Search Slopes Research Articles

When visual marking meets the attentional blink: More evidence for top-down, limited-capacity inhibition.

An attentional blink (AB) paradigm was used to investigate the attentional resources necessary for visual marking. The results showed that distractors presented inside the AB cannot easily be ignored despite participants anticipating a future target display. This supports the hypothesis that attentional resources are required for visual marking. In addition, probe dots were better detected on blinked distractors than on successfully ignored distractors, but only when the task required new items to be prioritized. In a final experiment, a stronger negative carry-over effect on search occurred for targets identical to distractors presented outside rather than inside the AB. This suggests that at least part of the inhibitory processes involved in visual marking are nonspatial. The study of visual selective attention focuses on our visual system’s ability to prioritize certain visual events over others. In brief, efficient prioritization depends on the spatial and temporal properties of, as well as the task constraints surrounding, the visual event. In the present study, we considered the interactions between these spatial and temporal factors. Visual selective attention has a strong spatial component. Typically, visual objects relevant to our behavior ( targets) occupy limited spatial regions in a cluttered visual field filled with numerous irrelevant objects (distractors) that are simultaneously present. Sometimes selection of a target is quite effortless. For instance, Treisman and Gelade (1980) found that observers were very efficient in searching for a blue T in a display filled with brown Ts and green Xs. In this single-feature search task it is as if the unique feature (color) of the target guides selection. Typically, therefore, the number of distractors (the display size) has little or no effect on search reaction times (RTs), creating flat slopes for the Display Size RT search functions. In other tasks, selection may be more effortful. For example, Treisman and Gelade found that search for a green T among brown Ts and green Xs was much less efficient than a single-feature search. In this conjunction search, visual attention cannot be guided by the target because the target is defined only by a combination of features it shares with both distractor types. Instead, it is as if attention has to be shifted around the display in an effortful way until the target is found. Typically, therefore, conjunction-search RTs are dependent on the number of items simultaneously present, resulting in a relatively steep search slope (see Wolfe, 1994, and Wolfe, Cave, & Franzel, 1989; for variations; but see Duncan & Humphreys, 1989, for a different explanation).

Role of structure and component in judgments of visual similarity of Chinese characters.

Skilled readers of Chinese participated in sorting and visual search experiments. The sorting results showed that under conditions of conflicting information about structure and component, subjective judgments of the visual similarity among characters were based on the characters' overall configurations (i.e., structures) rather than on the common components the characters possessed. In visual search, both structure and component contributed to the visual similarity reflected by the search efficiency. The steepest search slopes (thus the most similar target-distractor pairs) were found when the target and the distractor characters had the same structure and shared 1 common component, compared with when they had different structures and/or shared no common components. Results demonstrated that character structure plays a greater role in the visual similarity of Chinese characters than has been considered.

Is visual search really like foraging?

The visual-search paradigm provides a controlled and easy to implement experimental situation in which to study the search process. However, little work has been carried out in humans to investigate the extent to which traditional visual-search tasks are similar to more general search or foraging. Here we report results from a task in which search involves walking around a room and leaning down to inspect individual locations. Consistent with more traditional search tasks, search time increases linearly with display size, and the target-present to target-absent search slope is 1:2. However, although rechecking of locations did occur, compared to more traditional search it was relatively rare, suggesting an increased role for memory.

Visual search: efficiency continuum or distinct processes?

Whether visual search involves two distinct processes--traditionally referred to as serial and parallel--or a single process operating on a continuum of efficiency or speed, is an issue with a long history in the study of attention. On the basis of the unimodality of search slope distributions in a very large data set, Wolfe (1998) argued for a continuum model. Reanalysis of this data set using statistical procedures more appropriate for adjudicating between continuous and discontinuous models supports the existence of two distinct processes.

Visual search for a socially defined feature: what causes the search asymmetry favoring cross-race faces?

Levin (1996, 2000) reported that white subjects search for black targets more quickly than they search for white targets, suggesting that black faces are perceived as having a feature that is lacking in white faces. Here we test one of the implications of this asymmetry by having subjects search for same-race (SR) and cross-race (CR) faces that are distorted to look less like each other (producing caricatures that enhance race-specifying features), or are distorted to look more like each other (a prototypical distortion expected to reduce the salience of race-specifying features). Experiments 1 and 2 show that caricaturing the feature-positive CR distractors speeds search for the SR face and that prototypical distortion slows this search. The same distortions in SR faces did not affect the search slopes. However, these distortions also eliminated the overall advantage for CR faces. Experiment 3 shows that trial-to-trial variation in the specific distractors in each display can eliminate the asymmetry and suggests that this asymmetry depends on the subjects' ability to set a consistent a priori perceptual criterion when searching for a CR target, while the distortion effects emphasize the importance of distractor-rejection processes in determining the form of a serial search asymmetry.

The preattentive emperor has no clothes: A dynamic redressing.

Preattentive models of early vision have not been supported by the evidence. Instead, an input filtering system, which is dynamically reconfigured so as to optimize performance on the task at hand, is proposed. As a case in point, the authors examined Sagi and Julesz's (1985a) claim that detection tasks are processed preattentively and efficiently (shallow search slopes), whereas discrimination tasks require focal attention and yield inefficient steep slopes. In 5 visual search experiments, efficiency was found to depend not on the nature of the task but on whether the task is single or dual. The second component of a dual task, whether detection or discrimination, is performed inefficiently if it does not fit the configuration of the input system, which had been set optimally for the first component. But, even the second component is processed efficiently if there is enough time to reconfigure the system after processing the first component.

The preattentive emperor has no clothes: a dynamic redressing.

Preattentive models of early vision have not been supported by the evidence. Instead, an input filtering system, which is dynamically reconfigured so as to optimize performance on the task at hand, is proposed. As a case in point, the authors examined Sagi and Julesz's (1985a) claim that detection tasks are processed preattentively and efficiently (shallow search slopes), whereas discrimination tasks require focal attention and yield inefficient steep slopes. In 5 visual search experiments, efficiency was found to depend not on the nature of the task but on whether the task is single or dual. The second component of a dual task, whether detection or discrimination, is performed inefficiently if it does not fit the configuration of the input system, which had been set optimally for the first component. But, even the second component is processed efficiently if there is enough time to reconfigure the system after processing the first component.

A search asymmetry reversed by figure-ground assignment.

We report evidence demonstrating that a search asymmetry favoring concave over convex targets can be reversed by altering the figure-ground assignment of edges in shapes. Visual search for a concave target among convex distractors is faster than search for a convex target among concave distractors (a search asymmetry). By using shapes with ambiguous local figure-ground relations, we demonstrated that search can be efficient (with search slopes around 10 ms/item) or inefficient (with search slopes around 30-40 ms/item) with the same stimuli, depending on whether edges are assigned to concave or convex "figures." This assignment process can operate in a top-down manner, according to the task set. The results suggest that attention is allocated to spatial regions following the computation of figure-ground relations in parallel across the elements present. This computation can also be modulated by top-down processes.

Facial Expressions of Emotion: Are Angry Faces Detected More Efficiently?

The rapid detection of facial expressions of anger or threat has obvious adaptive value. In this study, we examined the efficiency of facial processing by means of a visual search task. Participants searched displays of schematic faces and were required to determine whether the faces displayed were all the same or whether one was different. Four main results were found: (1) When displays contained the faces, people were slower in detecting the absence of a discrepant face when the faces displayed angry (or sad/angry) rather than happy expressions. (2) When displays contained a discrepant face people were faster in detecting this when the discrepant face displayed an angry rather than a happy expression. (3) Neither of these patterns for same and different displays was apparent when face displays were inverted, or when just the mouth was presented in isolation. (4) The search slopes for angry targets were significantly lower than for happy targets. These results suggest that detection of angry facial expressions is fast and efficient, although does not “pop-out” in the traditional sense.

Robust representations for faces: evidence from visual search.

We report evidence from visual search that people can develop robust representations for highly overlearned faces. When observers searched for their own face versus the face of an unfamiliar observer, search slopes and intercepts revealed consistently faster processing of self than stranger. These processing advantages persisted even after hundreds of presentations of the unfamiliar face and even for atypical profile and upside-down views. Observers not only showed rapid asymptotic recognition of their own face as the target, but could reject their own face more quickly as the distractor. These findings suggest that robust representations for a highly overlearned face may (a) mediate rapid asymptotic visual processing, (b) require extensive experience to develop, (c) contain abstract or view-invariant information, (d) facilitate a variety of processes such as target recognition and distractor rejection, and (e) demand less attentional resources.

Visual search for global/local stimulus features in humans and baboons

Fagot and Deruelle (1997) demonstrated that, when tested with identical visual stimuli, baboons exhibit an advantage in processing local features, whereas humans show the “global precedence” effect initially reported by Navon (1977). In the present experiments, we investigated the cause of this species difference. Humans and baboons performed a visual search task in which the target differed from the distractors at either the global or the local level. Humans responded more quickly to global than to local targets, whereas baboons did the opposite (Experiment 1). Human response times (RTs) were independent of display size, for both local and global processing. Baboon RTs increased linearly with display size, more so for global than for local processing. The search slope for baboons disappeared for continuous targets (Experiment 2). That effect was not due to variations in stimulus luminance (Experiment 3). Finally, variations in stimulus density affected global search slopes in baboons but not in humans (Experiment 4). Overall, results suggest that perceptual grouping operations involved during the processing of hierarchical stimuli are attention demanding for baboons, but not for humans.

Visual Search Processes in Healthy Aging

It is well established that there are two limiting types of visual search, a pre-attentive parallel and an attention-related serial process. Such different types of processing may depend upon different regions of the visual cortex and such a measurable dissociation of function could provide a useful marker for particular types of cortical pathology, such as that associated with Parkinson's disease, aging, and Alzheimer's disease. For example, recent studies in this laboratory have shown that people with Parkinson's disease have abnormal parallel but normal serial search functions. Plude and Doussard-Roosevelt (1989 Psychology and Aging4 98 – 105) found no difference in parallel processing between older and young adults, but found that older adults performed worse on a conjunction task. The aim of the present experiment was to extend this research and look at the effect of aging on other types of visual search task and to compare these findings to patients with Parkinson's disease and Alzheimer's disease to determine if different patterns of visual function occur. Twenty-five young adults (mean age 32.6 years) and twenty-five older adults (mean age 63.4 years) performed both a conjunction task and a task involving the detection of a target only slightly larger than the distractors (the ‘size’ task). Our hypothesis was that for both types of visual search there would be an increase in search slope in the older adult group compared to the younger adults. Results of a 2-factor (1 between and 1 within) ANOVA performed on the slope values indicate statistically significant main effects of both age ( F=7.661, p<0.008) and search task ( F=25.426, p<0.0001), where in both the conjunction and size task the slope value was significantly greater for the older than for the younger adults. The slope for the size task was significantly greater than that for the conjunction task for both age groups. The results therefore support our hypothesis and further work is in progress to determine the effects of Alzheimer's disease on different types of visual search processing.

Measuring Preattentive Processes: When is Pop-out Not Enough?

Three behavioural tasks (standard visual search, multiple target search, and number discrimination)were tested with avariety of stimuli relevantto two-stage theories of vision. One purpose was to compare the two versions of the search task; a second was to explore the relations between processes of search and enumeration. The two search tasks were quite consistent overall, with strong correlations between search RT slope and multiple target RT gain across stimuli. Analysis of RT slopes for the 1–3 range in the number discrimination task revealed four categories of stimuli: Small slopes (5–40ms/item); moderately large slopes (100–200ms/item); very large slopes (400ms/item or more), and negative slopes (−214ms/item). Although the correlations between number discrimination and the two search tasks were positive, there were several interesting discrepancies, including stimuli with large search slopes and small number discrimination slopes, and those with small search slopes and large number discrimination slopes. These patterns suggest that, at least for some stimuli, subjects may be able to enumerate targets while they are also searching for them, and that search processes sometimes interfere with the processes required for enumeration.

Age differences in visual search for feature, conjunction, and triple-conjunction targets.

The authors examined the ability of younger and older adults to search for targets defined by single features (feature search), conjunctions of 2 features (conjunction search), and conjunctions of 3 features (triple-conjunction search). Feature search was relatively age-invariant, with both older and younger adults displaying shallow search slopes. However, older adults did show reduced search rates for many conjunction targets. Interestingly, both older and younger adults benefited equivalently when an extra feature was available to define the conjunction target. That is, the relative amount of improvement in search performance was similar for younger and older adults when the triple-conjunction search was compared to the conjunction search. These results are discussed in terms of age-related differences in the effectiveness of bottom-up and top-down mechanisms that support visual search.

Age differences in visual search for feature, conjunction, and triple-conjunction targets.

The authors examined the ability of younger and older adults to search for targets defined by single features (feature search), conjunctions of 2 features (conjunction search), and conjunctions of 3 features (triple-conjunction search). Feature search was relatively age-invariant, with both older and younger adults displaying shallow search slopes. However, older adults did show reduced search rates for many conjunction targets. Interestingly, both older and younger adults benefited equivalently when an extra feature was available to define the conjunction target. That is, the relative amount of improvement in search performance was similar for younger and older adults when the triple-conjunction search was compared to the conjunction search. These results are discussed in terms of age-related differences in the effectiveness of bottom-up and top-down mechanisms that support visual search.

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.

Intermodal Compensatory Effects in a Visual Search Task with Congenitally Deaf Adults

In a visual search paradigm we measured the stimulus onset asynchrony (SOA) between a stimulus and a mask that was required to reach 90% correct responses. This procedure has the advantage of taking into account the real processing time and excluding the time for the generation of the motor response. Twelve congenitally deaf adult subjects and twelve normal subjects were given a visual search task for a target letter O among a varying number of distractor letters Q and vice-versa. In both groups we found the asymmetrical visual search pattern classically observed with parallel processing for the search for the target Q and with serial processing for the search for the target O (Treisman, 1985 Computer Vision, Graphics, and Image Processing31 156 – 177). The difference between the mean search slopes for an O target was not statistically significant between the groups; this might be due to the variability within the groups. The visual search amidst the congenitally deaf does not seem to benefit from a compensatory effect in relation to the acoustic deprivation. Our results seem to confirm data reported by Neville (1990 Annals of the New York Academy of Science 71 – 91) obtained by an electrophysiological technique based on event-related potentials. Nevertheless, the deaf subjects were 2.5 times faster at the visual search task.

Visual search for colour targets that are or are not linearly separable from distractors

D'Zmura [(1991) Vision Research, 31, 951–966 reported qualitative differences in visual search rates for a target colour in a background of differently coloured distractors depending on their colour configuration in CIE( x, y) space. A target colour that was chromatically mid-way between the distractor colours resulted in steep search slopes. A target off the distractor-distracto line, “popped out”. We replicated his finding in several loci, investigated several potential confounds, and discovered boundary conditions for the phenomenon: for a given target, the effect of collinearity dissipates with increasing distractor-distractor colour difference. Furthermore, within limits, performance was dependent on the target to distractor-line distance.

Aging and individual differences in the development of skilled memory search performance.

We examined individual differences in measures of multiple intellectual abilities and performance on a pure memory search task over 5 experimental sessions. Old (n = 104) and young (n = 97) participants showed expected patterns of substantial improvement in memory search intercepts and slopes in consistently mapped (CM) conditions, relative to varied mapping (VM) conditions. Initial (unskilled) CM and VM memory search was highly correlated with a Semantic Memory Access Speed factor and moderately correlated with General Intelligence. Structural equation models showed that measures of Semantic Memory Access remained a strong predictor of skilled CM search performance in both age groups despite individual differences in CM memory search performance changes. These results indicate qualitative differences in the nature of automaticity between memory search and visual search and suggest age invariance in the mechanisms determining automaticity in memory search.

Dyslexie développementale et attention visuo-spatiale

Summary: Developmental dyslexia and visuo-spatial attention. This paper gives an overview of the literature on the visuo-perceptual and visuo-attentional deficits of dyslexic children. The hypothesis that visuo-attentional problems cooccur with dyslexia was assessed through a task of search for a target among distractors. Serial-attentional visual processing was required in one task condition (searchfor 0 among Qs) whereas the other condition involved parallel/automatic processing (search for Q among 0 s). Ten dyslexic subjects constituted the experimental group. Their performance was compared to that of 20 non-dyslexic children, 10 matched for chronological age and 10 for reading age. The performance of dyslexic subjects was similar to that of children matched for chronological age in the parallel/automatic condition. In contrast, their performance differed by an increased search slope front that of the two control groups in the seriallattentional condition. These results suggest the existence of visuo-attentional deficits in dyslexic children. Key words : visual search, parallel vs attentional processing, developmental dyslexia.