Space-based and object-based capacity limitations in visual search

Space-based accounts of visual attention assume that we select a limited spatial region independent of the number of objects it contains. In contrast, object-based accounts suggest that we select objects independent of their location. We investigated the boundary conditions on the selection modes of attention in a series of tachistoscopic visual search tasks, where the nature of capacity limitations on search was examined. Observers had to search for a horizontally oriented target ellipse among differently oriented distractor ellipses. Across four experiments, we orthogonally manipulated target-distractor (TD) similarity and distractor-distractor (DD) similarity. Each experiment consisted of a two-way design: Firstly, with a central cue, we indicated the spatial extent of the relevant search area. Secondly, we varied the number and spatial proximity of items in the display. Performance could be accounted for in terms of capacity limited object-based attention, assuming also that the spatial proximity of items enhances performance when there is high DD-similarity (and grouping). In addition, the cueing effect interacted with spatial proximity when DD-similarity was high, suggesting that grouping was influenced by attention. We propose that any capacity limits on visual search are due to object-based attention, and that the formation of perceptual objects and object groups is also subject to attentional modulation.     

Perceptual grouping and visual selective attention

The present study demonstrates that incongruent distractor letters at a constant distance from a target letter produce more response competition and negative priming when they share a target’s color than when they have a different color. Moreover, perceptual grouping by means of color, attenuated the effects of spatial proximity. For example, when all items were presented in the same color, near distractors produced more response competition and negative priming than far distractors (Experiment 3A). However, when near distractors were presented in a different color and far distractors were presented in the same color as the target, the response competition × distractor proximity interaction was eliminated and the proximity × negative priming interaction was reversed (Experiment 3B). A final experiment demonstrated that distractors appearing on the same object as a selected target produced comparable amounts of response competition and negative priming whether they were near or far from the target. This suggests that the inhibitory mechanisms of visual attention can be directed to perceptual groups/objects in the environment and not only to unsegmented regions of visual space.     

Visual attention and perceptual grouping.

Perceptual organization is thought to involve an analysis of both textural discontinuities and perceptual grouping. In earlier work, we found that textural discontinuities were detected normally even when visual attention was engaged elsewhere. Here we report how perceptual grouping is affected when visual attention is engaged by a concurrent visual task. To elicit perceptual grouping, we used the Gestalt demonstrations of grouping on the basis of proximity and of similarity. Four tasks were investigated, some requiring the observer to discriminate between horizontal and vertical grouping, and some requiring the observer to merely detect the presence or absence of grouping. Visual attention was engaged at the center of the display by a form identification task. The detection of a textural discontinuity served as a control task. Concurrent form identification conflicted with all four grouping tasks, resulting in a significant reduction of grouping performance in each case. No performance reduction was observed when either form identification or grouping discrimination was combined with the detection of a textural discontinuity. These results suggest that perceptual grouping and form identification compete for visual attention, whereas the detection of a textural discontinuity does not.     

Combined effects of perceptual grouping cues on object representation: Evidence from motion-induced blindness

We investigated the combined effects of perceptual grouping cues (proximity and contour closure/proximity and orientation similarity) on object representation, using motion-induced blindness, a phenomenon in which salient visual stimuli perceptually disappear when surrounded by moving patterns. We presented as visual targets two stimuli in which a solid square was embedded in an outlined square. Participants reported whether the targets disappeared independently or simultaneously. The results showed that a relatively high proximity cue (with a 0.2-deg separation between the targets) modulated the perceptions of the independent or simultaneous disappearances of targets, regardless of other grouping cues. The contour closure cue modulated these disappearances in the 0.4- to 0.8-deg separations. Finally, the orientation similarity cue began to modulate these disappearances in the 0.6- to 0.8-deg separations. We suggest that the separation between the visual stimuli modulates the combined effects of perceptual grouping cues on complete object representation.     

Visual attention and perceptual grouping

Perceptual organization is thought to involve an analysis of bothtextural discontinuities andperceptual grouping. In earlier work, we found that textural discontinuities were detected normally even when visual attention was engaged elsewhere. Here we report how perceptual grouping is affected when visual attention is engaged by a concurrent visual task. To elicit perceptual grouping, we used the Gestalt demonstrations of grouping on the basis of proximity and of similarity. Four tasks were investigated, some requiring the observer to discriminate between horizontal and vertical grouping, and some requiring the observer to merely detect the presence or absence of grouping. Visual attention was engaged at the center of the display by a form identification task. The detection of a textural discontinuity served as a control task. Concurrent form identification conflicted with all four grouping tasks, resulting in a significant reduction of grouping performance in each case. No performance reduction was observed when either form identification or grouping discrimination was combined with the detection of a textural discontinuity. These results suggest that perceptual grouping and form identification compete for visual attention, whereas the detection of a textural discontinuity does not.     

Rethinking perceptual organization: The role of uniform connectedness

A principle of perceptual organization, calleduniform connectedness (UC), is described, and a theoretical approach to perceptual organization is proposed in which this principle plays a fundamental role. The principle of UC states that closed regions of homogeneous properties—such as lightness, chromatic color, texture, and so forth—tend to be perceived initially as single units. We demonstrate its effects and show that they occur even when opposed by powerful grouping principles such as proximity and similarity. We argue that UC cannot be reduced to such grouping principles, because it is not a form of grouping at all. We then propose a theoretical framework within which UC accounts for the initial (orentry level) organization of the visual field into primitive units. Classical principles of grouping operate after UC, creating superordinate units consisting of two or more basic-level units. Parsing processes also operate after UC, dividing basic-level units into subordinate parts. UC in the retinal image is proposed to be a necessary, but not a sufficient, condition for unit formation, since connected elements on the retina that are perceived to lie in separate depth planes fail to be perceived as units. This fact, together with other evidence that the Gestalt principles of grouping are based onperceived (rather than retinal) relations, suggests that the organization of visual stimulation into UC objects is ultimately achieved within a relatively late, postconstancy representation of environmental surfaces. The implications of this possibility are discussed in light of present theories of visual perception.     

Grouping reduces visual extinction: Neuropsychological evidence for weight-linkage in visual selection

Abstract

We tested two subjects following damage to right parietal cortex to see if their failure to detect a left visual stimulus in the presence of a simultaneous right stimulus (visual extinction) could be modulated by perceptual grouping between the left and right stimuli. Subjects performed a simple detection task for brief displays in which items could appear in the left or right visual field, both fields, or neither field. On trials in which items appeared in both fields, we found that left omissions (extinction errors) were dramatically reduced when the two items formed a good perceptual group, either on the basis of Gestalt factors such as similarity and symmetry (Experiment 1). or by forming a familiar configuration (Experiment 2). We suggest that extinction may be a spatially specific exaggeration of a normal attention limitation, in which the contralesional item is disadvantaged in the competition for selection. However, this obstacle to selection can be overcome if, as a result of grouping, ipsilesional and contralesional items become allies rather than competitors for selection.     

Associative grouping: Perceptual grouping of shapes by association

Perceptual grouping is usually defined by principles that associate distinct elements by virtue of image properties, such as proximity, similarity, and occurrence within common regions. What role does learning play in forming a perceptual group? This study provides evidence that learning of shape associations leads to perceptual grouping. Subjects were repeatedly exposed to pairs of unique shapes that co-occurred within a common region. The common region cue was later removed in displays composed of these shapes, and the subjects searched the display for two adjacent shapes of the same color. The subjects were faster at locating the color repetition when the adjacent shapes with the same color came from the same trained groups than when they were composed of two shapes from different trained groups. The effects were perceptual in nature: Learned pairings produced spatial distortions similar to those observed for groups defined by perceptual similarity. A residual grouping effect was observed even when the shapes in the trained group switched their relative positions but was eliminated when each shape was inverted. These results indicate that statistical co-occurrence with explicit grouping cues may form an important component of perceptual organization, determining perceived scene structure solely on the basis of past experience.     

The interaction of objective and subjective organizations in a localization search task

We investigated how both objective and subjective organizations affect perceptual organization and how this perceptual organization, in turn, influences observers’ performance in a localization search task. Two groups of observers viewing exactly the same stimuli (objective organization) performed in significantly different ways, depending on how they were induced to parse the display (subjective organization). In Experiments 1 and 2, the observers were asked to describe the location of a tilted target among a varying number of vertical or horizontal distractors. Subjective organization was induced by instructing observers to parse the display into either three horizontal regions (rows) or three vertical regions (columns). The position of the target was critical: location performance, as assessed by reaction time and errors, was consistently impaired at the locations adjacent to the boundaries defining the regions, producing what we refer to as thesubjective boundary effect. Furthermore, the extent of this effect depended on whether the stimulus-driven and conceptually driven information concurred or conflicted. This made location information more or less accessible. In Experiment 1, the strength of objective grouping was a function of the proximity of the items (near or far conditions) and their orientation in a 6×6 matrix. In Experiment 2, the strength of objective grouping was a function of similarity of color (items were color coded by rows or by columns) and the orientation of the items in a 9×9 matrix. The subjective boundary effect was more pronounced when the display promoted grouping in the direction orthogonal to that of the task (e.g., when observers parsed by rows but vertical distractors were closer together [Experiment 1] or color coded [Experiment 2] to induce global columns). In contrast, this effect decreased when the direction of both objective and subjective organizations was parallel (e.g., when observers parsed by rows and horizontal distractors were closer together [Experiment 1] or were color coded [Experiment 2] to induce global rows). A localization search task proved to be an ideal forum in which objective and subjective organizations interacted. We discuss how these results indicated that observers’ performance in a localization task was determined by the interaction of objective and subjective organizations, and that the resulting perceptual organization constrained coarse location information.     

抑制控制能力衰退对知觉组织老年化的影响

通过比较注意线索对老年人和年轻人共线性和相似性知觉组织的影响,探讨了知觉组织老年化的原因。研究发现,注意线索对年轻人的助益更大,对老年人的助益较小,符合抑制控制理论的研究假设,同时本文还对加工速度、基本感觉能力等可能引起认知老年化的因素进行了分析。此外,还发现了知觉组织中与阅读习惯一致的方向偏好,无论在有无注意线索的条件下,135°方向上的知觉组织都快于45°方向上的知觉组织。     

Movement and proximity constrain miscombinations of colour and form.

A relatively frequent error when reporting brief visual displays is to combine presented features incorrectly. It has been proposed that Gestalt grouping constrains such errors so that miscombined features tend to come from the same perceptual group. In three experiments it was examined whether this principle applies to grouping by motion, and to grouping by proximity. Miscombinations of colour and form were more likely to consist of a colour and form that had moved in the same direction than features which had moved in opposite directions. Miscombinations were also more likely for adjacent items. The implications of these results for the mechanisms of feature integration are discussed.     

Subliminal Gestalt grouping: Evidence of perceptual grouping by proximity and similarity in absence of conscious perception

Previous studies making use of indirect processing measures have shown that perceptual grouping can occur outside the focus of attention. However, no previous study has examined the possibility of subliminal processing of perceptual grouping. The present work steps forward in the study of perceptual organization, reporting direct evidence of subliminal processing of Gestalt patterns. In two masked priming experiments, Gestalt patterns grouped by proximity or similarity that induced either a horizontal or vertical global orientation of the stimuli were presented as masked primes and followed by visible targets that could be congruent or incongruent with the orientation of the primes. The results showed a reliable priming effect in the complete absence of prime awareness for both proximity and similarity grouping principles. These findings suggest that a phenomenal report of the Gestalt pattern is not mandatory to observe an effect on the response based on the global properties of Gestalt stimuli.     

Top-down influences on perceptual grouping

Observers are faster to find and discriminate a target pair of adjacent, same-shaped objects that are within the same perceptual group (within group) versus within adjacent perceptual groups. Manipulating the probability of a within-group pair modulated this grouping effect such that it increased as the percentage of within-group trials (25%, 50%, or 75%) increased. This probability effect was stronger for extrinsic grouping factors (common region and connectedness) than for intrinsic factors (proximity and color similarity). Further experiments, however, suggested that this extrinsic-intrinsic difference was due to differences in overall speed with which the target pair was found. The results are interpreted as indicating that grouping is sensitive to top-down influences as long as there is sufficient time for feedback to operate.     

The Gestalt principle of similarity benefits visual working memory

Visual working memory (VWM) is essential for many cognitive processes, yet it is notably limited in capacity. Visual perception processing is facilitated by Gestalt principles of grouping, such as connectedness, similarity, and proximity. This introduces the question, do these perceptual benefits extend to VWM? If so, can this be an approach to enhance VWM function by optimizing the processing of information? Previous findings have demonstrated that several Gestalt principles (connectedness, common region, and spatial proximity) do facilitate VWM performance in change detection tasks (Jiang, Olson, & Chun, 2000; Woodman, Vecera, & Luck, 2003; Xu, 2002, 2006; Xu & Chun, 2007). However, one prevalent Gestalt principle, similarity, has not been examined with regard to facilitating VWM. Here, we investigated whether grouping by similarity benefits VWM. Experiment 1 established the basic finding that VWM performance could benefit from grouping. Experiment 2 replicated and extended this finding by showing that similarity was only effective when the similar stimuli were proximal. In short, the VWM performance benefit derived from similarity was constrained by spatial proximity, such that similar items need to be near each other. Thus, the Gestalt principle of similarity benefits visual perception, but it can provide benefits to VWM as well.     

Uniform connectedness and classical Gestalt principles of perceptual grouping.

We assessed whether uniform connectedness (UC; Palmer & Rock, 1994) operates prior to effects reflecting classical principles of grouping: proximity and similarity. In Experiments 1 and 2, reaction times to discriminate global letters (H vs. E), made up of small circles, were recorded. The small circles were respectively grouped by proximity, similarity of shapes, and by UC. The discrimination of stimuli grouped by similarity was slower than those grouped by proximity, and it was speeded up by the addition of UC. However, the discrimination of stimuli grouped by proximity was unaffected by connecting the local elements. In Experiment 3, similar results occurred in a task requiring discrimination of the orientation of grouped elements, except that the discrimination of stimuli grouped by UC was faster than that of those grouped by weak proximity. Experiment 4 further showed that subjects could respond to letters composed of discriminably separate local elements as fast as to those without separated local elements. The results suggest that grouping by similarity of shapes is perceived slower than grouping by UC, but grouping by proximity can be as fast and efficient as that by UC.     

Exogenous attention during perceptual group formation and dissolution

Under natural viewing conditions, a large amount of information reaches our senses, and the visual system uses attention and perceptual grouping to reduce the complexity of stimuli in order to make real-time perception possible. Prior studies have shown that attention and perceptual grouping operate in synergy; exogenous attention is deployed not only to the cued item, but also to the entire group. Here, we investigated how attention and perceptual grouping operate during the formation and dissolution of groups. Our results showed that reaction times are higher in the presence of perceptual groups than they are for ungrouped stimuli. On the other hand, attentional benefits of perceptual grouping were observed during both the formation and the dissolution of groups. The dynamics were similar during group formation and dissolution, showing a gradual effect that takes approximately half a second to reach its maximum level. In the case of group dissolution, the attentional benefits persisted for about a quarter of a second after dissolution of the group. Taken together, our results reveal the dynamics of how attention and grouping work in synergy during the transient period when groups form or dissolve.     

When does visual perceptual grouping affect multisensory integration?

Several studies have shown that the direction in which a visual apparent motion stream moves can influence the perceived direction of an auditory apparent motion stream (an effect known as crossmodal dynamic capture). However, little is known about the role that intramodal perceptual grouping processes play in the multisensory integration of motion information. The present study was designed to investigate the time course of any modulation of the cross-modal dynamic capture effect by the nature of the perceptual grouping taking place within vision. Participants were required to judge the direction of an auditory apparent motion stream while trying to ignore visual apparent motion streams presented in a variety of different configurations. Our results demonstrate that the cross-modal dynamic capture effect was influenced more by visual perceptual grouping when the conditions for intramodal perceptual grouping were set up prior to the presentation of the audiovisual apparent motion stimuli. However, no such modulation occurred when the visual perceptual grouping manipulation was established at the same time as or after the presentation of the audiovisual stimuli. These results highlight the importance of the unimodal perceptual organization of sensory information to the manifestation of multisensory integration.     

Visual search in divided areas: Dividers initially interfere with and later facilitate visual search

A common search paradigm requires observers to search for a target among undivided spatial arrays of many items. Yet our visual environment is populated with items that are typically arranged within smaller (subdivided) spatial areas outlined by dividers (e.g., frames). It remains unclear how dividers impact visual search performance. In this study, we manipulated the presence and absence of frames and the number of frames subdividing search displays. Observers searched for a target O among Cs, a typically inefficient search task, and for a target C among Os, a typically efficient search. The results indicated that the presence of divider frames in a search display initially interferes with visual search tasks when targets are quickly detected (i.e., efficient search), leading to early interference; conversely, frames later facilitate visual search in tasks in which targets take longer to detect (i.e., inefficient search), leading to late facilitation. Such interference and facilitation appear only for conditions with a specific number of frames. Relative to previous studies of grouping (due to item proximity or similarity), these findings suggest that frame enclosures of multiple items may induce a grouping effect that influences search performance.     

Uniform connectedness and classical gestalt principles of perceptual grouping

We assessed whether uniform connectedness (UC; Palmer & Rock, 1994) operates prior to effects reflecting classical principles of grouping: proximity and similarity. In Experiments 1 and 2, reaction times to discriminate global letters (H vs. E), made up of small circles, were recorded. The small circles were respectively grouped by proximity, similarity of shapes, and by UC. The discrimination of stimuli grouped by similarity was slower than those grouped by proximity, and it was speeded up by the addition of UC. However, the discrimination of stimuli grouped by proxhnity was unaffected by connecting the local elements. In Experiment 3, similar results occurred in a task requiring discrimination of the orientation of grouped elements, except that the discrimination of stimuli grouped by UC was faster than that of those grouped by weak proximity. Experiment 4 further showed that subjects could respond to letters composed of discriminably separate local elements as fast as to those without separated local elements. The results suggest that grouping by similarity of shapes is perceived slower than grouping by UC, but grouping by proximity can be as fast and efficient as that by UC.     

Principles of perceptual organization and spatial distortion: the gestalt illusions

In five interleaved experiments, conducted with 94 observers, it is shown that organization of the visual field according to gestalt principles results in measurable spatial distortions. Using the principles of proximity, similarity, good continuation, and two types of closure, it was found that interior distances (within a perceptual unit or group) are underestimated relative to exterior distances. The relationship between these spatial distortions and the resultant perceptual organization are discussed.