Concavities as basic features in visual search: evidence from search asymmetries

Concave cusps and negative curvature minima play an important role in many theories of visual shape perception. Cusps and minima are taken to be part boundaries, used to segment an object into parts. Because of their important role in determining object structure and because there is some evidence that object structure is processed in parallel, it might be expected that concave cusps and negative curvature minima are processed preferentially. We tested this conjecture in several visual search experiments. Visual search for a target with a concave cusp among totally convex distractors yields nearly flat slopes (< 10 msec/item) for both present and absent trials. Reversing the roles of the target and the distractor results in inefficient search. The same asymmetry is found when the concave cusp is replaced by other types of concavity. We conclude, therefore, the concavities can serve as basic features in visual search experiments. This conclusion implies that the unit of selection in a visual search task is an object, rather than a location.     

Attentional set for axis of symmetry in symmetry-defined visual search

Olivers and van der Helm (1998) showed that symmetry-defined visual search (for both symmetry and asymmetry) requires selective spatial attention. We hypothesize that an attentional set for the orientation of a symmetry axis also is involved in symmetry-defined visual search. We conducted three symmetry-defined visual search experiments with manipulations of the axis of symmetry orientations, and performance was better when the axis orientations within the search array were uniform, rather than a mixture of two orientations, and the attentional set for the axis orientation could be kept. In addition, search performance when the target was defined by the presence of symmetry was equivalent to that when the target was defined by a difference of symmetry axis orientation. These results suggest that attentional set for axis orientation plays a fundamental role in symmetry-defined visual search.     

Visual search for object orientation can be modulated by canonical orientation

The authors studied the influence of canonical orientation on visual search for object orientation. Displays consisted of pictures of animals whose axis of elongation was either vertical or tilted in their canonical orientation. Target orientation could be either congruent or incongruent with the object's canonical orientation. In Experiment 1, vertical canonical targets were detected faster when they were tilted (incongruent) than when they were vertical (congruent). This search asymmetry was reversed for tilted canonical targets. The effect of canonical orientation was partially preserved when objects were high-pass filtered, but it was eliminated when they were low-pass filtered, rendering them as unfamiliar shapes (Experiment 2). The effect of canonical orientation was also eliminated by inverting the objects (Experiment 3) and in a patient with visual agnosia (Experiment 4). These results indicate that orientation search with familiar objects can be modulated by canonical orientation, and they indicate a top-down influence on orientation processing.     

Structural salience and the nonaccidentality of a gestalt

We perceive structure through a process of perceptual organization. Here we report a new perceptual organization phenomenon-the facilitation of visual grouping by global curvature. Observers viewed patterns that they perceived as organized into collections of curves. The patterns were perceptually ambiguous such that the perceived orientation of the patterns varied from trial to trial. When patterns were sufficiently dense and proximity was equated for the predominant perceptual alternatives, observers tended to perceive the organization with the greatest curvature. This effect is tantamount to visual grouping by maximal curvature and thus demonstrates an unprecedented effect of global structure on perceptual organization. We account for this result with a model that predicts the perceived organization of a pattern as function of its nonaccidentality, which we define as the probability that it could have occurred by chance. Our findings demonstrate a novel relationship between the geometry of a pattern and the visual salience of global structure.     

Local and global orientation in visual search

Six experiments investigated the role of global (shape) and local (contour) orientation in visual search for an orientation target. Experiment 1 demonstrated thatsearch for a conjunction of local contours with a distinct global orientation was less efficient than search for a target featurally distinct in terms of both global and local contour orientation. However, Experiments 2 and 4 demonstrated that the presence of a unique line contour was neither sufficient nor necessary to allow efficient search. Experiment 5 found thatsearch for a local orientation difference was strongly impeded by irrelevant variation in global orientation, arguing for a preeminent role for global orientation. Finally, Experiment 6 demonstrated that the orientation search asymmetry holds for the global orientation of stimuli. Taken together, the results are consistent with visual search processes guided predominately by a representation of global orientation.     

Visual search for direction of shading is influenced by apparent depth.

Recent reports of rapid visual search for some feature conjunctions suggested that preattentive vision might be sensitive to scene-based as well as to image-based features (Enns & Rensink, 1990a, 1990b). This study examined visual search for targets defined by the direction of a luminance gradient, a conjunction of luminance and relative location that often corresponds to object curvature and direction of lighting in naturalistic scenes. Experiment 1 showed that such search is influenced by several factors, including the type of gradient, the shape of the contour enclosing the gradient, and the background luminance. These factors were varied systematically in Experiment 2 in a three-dimensionality rating task and in a visual-search task. The factors combined interactively in the rating task, supporting the presence of an emergent property of three-dimensionality. In contrast, each factor contributed only additively to the speed of the visual-search task. This is inconsistent with the view that search is guided by specialized detectors for surface curvature or direction of lighting. Rather, it is in keeping with the view that search is governed by a number of "quick and dirty" processes that are implemented rapidly and in parallel across the visual field.     

Visual search for direction of shading is influenced by apparent depth

Recent reports ofrapid visual search for some feature conjunctions-suggested that preattentive vision might be sensitive to scene-based as well as to image-based features (Enns & Rensink, 1990a, 1990b). This study examined visual search for targets defined by the direction of a luminance gradient, a conjunction of luminance and relative location that often: corresponds to ob-= ject curvature and direction of lighting in naturalistic scenes. Experiment 1 showed that such search is influenced by several factors, including the type of gradient, the shape of the contour enclosing the gradient, and the background luminance. These factors were varied systematically in Experiment 2 in a three-dimensionality rating task and in a visual-search task. The factors combined interactively in the rating task, supporting the presence of an emergent property of three-dimensionality. In contrast, each factor contributed only additively to the speed of the visualsearch task. This is inconsistent with the view that search is guided by specialized detectors for surface curvature or direction of lighting. Rather, it is in keeping with the view that search is governed by a number of “quick and dirty” processes that are implemented rapidly and in parallel across the visual field.     

Early computation of part structure: evidence from visual search

The visual system represents object shapes in terms of intermediate-level parts. The minima rule proposes that the visual system uses negative minima of curvature to define boundaries between parts. We used visual search to test whether part structures consistent with the minima rule are computed preattentively--or at least, rapidly and early in visual processing. The results of Experiments 1 and 2 showed that whereas the search for a non-minima-segmented shape is fast and efficient among minima-segmented shapes, the reverse search is slow and inefficient. This asymmetry is expected if parsing at negative minima occurs obligatorily. The results of Experiments 3 and 4 showed that although both minima- and non-minima-segmented shapes pop out among unsegmented shapes, the search for minima-segmented shapes is significantly slower. Together, these results demonstrate that the visual system segments shapes into parts, using negative minima of curvature, and that it does so rapidly in early stages of visual processing.     

Event-related brain potentials and the efficiency of visual search for vertically and horizontally oriented stimuli

Reports that visual search is more efficient for vertically than for horizontally shaded objects suggested that search is influenced by a priori knowledge about the source of light. In this study, we examined search for targets defined by the orientation of luminance gradients and measured event-related brain potentials (ERPs). In Experiment 1, we examined search for stimuli that comprised gradual luminance differences. Response times showed the expected orientation anisotropy effect. ERP amplitudes in the P1 latency range were slightly more positive in response to horizontally oriented stimuli, whereas P3 amplitudes were more positive in response to nonsingleton vertically oriented stimuli. Experiment 2 compared search for stimuli that comprised gradual versus step differences in luminance. All the anisotropies that we observed in Experiment 1 could be replicated in Experiment 2. Moreover, these anisotropies were not dependent on the type of the luminance gradient. This finding is inconsistent with the view that search efficiency is influenced by a priori knowledge about the source of light. The behavioral and electrophysiological data are consistent with a context model of visual search. We propose that contextual modulation reduces redundancy and contributes to computing the saliency of visual information by implementing divisive normalization and multiplicative filtering.     

Serial and parallel search in pattern vision?

The nature of the processing of combinations of stimulus dimensions in human vision has recently been investigated. A study is reported in which visual search for suprathreshold positional information--vernier offsets, stereoscopic disparity, lateral separation, and orientation--was examined. The initial results showed that reaction times for visual search for conjunctions of stereoscopic disparity and either vernier offsets or orientation were independent of the number of distracting stimuli displayed, suggesting that disparity was searched in parallel with vernier offsets or orientation. Conversely, reaction times for detection of conjunctions of vernier offsets and orientation, or lateral separation and each of the other positional judgements, were related linearly to the number of distractors, suggesting serial search. However, practice has a significant effect upon the results, indicative of a shift in the mode of search from serial to parallel for all conjunctions tested as well as for single features. This suggests a reinterpretation of these and perhaps other studies that use the Treisman visual search paradigm, in terms of perceptual segregation of the visual field by disparity, motion, color, and pattern features such as colinearity, orientation, lateral separation, or size.     

Costs in searching for two targets: dividing search across target types could improve airport security screening

The cost of searching for two visual targets simultaneously was compared against two separate single‐target searches using exposure time and accuracy measures within a staircase procedure. Dual‐target search for all stimuli (colour, shape and orientation) exhibited a loss of accuracy for one target. For orientation and shape, this dual‐target cost in accuracy was extreme, with chance‐level performance on one target. For colour, dual‐target search exhibited an additional cost in search time, with search requiring a longer exposure than the summed time required for two single‐target searches. An additional search‐time cost was also found for orientation targets when irrelevant colour variation was added to the display. In conclusion, dual‐target search for dissimilar targets is accompanied by an accuracy cost. Furthermore, colour variation, whether task‐relevant or not, leads to an additional cost in processing speed. The results suggest that a divided‐effort strategy would improve performance in search tasks such as X‐ray baggage screening. Copyright © 2006 John Wiley & Sons, Ltd.     

Nature and Dynamics of Reference Frames in Visual Search for Orientation: Implications for Early Visual Processing

Visual detection of a line target differing in orientation from a background of lines is assumed to occur early in vision and to involve filter mechanisms acting in parallel over the visual field in retinotopic maps (Foster & Ward, 1991; Treisman, 1985; Wolfe, 1994). But retinotopicality does not imply that early vision takes place in a retinal reference frame. This article briefly reviews experiments on this issue and analyzes their implications regarding early visual processing. The review shows that the spatial frame of reference used in visual search for orientation is dynamically determined by various sensory cues to orientation (Marendaz, Stivalet, Barraclough, & Walkowiac, 1993; Stivalet, Marendaz, Barraclough, & Mourareau, 1995). These findings suggest that intersensory integration and perceptual organization are involved at a processing level preceding visual search. This functional viewpoint is discussed in relation to data from neuroscience and psychology (visual search, microgravity).     

Visual marking inhibits singleton capture

This paper is concerned with how we prioritize the selection of new objects in visual scenes. We present four experiments investigating the effects of distractor previews on visual search through new objects. Participants viewed a set of to-be-ignored nontargets, with the task being to search for a target in a second set, added to the first after 1000ms. This second set could contain a salient feature singleton, defined in terms of its color, orientation, or both color and orientation. When the singleton was a distractor, search was slowed relative to when there was no singleton. Search was facilitated when the singleton was a target. Interestingly, both the interference and facilitation effects were modulated when the preview shared features with the singleton. Follow-up experiments showed that this reduction of singleton effects was not due to: (i) low-level sensory aspects of the displays, (ii) increased heterogeneity in the search set in the preview condition, or (iii) color-based grouping of old and new items. Instead, we suggest that there is an inhibitory carry-over from the first to the second set of items based on feature similarity. We suggest the suppression stems from a process termed visual marking, which suppresses irrelevant visual objects in anticipation of more relevant new objects (Watson & Humphreys, 1997). The findings argue against alternative explanations such as the automatic capture by abrupt new onsets account.     

The role of categorization in visual search for orientation.

Visual search for 1 target orientation is fast and virtually independent of set size if all of the distractors are of a single, different orientation. However, in the presence of distractors of several orientations, search can become inefficient and strongly dependent on set size (Exp. 1). Search can be inefficient even if only 2 distractor orientations are used and even if those orientations are quite remote from the target orientation (e.g. 20 degrees or even 40 degrees away, Exp. 2). Search for 1 orientation among heterogeneous distractor orientations becomes more efficient if the target orientation is the only item possessing a categorical attribute such as steep, shallow (Exp. 3), tilted left or tilted right (Exp. 4), or simply tilted (Exps. 5 and 6). Orientation categories appear to be 1 of several strategies used in visual search for orientation. These serve as a compromise between the limits on parallel visual processing and the demands of a complex visual world.     

Is apparent instability a guiding feature in visual search?

ABSTRACT

Humans are quick to notice if an object is unstable. Does that assessment require attention or can instability serve as a preattentive feature that can guide the deployment of attention? This paper describes a series of visual search experiments, designed to address this question. Experiment 1 shows that less stable images among more stable images are found more efficiently than more stable among less stable; a search asymmetry that supports guidance by instability. Experiment 2 shows efficient search but no search asymmetry when the orientation of the objects is removed as a confound. Experiment 3 independently varies the orientation cues and perceived stability and finds a clear main effect of apparent stability. Experiment 4 shows converging evidence for a role of stability using different stimuli that lack an orientation cue. However, here both search for stable and unstable targets is inefficient. Experiment 5 is a control for Experiment 4, showing that the stability effect in Experiment 4 is not simple side-effects of the geometry of the stimuli. On balance, the data support a role for instability in the guidance of attention in visual search. (184 words)     

Asymmetries in visual search: an introduction.

In visual search tasks, observers look for a target stimulus among distractor stimuli. A visual search asymmetry is said to occur when a search for stimulus A among stimulus B produces different results from a search for B among A. Anne Treisman made search asymmetries into an important tool in the study of visual attention. She argued that it was easier to find a target that was defined by the presence of a preattentive basic feature than to find a target defined by the absence of that feature. Four of the eight papers in this symposium in Perception & Psychophysics deal with the use of search asymmetries to identify stimulus attributes that behave as basic features in this context. Another two papers deal with the long-standing question of whether a novelty can be considered to be a basic feature. Asymmetries can also arise when one type of stimulus is easier to identify or classify than another. Levin and Angelone's paper on visual search for faces of different races is an examination of an asymmetry of this variety. Finally, Previc and Naegele investigate an asymmetry based on the spatial location of the target. Taken as a whole, these papers illustrate the continuing value of the search asymmetry paradigm.     

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.     

Are curves detected by 'curvature detectors'?

Five experiments which attempted to evaluate the relationship between orientation and curvature selectivity in human vision are described. In the first two experiments, threshold elevation for curved gratings was measured after exposure to similar gratings, with the use of either an adaptation (experiment 1) or a masking (experiment 2) paradigm. In both experiments threshold elevation occurred which was selective for both the degree and the direction of curvature of the adapting pattern. Experiment 3 compared the effects of adapting to tilted rectilinear or vertical curved gratings upon threshold for a vertical rectilinear grating. Threshold elevation declined systematically as the adapting gratings were either tilted or made more curved. Experiment 4 measured curvature selectivity as a function of the orientation of a curved adapting grating. Threshold elevation declined as the adapting grating was tilted more, but curvature selectivity remained. Experiment 5 measured the orientation tuning for curved gratings directly. Threshold elevation declined to 50% of its maximum value at an adpating orientation of about 28 degrees. This was constant for all values of curvature used. The resulsts are discussed with reference to the question of whether the human visual system contains 'curvature detectors' or linear-contour detectors which respond to the tangents of curves.     

Guided Search: The Effects of Learning

The previous report (Efron & Yund, 1996) offered an interpretation of the results of a number of search experiments within the theoretical context of the guided search model of Cave and Wolfe (1990) and Wolfe (1994). The present report extends this interpretation to the effects of extended practice when subjects search for a target defined by its orientation in the presence of a number of heterogeneous distractor items having differing orientations. Three experiments are described: The first revealed that over the course of 21 experimental sessions extending for a period of 6 weeks there were marked decreases in the magnitude of the reaction time gradient (RTG) and the right visual field superiority observed in the previous experiments. This learning persisted for more than 3 years. The second experiment revealed an interference in the capacity to learn to detect a target of one orientation when subjects had previously learned to detect a target of a different orientation at the same locations. The third experiment revealed that the learning was restricted to the area of the visual field where the target had been presented and that subjects could learn to detect two different targets concurrently. The results of these experiments indicate that the learning is orientation-specific and location-specific and is consistent with a localized increase in the selectivity of the top-down selection mechanism of the guided search model.