Theories Of Visual Search Research Articles

What are the shapes of response time distributions in visual search?

Many visual search experiments measure response time (RT) as their primary dependent variable. Analyses typically focus on mean (or median) RT. However, given enough data, the RT distribution can be a rich source of information. For this paper, we collected about 500 trials per cell per observer for both target-present and target-absent displays in each of three classic search tasks: feature search, with the target defined by color; conjunction search, with the target defined by both color and orientation; and spatial configuration search for a 2 among distractor 5s. This large data set allows us to characterize the RT distributions in detail. We present the raw RT distributions and fit several psychologically motivated functions (ex-Gaussian, ex-Wald, Gamma, and Weibull) to the data. We analyze and interpret parameter trends from these four functions within the context of theories of visual search.

Modeling automatic threat detection: Development of a face-in-the-crowd task.

An angry face is expected to be detected faster than a happy face because of an early, stimulus-driven analysis of threat-related properties. However, it is unclear to what extent results from the visual search approach-the face-in-the-crowd task-mirror this automatic analysis. The paper outlines a model of automatic threat detection that combines the assumption of a neuronal system for threat detection with contemporary theories of visual search. The model served as a guideline for the development of a new face-in-the-crowd task. The development involved three preliminary studies that provided a basis for the selection of angry and happy facial stimuli resembling each other in respect to perceptibility, homogeneity, and intensity. With these stimuli a signal detection version of the search task was designed and tested. For crowds composed of neutral faces, the sensitivity measure d' proved the expected detection advantage of angry faces compared to happy faces. However, the emotional expression made no difference if a neutral face had to be detected in crowd composed of either angry or happy faces. Results are in line with the assumption of a stimulus-driven shift of attention giving rise to the superior detection of angry target faces.

Collaboration during visual search

Two experiments examine how collaboration influences visual search performance. Working with a partner or on their own, participants reported whether a target was present or absent in briefly presented search displays. We compared the search performance of individuals working together (collaborative pairs) with the pooled responses of the individuals working alone (nominal pairs). Collaborative pairs were less likely than nominal pairs to correctly detect a target and they were less likely to make false alarms. Signal detection analyses revealed that collaborative pairs were more sensitive to the presence of the target and had a more conservative response bias than the nominal pairs. This pattern was observed even when the presence of another individual was matched across pairs. The results are discussed in the context of task-sharing, social loafing and current theories of visual search.

Bayesian theory of visual search

Human performance in searching for a tilted target among vertical distractors is well described by signal detection theory (SDT) models which combine the outputs of noisy detectors using a maximum or summation operation [1]. These theories assume that each detector is a neuron best tuned to the target, and that a detector’s response to the target or distractor follows a Gaussian distribution. However, recent work has questioned the validity of SDT measures in search tasks with more complex stimulus distributions, for instance ones in which distractors flank the target in orientation space [2]. Instead, a saliency-based signal-to-noise ratio was proposed to measure task difficulty. A drawback of this theory is that it cannot predict receiver operating characteristics (ROCs).

Hiding and finding: The relationship between visual concealment and visual search

As an initial step toward developing a theory of visual concealment, we assessed whether people would use factors known to influence visual search difficulty when the degree of concealment of objects among distractors was varied. In Experiment 1, participants arranged search objects (shapes, emotional faces, and graphemes) to create displays in which the targets were in plain sight but were either easy or hard to find. Analyses of easy and hard displays created during Experiment 1 revealed that the participants reliably used factors known to influence search difficulty (e.g., eccentricity, target-distractor similarity, presence/absence of a feature) to vary the difficulty of search across displays. In Experiment 2, a new participant group searched for the targets in the displays created by the participants in Experiment 1. Results indicated that search was more difficult in the hard than in the easy condition. In Experiments 3 and 4, participants used presence versus absence of a feature to vary search difficulty with several novel stimulus sets. Taken together, the results reveal a close link between the factors that govern concealment and the factors known to influence search difficulty, suggesting that a visual search theory can be extended to form the basis of a theory of visual concealment.

The course of visual searching to a target in a fixed location: Electrophysiological evidence from an emotional flanker task

The present study investigated the course of visual searching to a target in a fixed location, using an emotional flanker task. Event-related potentials (ERPs) were recorded while participants performed the task. Emotional facial expressions were used as emotion-eliciting triggers. The course of visual searching was analyzed through the emotional effects arising from these emotion-eliciting stimuli. The flanker stimuli showed effects at about 150–250 ms following the stimulus onset, while the effect of target stimuli showed effects at about 300–400 ms. The visual search sequence in an emotional flanker task moved from a whole overview to a specific target, even if the target always appeared at a known location. The processing sequence was “parallel” in this task. The results supported the feature integration theory of visual search.

Visual search-based design and evaluation of screen magnifiers for older and visually impaired users

The purpose of this study was to achieve the following goals: apply some principles in visual search theory to design and develop a screen magnifier; evaluate if applying such features could increase user performance; and make some recommendations for designing a screen magnifier according to the findings of the study. In order to achieve these goals, a screen magnifier and an experimental tool to present stimulus words were developed. Seventy-two elderly Chinese adults took part in the experiment. The findings indicate that, for a screen magnifier, the overlapping mode is superior to the parallel mode; the yellow-highlighted background is superior to the non-highlighted background; for lower-density text, the dual output mode (visual+auditory) is superior to the mono output mode (visual only), while for the higher-density text, no difference was found between the mono and dual output modes. Based on the findings, several recommendations are made. The overlapping mode should be set as the default working mode. Yellow could be a reasonable choice of background color for a magnifier when the text is black. Auditory output support should be used, and users should be advised to use dual output when the text density is lower and mono output when the text density is higher.

Attention operates on non-retinotopic feature maps in visual search

At the heart of feature integration theory is a master map which operates on dimension-based feature maps. It is assumed that these feature maps are organized retinotopically. If in conjunction search, for example, a horizontal green line has to be searched for, the master map 'looks' at the retinotopic position (x,y) in both the color and the orientation maps whether they contain a 'green' and a 'horizontal' entry, respectively. Here, we show evidence that attention operates on non-retinotopically organized feature maps. We presented a Ternus - Pikler display where three squares shifted horizontally back and forth by one inter-square distance. This created a periodic motion where two squares always overlapped in successive frames. Each square contained a different conjunction search display and the observers were asked to identify a conjunction target in the central square. Theories assuming retinotopically organized feature maps predict poor performance because of the spatial overlap of the different search displays in successive frames. However, we found a remarkably good performance. Moreover, observers clearly perceived the search display in the central square because, as we argue, perception and attention are based on non-retinotopic feature maps--contrary to most theories of visual search.

Scientific Problem Solving in a Virtual Laboratory: A Comparison Between Individuals and Pairs

This study compared individuals with pairs in a scientific problem-solving task. Participants interacted with a virtual psychological laboratory called Virtue to reason about a visual search theory. To this end, they created hypotheses, designed experiments, and analyzed and interpreted the results of their experiments in order to discover which of five possible factors affected the visual search process. Before and after their interaction with Virtue, participants took a test measuring theoretical and methodological knowledge. In addition, process data reflecting participants’ experimental activities and verbal data were collected. The results showed a significant but equal increase in knowledge for both groups. We found differences between individuals and pairs in the evaluation of hypotheses in the process data, and in descriptive and explanatory statements in the verbal data. Interacting with Virtue helped all students improve their domain-specific and domain-general psychological knowledge.

Collaboration during visual search

Two experiments examine how collaboration influences visual search performance. Working with a partner or on their own, participants reported whether a target was present or absent in briefly presented search displays. We compared the search performance of individuals working together (collaborative pairs) with the pooled responses of the individuals working alone (nominal pairs). Collaborative pairs were less likely than nominal pairs to correctly detect a target and they were less likely to make false alarms. Signal detection analyses revealed that collaborative pairs were more sensitive to the presence of the target and had a more conservative response bias than the nominal pairs. This pattern was observed even when the presence of another individual was matched across pairs. The results are discussed in the context of task-sharing, social loafing and current theories of visual search.

Visual search and selective attention

Visual search is a key paradigm in attention research that has proved to be a test bed for competing theories of selective attention. The starting point for most current theories of visual search has been Treisman's “feature integration theory” of visual attention (e.g., Treisman & Gelade, 1980). A number of key issues that have been raised in attempts to test this theory are still pertinent questions of research today: (1) The role and (mode of) function of bottom-up and top-down mechanisms in controlling or “guiding” visual search; (2) in particular, the role and function of implicit and explicit memory mechanisms; (3) the implementation of these mechanisms in the brain; and (4) the simulation of visual search processes in computational or, respectively, neurocomputational (network) models. This paper provides a review of the experimental work and the—often conflicting—theoretical positions on these thematic issues, and goes on to introduce a set of papers by distinguished experts in fields designed to provide solutions to these issues.

Visual search near threshold: Some features are more equal than others

While searching for objects, we combine information from multiple visual modalities. Classical theories of visual search assume that features are processed independently prior to an integration stage. Based on this, one would predict that features that are equally discriminable in single feature search should remain so in conjunction search. We test this hypothesis by examining whether search accuracy in feature search predicts accuracy in conjunction search. Subjects searched for objects combining color and orientation or size; eye movements were recorded. Prior to the main experiment, we matched feature discriminability, making sure that in feature search, 70% of saccades were likely to go to the correct target stimulus. In contrast to this symmetric single feature discrimination performance, the conjunction search task showed an asymmetry in feature discrimination performance: In conjunction search, a similar percentage of saccades went to the correct color as in feature search but much less often to correct orientation or size. Therefore, accuracy in feature search is a good predictor of accuracy in conjunction search for color but not for size and orientation. We propose two explanations for the presence of such asymmetries in conjunction search: the use of conjunctively tuned channels and differential crowding effects for different features.

Perceptual encoding efficiency in visual search.

The authors present 10 experiments that challenge some central assumptions of the dominant theories of visual search. Their results reveal that the complexity (or redundancy) of nontarget items is a crucial but overlooked determinant of search efficiency. The authors offer a new theoretical outline that emphasizes the importance of nontarget encoding efficiency, and they test this proposal using dot pattern stimuli adapted from W. R. Garner and D. E. Clement (1963). The results provide converging support for the importance of nontarget encoding efficiency in accounting for visual search performance.

Visual search asymmetry with uncertain targets.

The underlying mechanism of search asymmetry is still unknown. Many computational models postulate top-down selection of target-defining features as a crucial factor. This feature selection account implies, and other theories implicitly assume, that predefined target identity is necessary for search asymmetry. The authors tested the validity of the feature selection account using a singleton search task without a predefined target. Participants conducted a target-defined and a singleton search task with a circle (O) and a circle with a vertical bar (Q). Search asymmetry was observed in both tasks with almost identical magnitude. The results were not due to trial-by-trial feature selection, because search asymmetry persisted even when the target was completely unpredictable. Asymmetry in the singleton search was also observed with more complex stimuli, Kanji characters. These results suggest that feature selection is not necessary for search asymmetry, and they impose important constraints on current visual search theories.

System reconfiguration, not resource depletion, determines the efficiency of visual search

We examined two theories of visual search: resource depletion, grounded in a static, built-in brain architecture, with attention seen as a limited depletable resource, and system reconfiguration, in which the visual system is dynamically reconfigured from moment to moment so as to optimize performance on the task at hand. In a dual-task paradigm, a search display was preceded by a visual discrimination task and was followed, after a stimulus onset asynchrony (SOA) governed by a staircase procedure, by a pattern mask. Search efficiency, as indexed by the slope of the function relating critical SOA to number of distractors, was impaired under dual-task conditions for tasks that were performed efficiently (shallow search slope) when done singly, but not for tasks performed inefficiently (steep slope) when done singly. These results are consistent with system reconfiguration, but not with resource depletion, models and point to a dynamic, rather than a static, architecture of the visual system.

Efficient visual search without top-down or bottom-up guidance

Two types of mechanisms have dominated theoretical accounts of efficient visual search. The first are bottom-up processes related to the characteristics of retinotopic feature maps. The second are top-down mechanisms related to feature selection. To expose the potential involvement of other mechanisms, we introduce a new search paradigm whereby a target is defined only in a context-dependent manner by multiple conjunctions of feature dimensions. Because targets in a multiconjunction task cannot be distinguished from distractors either by bottom-up guidance or top-down guidance, current theories of visual search predict inefficient search. While inefficient search does occur for the multiple conjunctions of orientation with color or luminance, we find efficient search for multiple conjunctions of luminance/size, luminance/shape, and luminance/topology. We also show that repeated presentations of either targets or a set of distractors result in much faster performance and that bottom-up feature extraction and top-down selection cannot account for efficient search on their own. In light of this, we discuss the possible role of perceptual organization in visual search. Furthermore, multiconjunction search could provide a new method for investigating perceptual grouping in visual search.

Memory for rejected distractors in visual search?

Theories of visual search have generally assumed that rejected distractors are marked so as to avoid further processing of these items (memory-driven search). To test this assumption, Horowitz and Wolfe (1998) developed the randomized search paradigm, in which standard static search is compared to dynamic search where items are randomly replotted at new locations throughout a trial. Memory-driven search predicts that search slopes should double in the dynamic condition. After reviewing earlier findings that slopes were similar in the two conditions, we present two new experiments. Experiment 1 replicated and extended our previous findings using a larger range of set sizes, a slower rate of change, and adding a fixed location dynamic condition. Experiment 2 employed stimuli that required overt fixation. Neither experiment showed evidence for memory-driven search. We conclude that visual search is best understood as a series of successive judgements of the momentary probability of target presence.

How does attention attenuate target–distractor interference in vision?: Evidence from magnetoencephalographic recordings

This study used magnetoencephalographic and electroencephalographic recordings to investigate the neural mechanisms that underlie the attentional resolution of ambiguous feature coding in visual search. We addressed this issue by comparing neural activity related to target discrimination under conditions of more versus less feature overlap between the target and distractor items. The results show that increasing feature overlap leads to a focal enhancement of neural activity in ventral occipito-temporal areas, consistent with the larger need to attenuate distractor interference. Furthermore, the results suggest that distractor attenuation proceeds as a stepwise operation, with different spatial locations containing interfering features being suppressed successively. These findings support theories of visual search that emphasize location-based attentional selection as a key mechanism in resolving ambiguous feature coding in vision.

The role of priming in conjunctive visual search

To assess the role of priming in conjunctive visual search tasks, we systematically varied the consistency of the target and distractor identity between different conditions. Search was fastest in the standard conjunctive search paradigm where identities remained constant. Search was slowest when potential target identity varied predictably for each successive trial (the ‘switch’ condition). The role of priming was also demonstrated on a trial-by-trial basis in a ‘streak’ condition where target and distractor identity was unpredictable yet was consistent within streaks. When the target to be found was the same for a few trials in a row, search performance became similar to that when the potential target was the same on all trials. A similar pattern was found for the target absent trials, suggesting that priming is based on the whole search array rather than just the target in each case. Further analysis indicated that the effects of priming are sufficiently strong to account for the advantage seen for the conjunctive search task. We conclude that the role of priming in visual search is underestimated in current theories of visual search and that differences in search times often attributed to top-down guidance may instead reflect the benefits of priming.

The psychophysics of visual search

Most theories of visual search emphasize issues of limited versus unlimited capacity and serial versus parallel processing. In the present article, we suggest a broader framework based on two principles, one empirical and one theoretical. The empirical principle is to focus on conditions at the intersection of visual search and the simple detection and discrimination paradigms of spatial vision. Such simple search conditions avoid artifacts and phenomena specific to more complex stimuli and tasks. The theoretical principle is to focus on the distinction between high and low threshold theory. While high threshold theory is largely discredited for simple detection and discrimination, it persists in the search literature. Furthermore, a low threshold theory such as signal detection theory can account for some of the phenomena attributed to limited capacity or serial processing. In the body of this article, we compare the predictions of high threshold theory and three versions of signal detection theory to the observed effects of manipulating set size, discriminability, number of targets, response bias, external noise, and distractor heterogeneity. For almost all cases, the results are inconsistent with high threshold theory and are consistent with all three versions of signal detection theory. In the Discussion, these simple theories are generalized to a larger domain that includes search asymmetry, multidimensional judgements including conjunction search, response time, search with multiple eye fixations and more general stimulus conditions. We conclude that low threshold theories can account for simple visual search without invoking mechanisms such as limited capacity or serial processing.