidence for a pre-attentive origin of these effects.

Inter-trial and redundant-signals effects in visual search and discrimination tasks: separable pre-attentive and post-selective effects.

Feature singleton search is faster when the target-defining dimension is repeated, rather than changed, across trials (Found & Müller, 1996). A similar dimension repetition benefit has been observed in a non-search (discrimination) task with a single stimulus (Mortier, Theeuwes, & Starreveld, 2005). Two experiments examined whether these effects in the two tasks originate from the same or different processing stages. Experiment 1 revealed differential feature-specific effects, and Experiment 2 differential processing of dimensionally redundant target signals between the two types of task. These dissociations support the existence of separable, pre-attentive and post-selective sources of inter-trial effects in the two tasks.

The influence of alertness on spatial and nonspatial components of visual attention.

Three experiments investigated whether spatial and nonspatial components of visual attention would be influenced by changes in (healthy, young) subjects' level of alertness and whether such effects on separable components would occur independently of each other. The experiments used a no-cue/alerting-cue design with varying cue-target stimulus onset asynchronies in two different whole-report paradigms based on Bundesen's (1990) theory of visual attention, which permits spatial and nonspatial components of selective attention to be assessed independently. The results revealed the level of alertness to affect both the spatial distribution of attentional weighting and processing speed, but not visual short-term memory capacity, with the effect on processing speed preceding that on the spatial distribution of attentional weighting. This pattern indicates that the level of alertness influences both spatial and nonspatial component mechanisms of visual attention and that these two effects develop independently of each other; moreover, it suggests that intrinsic and phasic alertness effects involve the same processing route, on which spatial and nonspatial mechanisms are mediated by independent processing systems that are activated, due to increased alertness, in temporal succession.