Color in visual search

Ensemble percepts of colored targets among distractors are influenced by hue similarity, not categorical identity

Color can be used to group similar elements, and ensemble percepts of color can be formed for such groups. In real-life settings, however, elements of similar color are often spatially interspersed among other elements and seen against a background. Forming an ensemble percept of these elements would require the segmentation of the correct color signals for integration. Can the human visual system do this? We examined whether observers can extract the ensemble mean hue from a target hue distribution among distractors and whether a color category boundary between target and distractor hues facilitates ensemble hue formation. Observers were able to selectively judge the target ensemble mean hue, but the presence of distractor hues added noise to the ensemble estimates and caused perceptual biases. The more similar the distractor hues were to the target hues, the noisier the estimates became, possibly reflecting incomplete or inaccurate segmentation of the two hue ensembles. Asymmetries between nominally equidistant distractors and substantial individual variability, however, point to additional factors beyond simple mixing of target and distractor distributions. Finally, we found no evidence for categorical facilitation in selective ensemble hue formation.

Preserved recognition of basic visual features despite lack of awareness of shape: Evidence from a case of neglect

Human visual experience of objects comprises a combination of visual features, such as color, position, and shape. Spatial attention is thought to play a role in creating a coherent perceptual experience, integrating visual information coming from a given location, but the mechanisms underlying this process are not fully understood. Deficits of spatial attention in which this integration process does not occur normally, such as neglect, can provide insights regarding the mechanisms of spatial attention in visual object recognition. In this study, we describe a series of experiments conducted with an individual with neglect, DH. DH presents characteristic lack of awareness of the left side of individual objects, evidenced by poor object and face recognition, and impaired word reading. However, he exhibits intact recognition of color within the boundaries of the same objects he fails to recognize. Furthermore, he can also report the orientation and location of a colored region on the neglected left side despite lack of awareness of the shape of the region. Overall, DH shows selective lack of awareness of shape despite intact processing of basic visual features in the same spatial location. DH's performance raises intriguing questions and challenges about the role of spatial attention in the formation of coherent object percepts and visual awareness.

Contingent capture by color is sensitive to categorical color perception

Contingent capture (CC) theory postulates that attention can only be captured by top-down matching stimuli. Although the contingent capture of attention is a well-known and thoroughly studied phenomenon, there is still no consensus on the characteristics of the top-down template which guides the search for colors. We tried to replicate the classical contingent capture effect on color (Experiment 1) and then added linguistic processing to this perceptual effect (Experiment 2). In Experiment 1, attention was indeed captured by the cues of the same color as the target, while the cues of different colors were successfully ignored. In Experiment 2, the cue color was never identical to the target color but would either belong to the same linguistic category or not (i.e., linguistic matching and linguistic nonmatching cues). In both cases, cues were made to be equally perceptually distant from the target. Although, attention was captured by both cue types, the degree of capture was significantly higher for linguistic matching cues. Our research replicated the classic contingent capture effect but on color, and also demonstrated the effect of color categories in the search task. In short, we demonstrated the effect of color categories in the search task. Results show that the template for color search contains physical characteristics of color, as well as information about color category names.

The impact of display saturation on visual search performance in congenital colour vision deficiency

BACKGROUND

Colour-related search tasks are common in many professional fields. The study investigated whether increasing chromatic saturation can enhance the visual performance of individuals with colour vision deficiency (CVD) in colour-related search tasks.

METHODS

10 normal trichromats (5M, 5F; Mean (SD) age: 23.1 (3.3) years) and 15 individuals with CVD [8 deutans and 7 protans identified by HRR plates] (14M, 1F; aged 28.6 (8.7) years) participated in this study. Four naturalistic sceneries of everyday tasks/ birds, animals and flowers of 15 different colour combinations (1 pair of colours in each combination. e.g., 'brown/black' or 'red/green') were presented in 'low' saturation, 'original' (unaltered images) and 'high' saturation condition using the Psychopy program on a colour-calibrated monitor. On each trial, the subject was asked to identify a specific-coloured target.

RESULTS

Overall, the visual search performance index (expressed as product of accuracy and a reciprocal of reaction time (%correct*s-1) of the normal trichromats [Mean (SD):77.76% correct*s-1 (16.32)] was significantly higher than CVD [45.71% correct*s-1 (18.95)] in the "original" test images (p = 0.001), but in individuals with CVD, there was no significant difference between 'original' [45.71% correct*s-1 (18.95)] and 'high' saturation condition ([47.43% correct*s-1 (20.07)]; p > 0.05). However, colour-wise, increased saturation showed improvements (≥ 10%) in protans mainly for 'red' combinations with other colours such as white (i.e., 'red/white'), purple, orange, grey, green, brown and black.

CONCLUSION

The study suggests that increasing the saturation of certain colour combinations can potentially aid in the visual search performance of individuals with CVD. This knowledge will help in better counselling and management of the patients.

Good-enough attentional guidance

Theories of attention posit that attentional guidance operates on information held in a target template within memory. The template is often thought to contain veridical target features, akin to a photograph, and to guide attention to objects that match the exact target features. However, recent evidence suggests that attentional guidance is highly flexible and often guided by non-veridical features, a subset of features, or only associated features. We integrate these findings and propose that attentional guidance maximizes search efficiency based on a 'good-enough' principle to rapidly localize candidate target objects. Candidates are then serially interrogated to make target-match decisions using more precise information. We suggest that good-enough guidance optimizes the speed-accuracy-effort trade-offs inherent in each stage of visual search.

Evidence for top-down suppression of negative features in the target feature dimension

While searching for a goal-relevant object, an internal representation of the features necessary to identify the to-be-searched-for object (i.e., target) guides attention towards visual stimuli with matching properties. Recent evidence suggests that features that negatively define a target (i.e., negative features) also bias attentional allocation through top-down suppression. Since humans usually know what to look for, it will rarely, if ever, be the case that a negative feature defines a goal-relevant object alone. Thus, to better understand the relevance of top-down suppression, our participants searched for a target conjunctively defined by a positive (e.g., a blue bar) and a negative feature (e.g., a nonred bar) with both features realized within the same dimension (color in Experiments 1, 3 and 4, orientation in Experiment 2). Experiments 1 and 2 showed that reaction times were slower if cues with a negative feature preceded the target at the same versus a different position (i.e., validly vs. invalidly cued targets), indicating suppression. In contrast, cues with a task-irrelevant different-dimension feature elicited no significant reaction time difference between validly cued and invalidly cued trials. In addition, Experiment 3 showed that while negative cues were top-down suppressed, cues with a positive feature captured attention. This finding indicated that both positive and negative features guide visual attention through capture and suppression, respectively, during the search for a target defined by the presence of one and the absence of another feature from the same dimension. However, suppression seems to apply to the negative and all nontarget features in the task-relevant dimension. This was shown in Experiment 4, in which participants suppressed cues with a task-irrelevant color similarly to cues with a negative color.

Numerosity perception is tuned to salient environmental features

Numerosity perception is a key ability to guide behavior. However, current models propose that number units encode an abstract representation of numerosity regardless of the non-numerical attributes of the stimuli, suggesting rather coarse environmental tuning. Here we investigated whether numerosity systems spontaneously adapt to all visible items, or to subsets segregated by salient attributes such as color or pitch. We measured perceived numerosity after participants adapted to highly numerous stimuli with color either matched to or different from the test. Matched colors caused a 25% underestimation of numerosity, while different colors had virtually no effect. This was true both for physically different colors, and for the same colors perceived as different, via a color-assimilation illusion. A similar result occurred in the acoustic domain, where adaptation magnitude was halved when the adaptor and test differed in pitch. Taken together, our results support the idea that numerosity perception is selectively tuned to salient environmental attributes.

Facial hair may slow detection of happy facial expressions in the face in the crowd paradigm

Human visual systems have evolved to extract ecologically relevant information from complex scenery. In some cases, the face in the crowd visual search task demonstrates an anger superiority effect, where anger is allocated preferential attention. Across three studies (N = 419), we tested whether facial hair guides attention in visual search and influences the speed of detecting angry and happy facial expressions in large arrays of faces. In Study 1, participants were faster to search through clean-shaven crowds and detect bearded targets than to search through bearded crowds and detect clean-shaven targets. In Study 2, targets were angry and happy faces presented in neutral backgrounds. Facial hair of the target faces was also manipulated. An anger superiority effect emerged that was augmented by the presence of facial hair, which was due to the slower detection of happiness on bearded faces. In Study 3, targets were happy and angry faces presented in either bearded or clean-shaven backgrounds. Facial hair of the background faces was also systematically manipulated. A significant anger superiority effect was revealed, although this was not moderated by the target's facial hair. Rather, the anger superiority effect was larger in clean-shaven than bearded face backgrounds. Together, results suggest that facial hair does influence detection of emotional expressions in visual search, however, rather than facilitating an anger superiority effect as a potential threat detection system, facial hair may reduce detection of happy faces within the face in the crowd paradigm.

Hiding the Rabbit: Using a genetic algorithm to investigate shape guidance in visual search

During visual search, attention is guided by specific features, including shape. Our understanding of shape guidance is limited to specific attributes (closures and line terminations) that do not fully explain the richness of preattentive shape processing. We used a novel genetic algorithm method to explore shape space and to stimulate hypotheses about shape guidance. Initially, observers searched for targets among 12 random distractors defined, in radial frequency space, by the amplitude and phase of 10 radial frequencies. Reaction time (RT) was the measure of “fitness.” To evolve toward an easier search task, distractors with faster RTs survived to the next generation, “mated,” and produced offspring (new distractors for the next generation of search). To evolve a harder search, surviving distractors were those yielding longer RTs. Within eight generations of evolution, the method succeeds in producing visual searches either harder or easier than the starting search. In radial frequency space, easy distractors evolve amplitude × frequency spectra that are dissimilar to the target, whereas hard distractors evolve spectra that are more similar to the target. This method also works with naturally shaped targets (e.g., rabbit silhouettes). Interestingly, the most inefficient distractors featured a combination of a body and ear distractors that did not resemble the rabbit (visually or in spectrum). Adding extra ears to these distractors did not impact the search spectrally and instead made it easier to confirm a rabbit, once it was found. In general, these experiments show that shapes that are clearly distinct when attended are similar to each other preattentively.

A Neurodynamic Model of Saliency Prediction in V1

Lateral connections in the primary visual cortex (V1) have long been hypothesized to be responsible for several visual processing mechanisms such as brightness induction, chromatic induction, visual discomfort, and bottom-up visual attention (also named saliency). Many computational models have been developed to independently predict these and other visual processes, but no computational model has been able to reproduce all of them simultaneously. In this work, we show that a biologically plausible computational model of lateral interactions of V1 is able to simultaneously predict saliency and all the aforementioned visual processes. Our model's architecture (NSWAM) is based on Penacchio's neurodynamic model of lateral connections of V1. It is defined as a network of firing rate neurons, sensitive to visual features such as brightness, color, orientation, and scale. We tested NSWAM saliency predictions using images from several eye tracking data sets. We show that the accuracy of predictions obtained by our architecture, using shuffled metrics, is similar to other state-of-the-art computational methods, particularly with synthetic images (CAT2000-Pattern and SID4VAM) that mainly contain low-level features. Moreover, we outperform other biologically inspired saliency models that are specifically designed to exclusively reproduce saliency. We show that our biologically plausible model of lateral connections can simultaneously explain different visual processes present in V1 (without applying any type of training or optimization and keeping the same parameterization for all the visual processes). This can be useful for the definition of a unified architecture of the primary visual cortex.

Color scrambles reveal red and green half-wave linear mechanisms plus a mechanism selective for low chromatic contrast

This paper introduces a new method to determine how subjects make discriminations among red-green texture stimuli. More specifically, the method determines (1) the number of mechanisms in human vision sensitive to lights that vary along the constant-S cardinal axis (cSCA) of DKL space and (2) the sensitivity of each mechanism to cSCA lights. Each of five subjects was tested in four, separately-blocked tasks. In each task, the subject strove to detect the location of a patch of cSCA-scramble (a spatially random mixture of cSCA lights) in a large, annular background of cSCA-scramble with a different histogram. In different tasks the target patch was (1) redder, (2) greener, (3) higher in red-green contrast, and (4) lower in red-green contrast than the background. For each subject in each task, we measure how target salience is influenced by different cSCA lights. By assuming that in each task each subject uses a weighted sum of his-or-her available mechanisms to construct a "tool" that is optimal for detecting the target, we can derive the sensitivity functions of the mechanisms underlying performance. Results suggest that human vision possesses three mechanisms sensitive to cSCA lights: a red half-wave linear mechanism, a complementary green half-wave linear mechanism, and a third mechanism that is activated by color-scrambles with low chromatic contrast in high-chromatic-contrast backgrounds.

A comparison of the performance on extrinsic and intrinsic cartographic visualizations through correctness, response time and cognitive processing

The aim of this study was to compare the performance of two bivariate visualizations by measuring response correctness (error rate) and response time, and to identify the differences in cognitive processes involved in map-reading tasks by using eye-tracking methods. The present study is based on our previous research and the hypothesis that the use of different visualization methods may lead to significant cognitive-processing differences. We applied extrinsic and intrinsic visualizations in the study. Participants in the experiment were presented maps which depicted two variables (soil moisture and soil depth) and asked to identify the areas which displayed either a single condition (e.g., “find an area with low soil depth”) or both conditions (e.g., “find an area with high soil moisture and low soil depth”). The research sample was composed of 31 social sciences and humanities university students. The experiment was performed under laboratory conditions, and Hypothesis software was used for data collection. Eye-tracking data were collected for 23 of the participants. An SMI RED-m eye-tracker was used to determine whether either of the two visualization methods was more efficient for solving the given map-reading tasks. Our results showed that with the intrinsic visualization method, the participants spent significantly more time with the map legend. This result suggests that extrinsic and intrinsic visualizations induce different cognitive processes. The intrinsic method was observed to generally require more time and led to higher error rates. In summary, the extrinsic method was found to be more efficient than the intrinsic method, although the difference was less pronounced in the tasks which contained two variables, which proved to be better suited to intrinsic visualization.

The handicap of abnormal colour vision

All people with abnormal colour vision, except for a few mildly affected deuteranomals, report that they experience problems with colour in everyday life and at work. Contemporary society presents them with increasing problems because colour is now so widely used in printed materials and in computer displays. Equal opportunity law gives them protection against unfair discrimination in employment, so a decision to exclude a person from employment on the grounds of abnormal colour vision must now be well supported by good evidence and sound argument. This paper reviews the investigations that have contributed to understanding the nature and consequences of the problems they have. All those with abnormal colour vision are at a disadvantage with comparative colour tasks that involve precise matching of colours or discrimination of fine colour differences either because of their loss of colour discrimination or anomalous perception of metamers. The majority have problems when colour is used to code information, in man-made colour codes and in naturally occurring colour codes that signal ripeness of fruit, freshness of meat or illness. They can be denied the benefit of colour to mark out objects and organise complex visual displays. They may be unreliable when a colour name is used as an identifier. They are slower and less successful in search when colour is an attribute of the target object or is used to organise the visual display. Because those with the more severe forms of abnormal colour vision perceive a very limited gamut of colours, they are at a disadvantage in the pursuit and appreciation of those forms of art that use colour.

Testing the Cross-Cultural Generality of Hering's Theory of Color Appearance

This study examines the cross-cultural generality of Hering's (1878/1964) color-opponent theory of color appearance. English-speaking and Somali-speaking observers performed variants of two paradigms classically used to study color-opponency. First, both groups identified similar red, green, blue, and yellow unique hues. Second, 25 English-speaking and 34 Somali-speaking observers decomposed the colors present in 135 Munsell color samples into their component Hering elemental sensations-red,green,blue, yellow, white, and black-or else responded "no term." Both groups responded no term for many samples, notably purples. Somali terms for yellow were often used to name colors all around the color circle, including colors that are bluish according to Hering's theory. Four Somali Grue speakers named both green and blue elicitation samples by their term for green. However, that term did not name the union of all samples called blue or green by English speakers. A similar pattern was found among three Somali Achromatic speakers, who called the blue elicitation sample black or white. Thus, color decomposition by these Somali-speaking observers suggests a lexically influenced re-dimensionalization of color appearance space, rather than a simple reduction of the one proposed by Hering. Even some Somali Green-Blue speakers, whose data were otherwise similar to English, showed similar trends in yellow and blue usage. World Color Survey data mirror these results. These within- and cross-cultural violations of Hering's theory do not challenge the long-standing view that universal sensory processes mediate color appearance. However, they do demonstrate an important contribution of language in the human understanding of color.

Perceptual grouping constrains inhibition in time-based visual selection

In time-based visual selection, task-irrelevant, old stimuli can be inhibited in order to allow the selective processing of new stimuli that appear at a later point in time (the preview benefit; Watson & Humphreys, 1997). The current study investigated if illusory and non-illusory perceptual groups influence the ability to inhibit old and prioritize new stimuli in time-based visual selection. Experiment 1 showed that with Kanizsa-type illusory stimuli, a preview benefit occurred only when displays contained a small number of items. Experiment 2 demonstrated that a set of Kanizsa-type illusory stimuli could be selectively searched amongst a set of non-illusory distractors with no additional preview benefit obtained by separating the two sets of stimuli in time. Experiment 3 showed that, similarly to Experiment 1, non-illusory perceptual groups also produced a preview benefit only for a small number of number of distractors. Experiment 4 demonstrated that local changes to perceptually grouped old items eliminated the preview benefit. The results indicate that the preview benefit is reduced in capacity when applied to complex stimuli that require perceptual grouping, regardless of whether the grouped elements elicit illusory contours. Further, inhibition is applied at the level of grouped objects, rather than to the individual elements making up those groups. The findings are discussed in terms of capacity limits in the inhibition of old distractor stimuli when they consist of perceptual groups, the attentional requirements of forming perceptual groups and the mechanisms and efficiency of time-based visual selection.

The role of hue in visual search for texture differences: Implications for camouflage design

The purpose of camouflage is to be inconspicuous against a given background. Colour is an important component of camouflage, and the task of designing a single camouflage pattern for use against multiple different backgrounds is particularly challenging. As it is impossible to match the colour gamut of each background exactly, the question arises which colours from the different backgrounds should be incorporated in a camouflage pattern to achieve optimal concealment. Here, we used a visual search paradigm to address this question. Observers searched multi-coloured continuous textures for target regions defined by either the presence or absence of additional hues. Targets could be either a combination of five hues against a four-hued background ("patches"), or a combination of four hues against a five-hued background ("holes"). In Experiment 1, a search asymmetry was observed for the different targets, as observers were less accurate and slower at detecting holes than patches. Additionally, we observed a linear separability effect: search for a target was more difficult when the hue that defined the target was within the gamut of distractor colours (e.g. orange amongst reds and yellows). In Experiment 2, we further investigated "hole" targets designed for two different backgrounds and found that optimal concealment against both backgrounds was achieved by including intermediate colours that represented a compromise between the common colours and the unique colours of each background. The findings provide insights into how search asymmetries can be extended to complex texture properties and help inform the design process of camouflage for multiple backgrounds.

Flexible target templates improve visual search accuracy for faces depicting emotion

Theories of visual attention hypothesize that target selection depends upon matching visual inputs to a memory representation of the target - i.e., the target or attentional template. Most theories assume that the template contains a veridical copy of target features, but recent studies suggest that target representations may shift "off veridical" from actual target features to increase target-to-distractor distinctiveness. However, these studies have been limited to simple visual features (e.g., orientation, color), which leaves open the question of whether similar principles apply to complex stimuli, such as a face depicting an emotion, the perception of which is known to be shaped by conceptual knowledge. In three studies, we find confirmatory evidence for the hypothesis that attention modulates the representation of an emotional face to increase target-to-distractor distinctiveness. This occurs over-and-above strong pre-existing conceptual and perceptual biases in the representation of individual faces. The results are consistent with the view that visual search accuracy is determined by the representational distance between the target template in memory and distractor information in the environment, not the veridical target and distractor features.

Examining the influence of different types of dynamic change in a visual search task

It has been repeatedly demonstrated that when performing a visual search task, items can pop out of a display such that they are identified rapidly, independent of the number of distractors present. It has been less clear whether this type of pop-out is limited to static displays (e.g., images) or whether it can also occur in scenes containing movement, more akin to how we experience the real world. Recently, Jardine and Moore (Journal of Experimental Psychology: Human Perception and Performance, 42, 617-630, 2016) examined whether pop-out also occurs in displays consisting of dynamic motion - wherein items in the display rotated continuously until a critical frame that would elicit pop-out under static presentation conditions - and found that search was greatly impaired. It remains unclear, however, whether such impairment is exerted equivalently across all types of dynamic motions or if it is specific to orientation. In the present study, we replicate the original Jardine and Moore (Journal of Experimental Psychology: Human Perception and Performance, 42, 617-630, 2016) finding and extend this examination to another dimension - color change. We also explore whether search efficiency can be improved with dynamic context if aspects of the display become predictable. The results suggest that not all types of dynamic change impair search performance. Specifically, oddball color targets continue to pop out even when the items in the display are dynamic. Interestingly, adding predictable context did not aid search accuracy as expected, rather resulting in poorer performance. Taken together, the findings suggest that the influence of dynamic context on search performance is not absolute.

Conjunction search: Can we simultaneously bias attention to features and relations?

Attention allows selection of sought-after objects by tuning attention in a top-down manner to task-relevant features. Among other possible search modes, attention can be tuned to the exact feature values of a target (e.g., red, large), or to the relative target feature (e.g., reddest, largest item), in which case selection is context dependent. The present study tested whether we can tune attention simultaneously to a specific feature value (e.g., specific size) and a relative target feature (e.g., relative color) of a conjunction target, using a variant of the spatial cueing paradigm. Tuning to the specific feature of the target was encouraged by randomly presenting the conjunction target in a varying context of nontarget items, and feature-specific versus relational tuning was assessed by briefly presenting conjunction cues that either matched or mismatched the relative versus physical features of the target. The results showed that attention could be biased to the specific size and the relative color of the conjunction target or vice versa. These results suggest the existence of local and relatively low-level attentional control mechanisms that operate independently of each other in separate feature dimensions (color, size) to choose the best search strategy in line with current top-down goals.

Neural differences between chromatic- and luminance-driven attentional salience in visual search

Previous electroencephalographic research on attentional salience did not fully capture the complexities of low-level vision, which relies on both cone-opponent chromatic and cone-additive luminance mechanisms. We systematically varied color and luminance contrast using a visual search task for a higher contrast target to assess the degree to which the salience-computing attentional mechanisms are constrained by low-level visual inputs. In our first experiment, stimuli were defined by contrast that isolated chromatic or luminance mechanisms. In our second experiment, targets were defined by contrasts that isolated or combined achromatic and chromatic mechanisms. In both experiments, event-related potential waveforms contralateral and ipsilateral to the target were qualitatively different for chromatic- compared to luminance-defined stimuli. The same was true of the difference waves computed from these waveforms, with isoluminant stimuli eliciting a mid-latency posterior contralateral negativity (PCN) component and achromatic stimuli eliciting a complex of multiple components, including an early posterior contralateral positivity followed by a late-latency PCN. Combining color with luminance resulted in waveform and difference wave patterns equivalent to those of achromatic stimuli. When large levels of chromaticity contrast were added to targets with small levels of luminance contrast, PCN latency was speeded. In conclusion, the mechanisms underlying attentional salience are constrained by the low-level inputs they receive. Furthermore, speeded PCN latencies for stimuli that combine color and luminance signals compared to stimuli that contain luminance alone demonstrate that color and luminance channels are integrated during pre-attentive visual processing, before top-down allocation of attention is triggered.

Dual-target hazard perception: Could identifying one hazard hinder a driver's capacity to find a second?

Low-level cognitive processes like visual search are crucial for hazard detection. In dual-target searches, subsequent search misses (SSMs) are known to occur when the identification of one target impedes detection of another that is concurrently presented. Despite the high likelihood of concurrent hazards in busy driving environments, SSMs have not been empirically investigated in driving. In three studies, participants were asked to identify safety-related target(s) in simulated traffic scenes that contained zero, one, or two target(s) of low or high perceptual saliency. These targets were defined as objects or events that would have prevented safe travel in the direction indicated by an arrow preceding the traffic scene. Findings from the pilot study (n = 20) and Experiment 1 (n = 29) demonstrated that detecting one target hindered drivers' abilities to find a second from the same scene. In Experiment 2 (n = 30), explicit instructions regarding the level of risk were manipulated. It was found that search times were affected by the instructions, though SSMs persisted. Implications of SSMs in understanding the causes of some crashes are discussed, as well as future directions to improve ecological and criterion validity and to explore the roles of expertise and cognitive capabilities in multi-hazard detection.

What is a preattentive feature?

The concept of a preattentive feature has been central to vision and attention research for about half a century. A preattentive feature is a feature that guides attention in visual search and that cannot be decomposed into simpler features. While that definition seems straightforward, there is no simple diagnostic test that infallibly identifies a preattentive feature. This paper briefly reviews the criteria that have been proposed and illustrates some of the difficulties of definition.

Variation in target and distractor heterogeneity impacts performance in the centroid task

In a selective centroid task, the participant views a brief cloud of items of different types-some of which are targets, the others distractors-and strives to mouse-click the centroid of the target items, ignoring the distractors. Advantages of the centroid task are that multiple target types can appear in the same display and that influence functions, which estimate the weight of each stimulus type in the cloud on the perceived centroid for each participant, can be obtained easily and efficiently. Here we document the strong, negative impact on performance that results when the participant is instructed to attend to target dots that consist of two or more levels of a single feature dimension, even when those levels differ categorically from those of the distractor dots. The results also show a smaller, but still observable decrement in performance that results when there is heterogeneity in the distractor dots.

We can guide search by a set of colors, but are reluctant to do it

For some real-world color searches, the target colors are not precisely known, and any item within a range of color values should be attended. Thus, a target representation that captures multiple similar colors would be advantageous. If such a multicolor search is possible, then search for two targets (e.g., Stroud, Menneer, Cave, and Donnelly, Journal of Experimental Psychology: Human Perception and Performance, 38(1): 113-122, 2012) might be guided by a target representation that included the target colors as well as the continuum of colors that fall between the targets within a contiguous region in color space. Results from Stroud, Menneer, Cave, and Donnelly, Journal of Experimental Psychology: Human Perception and Performance, 38(1): 113-122, (2012) suggest otherwise, however. The current set of experiments show that guidance for a set of colors that are all from a single region of color space can be reasonably effective if targets are depicted as specific discrete colors. Specifically, Experiments 1-3 demonstrate that a search can be guided by four and even eight colors given the appropriate conditions. However, Experiment 5 gives evidence that guidance is sometimes sensitive to how informative the target preview is to search. Experiments 6 and 7 show that a stimulus showing a continuous range of target colors is not translated into a search target representation. Thus, search can be guided by multiple discrete colors that are from a single region in color space, but this approach was not adopted in a search for two targets with intervening distractor colors.

The attentional template is shifted and asymmetrically sharpened by distractor context

Theories of attention hypothesize the existence of an "attentional template" that contains target features in working or long-term memory. It is often assumed that the template contents are veridical, but recent studies have found that this is not true when the distractor set is linearly separable from the target (e.g., all distractors are "yellower" than an orange-colored target). In such cases, the target representation in memory shifts away from distractor features (Navalpakkam & Itti, 2007) and develops a sharper boundary with distractors (Geng, DiQuattro, & Helm, 2017). These changes in the target template are presumed to increase the target-to-distractor psychological distinctiveness and lead to better attentional selection, but it remains unclear what characteristics of the distractor context produce shifting versus sharpening. Here, we tested the hypothesis that the template representation shifts whenever the distractor set (i.e., all of the distractors) is linearly separable from the target but asymmetrical sharpening occurs only when linearly separable distractors are highly target-similar. Our results were consistent, suggesting that template shifting and asymmetrical sharpening are 2 mechanisms that increase the representational distinctiveness of targets from expected distractors and improve visual search performance. (PsycINFO Database Record (c) 2019 APA, all rights reserved).

Attention to colors induces surround suppression at category boundaries

We investigated how attention to a visual feature modulates representations of other features. The feature-similarity gain model predicts a graded modulation, whereas an alternative model asserts an inhibitory surround in feature space. Although evidence for both types of modulations can be found, a consensus has not emerged in the literature. Here, we aimed to reconcile these different views by systematically measuring how attention modulates color perception. Based on previous literature, we also predicted that color categories would impact attentional modulation. Our results showed that both surround suppression and feature-similarity gain modulate perception of colors but they operate on different similarity scales. Furthermore, the region of the suppressive surround coincided with the color category boundary, suggesting a categorical sharpening effect. We implemented a neural population coding model to explain the observed behavioral effects, which revealed a hitherto unknown connection between neural tuning shift and surround suppression.

Surface color and predictability determine contextual modulation of V1 firing and gamma oscillations

The integration of direct bottom-up inputs with contextual information is a core feature of neocortical circuits. In area V1, neurons may reduce their firing rates when their receptive field input can be predicted by spatial context. Gamma-synchronized (30–80 Hz) firing may provide a complementary signal to rates, reflecting stronger synchronization between neuronal populations receiving mutually predictable inputs. We show that large uniform surfaces, which have high spatial predictability, strongly suppressed firing yet induced prominent gamma synchronization in macaque V1, particularly when they were colored. Yet, chromatic mismatches between center and surround, breaking predictability, strongly reduced gamma synchronization while increasing firing rates. Differences between responses to different colors, including strong gamma-responses to red, arose from stimulus adaptation to a full-screen background, suggesting prominent differences in adaptation between M- and L-cone signaling pathways. Thus, synchrony signaled whether RF inputs were predicted from spatial context, while firing rates increased when stimuli were unpredicted from context.

Can we perceive two colors at the same time? A direct test of Huang and Pashler's (2007) Boolean map theory of visual attention

Can observers access two spatially separated color targets (e.g., red and green) at the same time (i.e., in parallel)? According to the Boolean map theory of visual attention (Huang & Pashler, Psychological Review, 114(3), 599-631, 2007), access to two different features that belong to the same dimension (e.g., red and green targets) is limited and therefore can be held only in a serial fashion. The current study proposes a strong test of the Boolean map theory of attention through the application of two of the most rigorous stochastic approaches to response times modeling-the system factorial technology (Townsend & Nozawa, 1995) and the logical rule models (Fifić, Little, & Nosofsky, Psychological Review, 117, 309-348, 2010). These approaches allowed identification of serial, parallel, and coactive architectures in the processing of multicolor targets. The results showed that multiple-color targets are processed serially when observers are required to process all the targets in the display (i.e., an exhaustive stopping rule), and in parallel or coactively when observers can terminate the search when one of the targets is found (i.e., self-terminating stopping rule). These results are generally inconsistent with predictions of the Boolean map theory. They highlight the role of stopping rules in multicolor visual search, as well as the flexibility of the attentional system in shifting between processing architectures.

Biasing Allocations of Attention via Selective Weighting of Saliency Signals: Behavioral and Neuroimaging Evidence for the Dimension-Weighting Account

Objects that stand out from the environment tend to be of behavioral relevance, and the visual system is tuned to preferably process these salient objects by allocating focused attention. However, attention is not just passively (bottom-up) driven by stimulus features, but previous experiences and task goals exert strong biases toward attending or actively ignoring salient objects. The core and eponymous assumption of the dimension-weighting account (DWA) is that these top-down biases are not as flexible as one would like them to be; rather, they are subject to dimensional constraints. In particular, DWA assumes that people can often not search for objects that have a particular feature but only for objects that stand out from the environment (i.e., that are salient) in a particular feature dimension. We review behavioral and neuroimaging evidence for such dimensional constraints in three areas: search history, voluntary target enhancement, and distractor handling. The first two have been the focus of research on DWA since its inception and the latter the subject of our more recent research. Additionally, we discuss various challenges to the DWA and its relation to other prominent theories on top-down influences in visual search.

Psychophysical evaluation of individual low-level feature influences on visual attention

In this study we provide the analysis of eye movement behavior elicited by low-level feature distinctiveness with a dataset of synthetically-generated image patterns. Design of visual stimuli was inspired by the ones used in previous psychophysical experiments, namely in free-viewing and visual searching tasks, to provide a total of 15 types of stimuli, divided according to the task and feature to be analyzed. Our interest is to analyze the influences of low-level feature contrast between a salient region and the rest of distractors, providing fixation localization characteristics and reaction time of landing inside the salient region. Eye-tracking data was collected from 34 participants during the viewing of a 230 images dataset. Results show that saliency is predominantly and distinctively influenced by: 1. feature type, 2. feature contrast, 3. temporality of fixations, 4. task difficulty and 5. center bias. This experimentation proposes a new psychophysical basis for saliency model evaluation using synthetic images.

Using Rapid Serial Visual Presentation to Measure Set-Specific Capture, a Consequence of Distraction While Multitasking

This method uses a rapid serial visual presentation (RSVP) paradigm to measure the cost of distraction when participants maintain multiple search goals. The protocol identifies two types of distraction within a single task - contingent attentional capture and set-specific capture - that represent different types of limitations of cognitive processing. Participants search for letters in two or more "target" ink colors (e.g., green and orange) within a continuous RSVP stream of heterogeneously colored letters, while ignoring two peripheral RSVPs of letters. Upon detecting a target, participants are to identify the letter. On some trials, target-colored distractors appear in the periphery just prior to the presentation of a target, causing a drop in target identification performance. Contingent attentional capture is observed by examining performance on trials in which the peripheral distractor is the same color as the target on that trial (e.g., both orange). Set-specific capture is represented by performance on trials in which the peripheral distractor is target-colored (e.g., orange), but not the same color as the target on that trial (e.g., green.) By varying the amount of time (i.e., the number of stimuli appearing) between the presentation of the distractor and the target, researchers can observe how participants recover from these distraction costs over time. As compared to static displays that are often used to measure contingent attentional capture, the dynamic display produces much larger effects, allowing the researcher to identify subtle effects of smaller manipulations. An unusual aspect of our design is that it employs a continuous display; "filler" stimuli connect one trial to the next seamlessly, and participants respond during this interval whenever they detect a target. The continuous display reduces chance performance to near-zero levels (rather than 50%) and provides researchers with a more sensitive measure of performance differences across trial types.

Effects of absolute luminance and luminance contrast on visual search in low mesopic environments

Diverse adaptive visual processing mechanisms allow us to complete visual search tasks in a wide visual photopic range (>0.6 cd/m²). Whether search strategies or mechanisms known from this range extend below, in the mesopic and scotopic luminance spectra (<0.6 cd/m²), has yet to be addressed. Based on a study that addressed simple target discrimination in luminance environments using contrast-dependent behavioral efficiency functions, we assessed visual search in more complex-feature and conjunction-search paradigms. The results verify the previously reported deficiency windows defined by an interaction of base luminance and luminance contrast for more complex visual-search tasks. Based on significant regression analyses, a more precise definition of the magnitude of contribution of different contrast parameters. Characterized feature search patterns had approximately a 2.5:1 ratio of contribution from the Michelson contrast property relative to Weber contrast, whereas the ratio was approximately 1:1 in a serial-search condition. The results implicate near-complete magnocellular isolation in a visual-search paradigm that has yet to be demonstrated. Our analyses provide a new method of characterizing visual search and the first insight in its underlying mechanisms in luminance environments in the low mesopic and scotopic spectra.

Perceptual Color Space Representations in the Oculomotor System Are Modulated by Surround Suppression and Biased Selection

The oculomotor system utilizes color extensively for planning saccades. Therefore, we examined how the oculomotor system actually encodes color and several factors that modulate these representations: attention-based surround suppression and inherent biases in selecting and encoding color categories. We measured saccade trajectories while human participants performed a memory-guided saccade task with color targets and distractors and examined whether oculomotor target selection processing was functionally related to the CIE (x,y) color space distances between color stimuli and whether there were hierarchical differences between color categories in the strength and speed of encoding potential saccade goals. We observed that saccade planning was modulated by the CIE (x,y) distances between stimuli thus demonstrating that color is encoded in perceptual color space by the oculomotor system. Furthermore, these representations were modulated by (1) cueing attention to a particular color thereby eliciting surround suppression in oculomotor color space and (2) inherent selection and encoding biases based on color category independent of cueing and perceptual discriminability. Since surround suppression emerges from recurrent feedback attenuation of sensory projections, observing oculomotor surround suppression suggested that oculomotor encoding of behavioral relevance results from integrating sensory and cognitive signals that are pre-attenuated based on task demands and that the oculomotor system therefore does not functionally contribute to this process. Second, although perceptual discriminability did partially account for oculomotor processing differences between color categories, we also observed preferential processing of the red color category across various behavioral metrics. This is consistent with numerous previous studies and could not be simply explained by perceptual discriminability. Since we utilized a memory-guided saccade task, this indicates that the biased processing of the red color category does not rely on sustained sensory input and must therefore involve cortical areas associated with the highest levels of visual processing involved in visual working memory.

Practice reduces set-specific capture costs only superficially

Contingent attentional capture costs are doubled or tripled under certain conditions when multiple attentional sets guide visual search (e.g., "search for green letters" and "search for orange letters"). Such "set-specific" capture occurs when a potential target that matches one attentional set (e.g., a green stimulus) impairs the ability to identify a temporally proximal target that matches another attentional set (e.g., an orange stimulus). In the present study, we examined whether these severe set-specific capture effects could be attenuated through training. In Experiment 1, half of participants experienced training consisting of mostly trials involving a set switch from distractor to target, while the other half experienced training consisting of mostly trials in which a set switch was not required. Upon test, participants trained on set switches produced greatly reduced set-specific capture effects compared to their own pretraining levels and compared to participants trained on trials without a set switch. However, in Experiments 2 and 3, we found that these training effects did not transfer to a new color context or even a single new target color, indicating that they were specific and involved low-level associative learning. We concluded that set-specific capture is pervasive and largely immutable, even with practice.

Competition in saccade target selection reveals attentional guidance by simultaneously active working memory representations

The content of visual working memory (VWM) guides attention, but whether this interaction is limited to a single VWM representation or functional for multiple VWM representations is under debate. To test this issue, we developed a gaze-contingent search paradigm to directly manipulate selection history and examine the competition between multiple cue-matching saccade target objects. Participants first saw a dual-color cue followed by two pairs of colored objects presented sequentially. For each pair, participants selectively fixated an object that matched one of the cued colors. Critically, for the second pair, the cued color from the first pair was presented either with a new distractor color or with the second cued color. In the latter case, if two cued colors in VWM interact with selection simultaneously, we expected the second cued color object to generate substantial competition for selection, even though the first cued color was used to guide attention in the immediately previous pair. Indeed, in the second pair, selection probability of the first cued color was substantially reduced in the presence of the second cued color. This competition between cue-matching objects provides strong evidence that both VWM representations interacted simultaneously with selection. (PsycINFO Database Record

On the Role of Symmetry in Visual Search

It is known that the efficiency of visual search for a target item among distractor items increases when distractors are similar to each other and decreases when target and distractors are similar. Here we show that symmetry relations between targets and distractors can alter search efficiency. When distractors form a background texture symmetrical about a vertical axis, search is easier than when they do not. In contrast, when some distractors are symmetrical with the target, search is more difficult than when they are not. These results suggest (1) that symmetry relations are processed in parallel and can help to distinguish a target from a distracting background and (2) that stimulus similarity can have several components even for a single feature (here, orientation).

Color-coded visualization of magnetic resonance imaging multiparametric maps

Multiparametric magnetic resonance imaging (mpMRI) data are emergingly used in the clinic e.g. for the diagnosis of prostate cancer. In contrast to conventional MR imaging data, multiparametric data typically include functional measurements such as diffusion and perfusion imaging sequences. Conventionally, these measurements are visualized with a one-dimensional color scale, allowing only for one-dimensional information to be encoded. Yet, human perception places visual information in a three-dimensional color space. In theory, each dimension of this space can be utilized to encode visual information. We addressed this issue and developed a new method for tri-variate color-coded visualization of mpMRI data sets. We showed the usefulness of our method in a preclinical and in a clinical setting: In imaging data of a rat model of acute kidney injury, the method yielded characteristic visual patterns. In a clinical data set of N = 13 prostate cancer mpMRI data, we assessed diagnostic performance in a blinded study with N = 5 observers. Compared to conventional radiological evaluation, color-coded visualization was comparable in terms of positive and negative predictive values. Thus, we showed that human observers can successfully make use of the novel method. This method can be broadly applied to visualize different types of multivariate MRI data.

Subset selective search on the basis of color and preview

In the preview paradigm observers are presented with one set of elements (the irrelevant set) followed by the addition of a second set among which the target is presented (the relevant set). Search efficiency in such a preview condition has been demonstrated to be higher than that in a full-baseline condition in which both sets are simultaneously presented, suggesting that a preview of the irrelevant set reduces its influence on the search process. However, numbers of irrelevant and relevant elements are typically not independently manipulated. Moreover, subset selective search also occurs when both sets are presented simultaneously but differ in color. The aim of the present study was to investigate how numbers of irrelevant and relevant elements contribute to preview search in the absence and presence of a color difference between subsets. In two experiments it was demonstrated that a preview reduced the influence of the number of irrelevant elements in the absence but not in the presence of a color difference between subsets. In the presence of a color difference, a preview lowered the effect of the number of relevant elements but only when the target was defined by a unique feature within the relevant set (Experiment 1); when the target was defined by a conjunction of features (Experiment 2), search efficiency as a function of the number of relevant elements was not modulated by a preview. Together the results are in line with the idea that subset selective search is based on different simultaneously operating mechanisms.

Distractor Heterogeneity versus Linear Separability in Colour Visual Search

D'Zmura, and Bauer, Jolicoeur, and Cowan demonstrated that a target whose chromaticity was linearly separable from distractor chromaticities was relatively easy to detect in a search display, whereas a target that was not linearly separable from the distractor chromaticities resulted in steep search slopes. This linear separability effect suggests that efficient colour visual search is mediated by a chromatically linear mechanism. Failure of this mechanism leads to search performance strongly influenced by number of search items (set size). In their studies, linear separability was confounded with distractor heterogeneity and thus the results attributed to linear separability were also consistent with the model of visual search proposed by Duncan and Humphreys in which search performance is determined in part by distractor heterogeneity. We contrasted the predictions based on linear separability and on the Duncan and Humphreys model by varying the ratios of the quantities of the two distractors and demonstrated the potent effects of linear separability in a design that deconfounded linear separability and distractor heterogeneity.

The Role of Color in Search Templates for Real-world Target Objects

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 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, whereas 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 second and third display of each trial run relative to the first display, demonstrating that pictures are more effective than word cues in guiding search. N2pc components were triggered more rapidly for targets in the second and third 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 noncolored 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.

Attentional guidance by relative features: Behavioral and electrophysiological evidence

Our ability to select task-relevant information from cluttered visual environments is widely believed to be due to our ability to tune attention to the particular elementary feature values of a sought-after target (e.g., red, orange, yellow). By contrast, recent findings showed that attention is often tuned to feature relationships, that is, features that the target has relative to irrelevant features in the context (e.g., redder, yellower). However, the evidence for such a relational account is so far exclusively based on behavioral measures that do not allow a safe inference about early perceptual processes. The present study provides a critical test of the relational account, by measuring an electrophysiological marker in the EEG of participants (N2pc) in response to briefly presented distractors (cues) that could either match the physical features of the target or its relative features. In a first experiment, the target color and nontarget color were kept constant across trials. In line with a relational account, we found that only cues with the same relative color as the target were attended, regardless of whether the cues had the same physical color as the target. In a second experiment, we demonstrate that attention is biased to the exact target feature value when the target is embedded in a randomly varying context. Taken together, these results provide the first electrophysiological evidence that attention can modulate early perceptual processes differently; in a context-dependent manner versus a context-independent manner, resulting in marked differences in the range of colors that can attract attention.