Standard models of visual search have focused upon asking participants to search for a single target in displays where the objects do not overlap one another, and where the objects are presented on a single depth plane. This stands in contrast to many everyday visual searches wherein variations in overlap and depth are the norm, rather than the exception. Here, we addressed whether presenting overlapping objects on different depths planes to one another can improve search performance. Across 4 different experiments using different stimulus types (opaque polygons, transparent polygons, opaque real-world objects, and transparent X-ray images), we found that depth was primarily beneficial when the displays were transparent, and this benefit arose in terms of an increase in response accuracy. Although the benefit to search performance only appeared in some cases, across all stimulus types, we found evidence of marked shifts in eye-movement behavior. Our results have important implications for current models and theories of visual search, which have not yet provided detailed accounts of the effects that overlap and depth have on guidance and object identification processes. Moreover, our results show that the presence of depth information could aid real-world searches of complex, overlapping displays. (PsycINFO Database Record

Previous research shows that visual search for two different targets is less efficient than search for a single target. Stroud, Menneer, Cave and Donnelly (2012) concluded that two target colours are represented separately based on modeling the fixation patterns. Although those analyses provide evidence for two separate target representations, they do not show whether participants search simultaneously for both targets, or first search for one target and then the other. Some studies suggest that multiple target representations are simultaneously active, while others indicate that search can be voluntarily simultaneous, or switching, or a mixture of both. Stroud et al.'s participants were not explicitly instructed to use any particular strategy. These data were revisited to determine which strategy was employed. Each fixated item was categorised according to whether its colour was more similar to one target or the other. Once an item similar to one target is fixated, the next fixated item is more likely to be similar to that target than the other, showing that at a given moment during search, one target is generally favoured. However, the search for one target is not completed before search for the other begins. Instead, there are often short runs of one or two fixations to distractors similar to one target, with each run followed by a switch to the other target. Thus, the results suggest that one target is more highly weighted than the other at any given time, but not to the extent that search is purely successive.

The efficiency of visual search for one (single-target) and either of two (dual-target) unfamiliar faces was explored to understand the manifestations of capacity and guidance limitations in face search. The visual similarity of distractor faces to target faces was manipulated using morphing (Experiments 1 and 2) and multidimensional scaling (Experiment 3). A dual-target cost was found in all experiments, evidenced by slower and less accurate search in dual- than single-target conditions. The dual-target cost was unequal across the targets, with performance being maintained on one target and reduced on the other, which we label "preferred" and "non-preferred" respectively. We calculated the capacity for each target face and show reduced capacity for representing the non-preferred target face. However, results show that the capacity for the non-preferred target can be increased when the dual-target condition is conducted after participants complete the single-target conditions. Analyses of eye movements revealed evidence for weak guidance of fixations in single-target search, and when searching for the preferred target in dual-target search. Overall, the experiments show dual-target search for faces is capacity- and guidance-limited, leading to superior search for 1 face over the other in dual-target search. However, learning faces individually may improve capacity with the second face. (PsycINFO Database Record

An important question about eye-movement behavior is when the decision is made to terminate a fixation and program the following saccade. Different approaches have found converging evidence in favor of a mixed-control account, in which there is some overlap between processing information at fixation and planning the following saccade. We examined one interesting instance of mixed control in visual search: lag-2 revisits, during which observers fixate a stimulus, move to a different stimulus, and then revisit the first stimulus on the next fixation. Results show that the probability of lag-2 revisits occurring increased with the number of target-similar stimuli, and revisits were preceded by a brief fixation on the intervening distractor stimulus. We developed the Efficient Visual Sampling (EVS) computational model to simulate our findings (fixation durations and fixation locations) and to provide insight into mixed control of fixations and the perceptual, cognitive, and motor processes that produce lag-2 revisits.

The Functional Visual Field (FVF) offers explanatory power. To us, it relates to existing literature on the flexibility of attentional focus in visual search and reading (Eriksen & St. James 1986; McConkie & Rayner 1975). The target article promotes reflection on existing findings. Here we consider the FVF as a mechanism in the Prevalence Effect (PE) in visual search.

Previous work showed that prior experience with discriminations requiring configural solutions (e.g. biconditional discrimination) confers an advantage for the learning of new configural discriminations (e.g. negative patterning) in comparison to prior experience with elemental discriminations. This effect is well established but its mechanism is not well understood. In the studies described below we assessed whether the saliences of configural and element cues were affected by prior training. We observed positive transfer to a new configural discrimination after configural pre-training but we were unable to find evidence for changes in cue salience using a signal-detection task. Our results confirm previous work by demonstrating experience-dependent flexibility in cue processing but they also suggest that this flexibility occurs at a point in the stimulus processing pipeline later than 1-2 s after the presentation of stimulus inputs. (138 words).

Behavior in visual search tasks is influenced by the proportion of trials on which a target is presented (the target prevalence). Previous research has shown that when target prevalence is low (2 % prevalence), participants tend to miss targets, as compared with higher prevalence levels (e.g. 50 % prevalence). There is an ongoing debate regarding the relative contributions of target repetition and the expectation that a target will occur in the emergence of prevalence effects. In order to disentangle these two factors, we went beyond previous studies by directly manipulating participants' expectations regarding how likely a target was to appear on a given trial. This we achieved without using cues or feedback. Our results indicated that both target repetition and target expectation contribute to the emergence of the prevalence effect.

Visual search is one of the most widely studied topics in vision science, both as an independent topic of interest, and as a tool for studying attention and visual cognition. A wide literature exists that seeks to understand how people find things under varying conditions of difficulty and complexity, and in situations ranging from the mundane (e.g. looking for one's keys) to those with significant societal importance (e.g. baggage or medical screening). A primary determinant of the ease and probability of success during search are the similarity relationships that exist in the search environment, such as the similarity between the background and the target, or the likeness of the non-targets to one another. A sense of similarity is often intuitive, but it is seldom quantified directly. This presents a problem in that similarity relationships are imprecisely specified, limiting the capacity of the researcher to examine adequately their influence. In this article, we present a novel approach to overcoming this problem that combines multi-dimensional scaling (MDS) analyses with behavioral and eye-tracking measurements. We propose a method whereby MDS can be repurposed to successfully quantify the similarity of experimental stimuli, thereby opening up theoretical questions in visual search and attention that cannot currently be addressed. These quantifications, in conjunction with behavioral and oculomotor measures, allow for critical observations about how similarity affects performance, information selection, and information processing. We provide a demonstration and tutorial of the approach, identify documented examples of its use, discuss how complementary computer vision methods could also be adopted, and close with a discussion of potential avenues for future application of this technique.

Previous research has shown that during visual search tasks target prevalence (the proportion of trials in which a target appears) influences both the probability that a target will be detected, and the speed at which participants will quit searching and provide an 'absent' response. When prevalence is low (e.g. target presented on 2 % of trials), participants are less likely to detect the target than when prevalence is higher (e.g. 50 % of trials). In the present set of experiments, we examined perceptual failures to detect low prevalence targets in visual search. We used a relative prevalence search task in order to be able to present an overall 50 % target prevalence and thereby prevent the results being accounted for by early quitting behavior. Participants searched for two targets, one of which appeared on 45 % of trials and another that appeared on 5 % of trials, leaving overall target prevalence at 50 %. In the first experiment, participants searched for two dissimilar targets; in the second experiment, participants searched for two similar targets. Overall, the results supported the notion that a reduction in prevalence primarily influenced perceptual failures of identification, rather than of selection. Together, these experiments add to a growing body of research exploring how and why observers fail to detect low prevalence targets, especially in real-world tasks in which some targets are more likely to appear than others.

The proportion of trials on which a target is presented (referred to as the target prevalence) during visual search influences the probability that the target will be detected. As prevalence increases, participants become biased toward reporting that the target is present. This bias results in an increase in detection rates for the target, coupled with an increased likelihood of making a false alarm. Previous work has demonstrated that, as prevalence increases, participants spend an increasing period of time searching on target-absent trials. The goal of the present study was to determine the information processing during the additional time spent searching on target-absent trials as prevalence increased. We recorded participants' eye movement behavior as they were engaged in low-prevalence (25% target-present trials), medium-prevalence (50%), or high-prevalence (75%) search. Increased prevalence primarily influenced search by increasing the time spent examining objects in the display, rather than by increasing the proportion of objects examined in each display. In addition, the additional time spent examining objects in high-prevalence target-absent trials was the result of revisiting objects. We discuss the implications of these results in relation to current models of search as well as ongoing efforts to alleviate the prevalence effect.

The Thatcher illusion (Thompson in Perception, 9, 483-484, 1980) is often explained as resulting from recognising a distortion of configural information when 'Thatcherised' faces are upright but not when inverted. However, recent behavioural studies suggest that there is an absence of perceptual configurality in upright Thatcherised faces (Donnelly et al. in Attention, Perception & Psychophysics, 74, 1475-1487, 2012) and both perceptual and decisional sources of configurality in behavioural tasks with Thatcherised stimuli (Mestry, Menneer et al. in Frontiers in Psychology, 3, 456, 2012). To examine sources linked to the behavioural experience of the illusion, we studied inversion and Thatcherisation of faces (comparing across conditions in which no features, the eyes, the mouth, or both features were Thatcherised) on a set of event-related potential (ERP) components. Effects of inversion were found at the N170, P2 and P3b. Effects of eye condition were restricted to the N170 generated in the right hemisphere. Critically, an interaction of orientation and eye Thatcherisation was found for the P3b amplitude. Results from an individual with acquired prosopagnosia who can discriminate Thatcherised from typical faces but cannot categorise them or perceive the illusion (Mestry, Donnelly et al. in Neuropsychologia, 50, 3410-3418, 2012) only differed from typical participants at the P3b component. Findings suggest the P3b links most directly to the experience of the illusion. Overall, the study showed evidence consistent with both perceptual and decisional sources and the need to consider both in relation to configurality.

It has previously been argued that, during visual search, eye movement behavior is indicative of an underlying scanning "strategy" that starts on a global, or "coarse," scale but then progressively focuses to a more local, or "fine," scale. This conclusion is motivated by the finding that, as a trial progresses, fixation durations tend to increase and saccade amplitudes tend to decrease. In the present study, we replicate these effects but offer an alternative explanation for them-that they emerge from a few stochastic factors that control eye movement behavior. We report the results of a simulation supporting this hypothesis and discuss implications for future models of visual search.

Using a visual search task, we explored how behavior is influenced by both visual and semantic information. We recorded participants' eye movements as they searched for a single target number in a search array of single-digit numbers (0-9). We examined the probability of fixating the various distractors as a function of two key dimensions: the visual similarity between the target and each distractor, and the semantic similarity (i.e. the numerical distance) between the target and each distractor. Visual similarity estimates were obtained using multidimensional scaling based on the independent observer similarity ratings. A linear mixed-effects model demonstrated that both visual and semantic similarity influenced the probability that distractors would be fixated. However, the visual similarity effect was substantially larger than the semantic similarity effect. We close by discussing the potential value of using this novel methodological approach and the implications for both simple and complex visual search displays.

Detection of the Thatcher illusion (Thompson, Perception, 9:483-484, 1980) is widely upheld as being dependent on configural processing (e.g. Bartlett & Searcy, Cognitive Psychology, 25:281-316, 1993; Boutsen, Humphreys, Praamstra, & Warbrick, NeuroImage, 32:352-367, 2006; Donnelly & Hadwin, Visual Cognition, 10:1001-1017, 2003; Leder & Bruce, Quarterly Journal of Experimental Psychology, 53A:513-536, 2000; Lewis, Perception, 30:769-774, 2001; Maurer, Grand, & Mondloch, Trends in Cognitive Sciences, 6:255-260, 2002; Stürzel & Spillmann, Perception, 29:937-942, 2000). Given that supercapacity processing accompanies configural processing (see Wenger & Townsend, 2001), supercapacity processing should occur in the processing of Thatcherised upright faces. The purpose of this study was to test for evidence that the grotesqueness of upright Thatcherised faces results from supercapacity processing. Two tasks were employed: categorisation of a single face as odd or normal, and a same/different task for sequentially presented faces. The stimuli were typical faces, partially Thatcherised faces (either eyes or mouth inverted) and fully Thatcherised faces. All of the faces were presented upright. The data from both experiments were analysed using mean response times and a number of capacity measures (capacity coefficient, the Miller and Grice inequalities, and the proportional-hazards ratio). The results of both experiments demonstrated some evidence of a redundancy gain for the redundant-target condition over the single-target condition, especially in the response times in Experiment 1. However, there was very limited evidence, in either experiment, that the redundancy gains resulted from supercapacity processing. We concluded that the oddity signalled by inversion of eyes and mouths does not arise from positive interdependencies between these features.

We report data from a prosopagnosic patient (PHD), and aged-matched control participants, from experiments where participants categorised individually presented emotional faces (experiment 1) and Thatcherised (from typical) faces (experiment 2). In experiment 2 participants also discriminated between simultaneously presented Thatcherised and typical faces. PHD was at chance categorising Thatcherised from typical faces. He was, however, able to discriminate between Thatcherised and typical faces, and partially able to categorise emotional faces. The results are discussed in terms of a loss of configural processing but preserved feature processing in PHD. The loss of configural processing impacts his categorisation of Thatcherised and typical faces, and his emotion processing, while his preserved feature processing supports his ability to categorise some emotional faces and his ability to discriminate between Thatcherised and typical faces.

Participants performed three feature-complete face processing tasks involving detection of changes in: (1) feature size and (2) feature identity in successive matching tasks, and (3) feature orientation. In each experiment, information in the top (eyes) and bottom (mouths) parts of faces were manipulated. All tasks were performed with upright and inverted faces. Data were analyzed first using group-based analysis of signal detection measures (sensitivity and bias), and second using analysis of multidimensional measures of sensitivity and bias along with probit regression models in order to draw inferences about independence and separability as defined within general recognition theory (Ashby and Townsend, 1986). The results highlighted different patterns of perceptual and decisional influences across tasks and orientations. There was evidence of orientation specific configural effects (violations of perceptual independence, perceptual seperability and decisional separabilty) in the Feature Orientation Task. For the Feature Identity Task there were orientation specific performance effects and there was evidence of configural effects (violations of decisional separability) in both orientations. Decisional effects are consistent with previous research (Wenger and Ingvalson, 2002, 2003; Richler et al. 2008; Cornes et al. 2011). Crucially, the probit analysis revealed violations of perceptual independence that remain undetected by marginal analysis.

Searching simultaneously for metal threats (guns and knives) and improvised explosive devices (IEDs) in X-ray images is less effective than 2 independent single-target searches, 1 for metal threats and 1 for IEDs. The goals of this study were to (a) replicate this dual-target cost for categorical targets and to determine whether the cost remains when X-ray images overlap, (b) determine the role of attentional guidance in this dual-target cost by measuring eye movements, and (c) determine the effect of practice on guidance. Untrained participants conducted 5,376 trials of visual search of X-ray images, each specializing in single-target search for metal threats, single-target search for IEDs, or dual-target search for both. In dual-target search, only 1 target (metal threat or IED) at most appeared on any 1 trial. Eye movements, response time, and accuracy were compared across single-target and dual-target searches. Results showed a dual-target cost in response time, accuracy, and guidance, with fewer fixations to target-color objects and disproportionately more to non-target-color objects, compared with single-target search. Such reduction in guidance explains why targets are missed in dual-target search, which was particularly noticeable when objects overlapped. After extensive practice, accuracy, response time, and guidance remained better in single-target search than in dual-target search. The results indicate that, when 2 different target representations are required for search, both representations cannot be maintained as accurately as in separate single-target searches. They suggest that baggage X-ray security screeners should specialize in one type of threat, or be trained to conduct 2 independent searches, 1 for each threat item.

Eye movements were monitored to examine search efficiency and infer how color is mentally represented to guide search for multiple targets. Observers located a single color target very efficiently by fixating colors similar to the target. However, simultaneous search for 2 colors produced a dual-target cost. In addition, as the similarity between the 2 target colors decreased, search efficiency suffered, resulting in more fixations on colors dissimilar to both target colors, which we describe as a "split-target cost." the patterns of fixations provide evidence to the type of mental representations guiding search. When the 2 targets are dissimilar, they are apparently encoded as separate and discrete representations. The fixation patterns for more similar targets can be explained with either 2 discrete target representations or a single, unitary range containing the target colors as well as the colors between them in color space.

Previous studies have demonstrated a dual-target cost in visual search. In the current study, the relationship between search for one and search for two targets was investigated to examine the effects of target prevalence and practice. Color-shape conjunction stimuli were used with response time, accuracy and signal detection measures. Performance was lower in dual-target search compared with the combined performance for two independent single-target searches. The cost in response time slope disappeared with practice, but the cost in accuracy remained. Sensitivity was lower and the decision criterion more conservative in dual-target search than in single-target searches, suggesting that the representation of the target was less effective in dual-target search than in single-target search. Manipulation of target prevalence induced a bias in favor of the more likely correct response: target-present responses were likely under high target prevalence and target-absent responses were likely under low target prevalence. The prevalence effect was greater in dual-target search than single-target search, causing the dual-target cost to be larger under target prevalences that differed from 50%. These findings are important for applied tasks in which targets appear rarely and can differ from each other. For example, the low target prevalence in X-ray security searches may magnify the dual-target cost implicated in previous research with X-ray images (see Menneer, Cave, & Donnelly, 2009). Such a result would increase the need for security personnel to consider alternatives to dual-target search, such as specialization in detecting one target type or training to encourage independent searches for each target.

The probability of target presentation in visual search tasks influences target detection performance: this is known as the prevalence effect (Wolfe et al. 2005). Additionally, searching for several targets simultaneously reduces search performance: this is known as the dual-target cost (DTC: Menneer et al. 2007). The interaction between the DTC and prevalence effect was investigated in a single study by presenting one target in dual-target search at a higher level of prevalence than the other target (Target A: 45% Prevalence; Target B: 5% Prevalence). An overall DTC was found for both RTs and response accuracy. Furthermore, there was an effect of target prevalence in dual-target search, suggesting that, when one target is presented at a higher level of prevalence than the other, both the dual-target cost and the prevalence effect contribute to decrements in performance. The implications for airport X-ray screening are discussed.

With the use of X-ray images, performance in the simultaneous search for two target categories was compared with performance in two independent searches, one for each category. In all cases, displays contained one target at most. Dual-target search, for both categories simultaneously, produced a cost in accuracy, although the magnitude of this dual-target cost was affected by the nature of the targets. When target feature sets shared values, accuracy in dual-target search was equivalent to that in the less accurate of the two single-target searches. However, when targets comprised different feature sets, accuracy in dual-target search was lower than in either single-target search. These results held after practice. In conclusion, dual-target search performance depends on the target representations required for search. When combined representations contain conflicting values within the most informative feature dimensions, then there is a cost in performance. When target representations share features, the search can be guided by the common values so that resources are not wasted on irrelevant distractors. The implication is that security screener performance might be improved by specializing in searching for threat categories that share features. (PsycINFO Database Record (c) 2009 APA, all rights reserved).