Access to Global and Local Properties in Visual Search for Compound Stimuli

Adults’ Markers of Face Processing Are Present at Age 6 and Are Interconnected Along Development

Recent studies on the development of face processing argue for a late, quantitative, domain-specific development of face processing, and face memory in particular. Most previous findings were based on separately tracking the developmental course of face perception skills, comparing performance across different age groups. Here, we adopted a different approach studying the mechanisms underlying the development of face processing by focusing on how different face skills are interrelated over the years (age 6 to adulthood). Specifically, we examined correlations within and between different categories of tasks: face domain-specific skills involving face recognition based on long-term representations (famous face), and short-term memory retention (Cambridge Face Memory Test), perceptual face-specific marker (inversion effect), global effects in scene perception (global–local task), and the perception of facial expressions. Factor analysis revealed that face identity skills have a similar pattern of interrelations throughout development, identifying two factors: a face domain-specific factor comprising adultlike markers of face processing and a general factor incorporating related, but nonspecific perceptual skills. Domain-specific age-related changes in face recognition entailing short- and long-term retention of face representations were observed, along with mature perceptual face-specific markers and more general perceptual effects predicting face perception skills already at age 6. The results suggest that the domain-specific changes in face processing are unlikely to result from developmental changes in perceptual skills driving face recognition. Instead, development may either involve improvement in the ability to retain face representations in memory or changes in the interactions between the perceptual representations of faces and their representations in long-term memory.

Statistical regularities guide the spatial scale of attention

Visual scenes contain information on both a local scale (e.g., a tree) and a global scale (e.g., a forest). The question of whether the visual system prioritizes local or global elements has been debated for over a century. Given that visual scenes often contain distinct individual objects, here we examine how regularities between individual objects prioritize local or global processing. Participants viewed Navon-like figures consisting of small local objects making up a global object, and were asked to identify either the shape of the local objects or the shape of the global object, as fast and accurately as possible. Unbeknown to the participants, local regularities (i.e., triplets) or global regularities (i.e., quadruples) were embedded among the objects. We found that the identification of the local shape was faster when individual objects reliably co-occurred immediately next to each other as triplets (local regularities, Experiment 1). This result suggested that local regularities draw attention to the local scale. Moreover, the identification of the global shape was faster when objects co-occurred at the global scale as quadruples (global regularities, Experiment 2). This result suggested that global regularities draw attention to the global scale. No participant was explicitly aware of the regularities in the experiments. The results suggest that statistical regularities can determine whether attention is directed to the individual objects or to the entire scene. The findings provide evidence that regularities guide the spatial scale of attention in the absence of explicit awareness.

Grasp preparation modulates early visual processing of size and detection of local/global stimulus features

Preparing to grasp objects facilitates visual processing of object location, orientation and size, compared to preparing actions such as pointing. This influence of action on perception reflects mechanisms of selection in visual perception tuned to current action goals, such that action relevant sensory information is prioritized relative to less relevant information. In three experiments, rather than varying movement type (grasp vs point), the magnitude of a prepared movement (power vs precision grasps) was manipulated while visual processing of object size, as well as local/global target detection was measured. Early event-related potentials (ERP) elicited by task-irrelevant visual probes were enhanced for larger probes during power grasp preparation and smaller probes during precision grasp preparation. Local targets were detected faster following precision, relative to power grasp cues. The results demonstrate a direct influence of grasp preparation on sensory processing of size and suggest that the hierarchical dimension of objects may be a relevant perceptual feature for grasp programming. To our knowledge, this is the first evidence that preparing different magnitudes of the same basic action has systematic effects on visual processing.

Fixations are not all created equal: An objection to mindless visual search

This call to revolution in theories of visual search does not go far enough. Treating fixations as uniform is an oversimplification that obscures the critical role of the mind. We remind readers that what happens during a fixation depends on mindset, as shown in studies of search strategy and of humans' ability to rapidly resume search following an interruption.

A developmental change in selective attention and global form perception

The primary purpose of the present study was to examine the processing of local and global perception in relation to selective attention during development from childhood to early adulthood. Filtering was the specific component of selective attention that was examined. The influence of varying distractor congruency and compatibility on relative local-global processing was also examined. Distractor congruency and compatibility did not differentially affect local and global processing. With the presence of neutral distractors, however, 6- and 8-year-old participants demonstrated a greater increase in RTs for global targets relative to local targets whereas older children and adults showed the same pattern of RTs for both local and global targets. The results are suggestive of separate developmental trajectories for global and local level processes, with global processing undergoing developmental change at least until 8 years of age.

Clusters Precede Shapes in Perceptual Organization

Does perceptual grouping require attention? Recent controversy on this question may be caused by a conflation of two aspects of grouping: element clustering (determining which elements belong together) and shape formation (determining cluster boundaries). In Experiment I, observers enumerated diamonds that were drawn with either lines or dots. These two types of stimuli were subitized (enumerated rapidly and accurately in the range from one to three items) equally well, suggesting that clustering dots into countable entities did not demand attention. In contrast, when target diamonds were enumerated among distractor squares in Experiment 2, only line-drawn items could be subitized. We propose that clustering and shape formation not only involve different perceptual processes, but play different functional roles in vision.

Perceptual grouping operates independently of attentional selection: evidence from hemispatial neglect

To what extent can human observers process visual information that is not currently the focus of attention? We evaluated the extent to which unattended visual information (i.e., that which appears on the neglected side of space in individuals with hemispatial neglect) is perceptually organized and influences the perceptual processing of information on the attended side. To examine this, patients (and matched controls) judged whether successive, complex checkerboard stimuli (targets), presented entirely to their intact side of space, were the same or different. Concurrent with this demanding task, irrelevant distractor elements appeared on the unattended side and either changed or retained their perceptual grouping on successive displays, independently of changes in the ipsilesional task-relevant target. Changes in the grouping of the unattended task-irrelevant distractor elements produced congruency effects on the attended target-change judgment to the same extent in the neglect patients as in the control participants, and this was true even in those patients with severe attentional deficits. These results suggest that some perceptual processes, such as grouping, can operate in the absence of attention.

Event-related potential evidence for a dual-locus model of global/local processing

We investigated the perceptual time course of global/local processing using event-related potentials (ERPs). Participants discriminated the global or local level of hierarchical letters of different sizes and densities. Participants were faster to discriminate the local level of large/sparse letters and the global level of small/dense letters. This was mirrored in early ERP components: The N1/N2 had smaller peak amplitudes when participants made discriminations at the level that took precedence. Only global discriminations for large/sparse letters led to amplitude enhancement of the later P3 component, suggesting that additional attention-demanding processes are involved in discriminating the global level of these stimuli. Our findings suggest a dual-locus time course for global/local processing: (a) Level precedence occurs early in visual processing; (b) extra processing is required at a later stage, but only for global discriminations of large, sparse, stimuli, which may require additional attentional resources for active grouping.

The contrasting impact of global and local object attributes on Kanizsa figure detection

Studies on the involvement of object completions in search for illusory figures have so far reported equivocal results. We have addressed this issue by investigating at which level object attributes in Kanizsa figures influence search. Employing a paradigm that investigated global and local attributes in the composition of distractors with relation to target composition, we report a selective involvement of multilevel processing upon detection. Four experiments demonstrate that global surface information, but not the surrounding global contour, determines the speed of Kanizsa figure detection. By contrast, local inducer information is encoded far less efficiently in search than processes computing the global object. Our conclusions are that surface filling-in acts as a major determinant of search, but depends on the relevance of the particular hierarchical level (local or global) coding the target.

The Power of the Positive: Revisiting Weak Coherence in Autism Spectrum Disorders

This paper reexamines Frith's original concept of weak coherence, its historical origins, recent reformulations, and alternative accounts. We suggest that the key notion of reduced global integration of information, which Frith proposed to underlie the assets in local processing, has been neglected in recent accounts of autism spectrum disorders (ASD). In fact, most paradigms used to test weak coherence conflate global and local processing, often placing them in direct trade-off, so that it is not possible to tell whether patterns of performance in ASD reflect reduced global processing, increased local processing, or both. We review the literature from typical development and ASD that may be pertinent to this distinction and examine some data from our own studies. Only once tasks are devised that measure separately the effects of reduced global processing and increased local processing will it be possible to test the on-line and developmental relations between these two aspects of “weak coherence”. Some preliminary ideas about these relationships are discussed, and suggestions are made for why disentangling two possibly independent dimensions of weak coherence may be timely and productive.

Prior experience affects amodal completion in pigeons

In a three-alternative forced-choice task, 4 pigeons were trained to discriminate a target stimulus consisting of two colored shapes, one of which partially occluded the other, from two foil stimuli that portrayed either a complete or an incomplete version of the occluded shape. The dependent measure was the percentage of total errors that the birds committed to the complete foil. At the outset of training, the pigeons committed approximately 50% of total errors to the complete foil, but as training progressed, the percentage of errors to the complete foil rose. When the pigeons were given a second exposure to the initial set of stimuli, they committed 70% of total errors to the complete foil, suggesting that they now saw the complete foil as more similar to the occluded target than the incomplete foil. These results suggest that experience with 2-D images may facilitate amodal completion in pigeons, perhaps via perceptual learning.

A single-element impact in global/local processing: the roles of element centrality and diagnosticity

A modification of the compound stimuli paradigm has been used to measure the impact of a certain single element on the local-to-global effect and to compare the measured impacts of central and non-central elements matched on diagnosticity. In addition to global letters made of identical response-associated elements, some global letters comprised of only one response-associated element at a specific location (with all other ones being response-neutral), and in some other global letters that critical element was rather response-neutral (with all other ones being response-associated). Experiment 1 showed that the contribution of a central element that served as a distinctive feature was as large as the joint contribution of all other elements. Experiment 2 (as well as Experiment 4) showed that, in contrast, a non-central element that served as a distinctive feature did not contribute at all to the effect. Experiment 3 showed that the contribution of a central element was still as large as the joint contribution of all other elements even when it was completely irrelevant for selecting the response.

Seeing trees OR seeing forests in simultanagnosia: Attentional capture can be local or global

Patients with simultanagnosia often demonstrate 'local capture', meaning that they identify only the local elements of stimuli that contain a hierarchy of both local and global structures. Recent studies, however, have found that these patients may implicitly process the global form. We examined the general applicability of the concept of local capture, and specifically whether the global level of stimuli can be explicitly reported by patients with simultanagnosia. We tested a patient with simultanagnosia with globally biased stimuli such as hierarchical Arcimboldo faces and small, dense Navon letters. With Arcimboldo faces our patient often reported only the face and not the local elements--the first demonstration of global rather than local capture. With Navon letters, the patient's ability to report the global letter varied with stimulus density and inversely with stimulus size, so that local capture was found only with large and sparse stimuli. With both faces and letters, the likelihood of global capture by the patient was related to the ease of global reporting in controls, as indexed by their reaction times. This suggests that the patient's global perception is influenced by the same factors operating in healthy individuals. We conclude that attentional capture in simultanagnosia can be either global or local. Capture likely occurs because of a pathological restriction and/or rigidity of attention, but the type of capture depends upon the competitive balance between global and local salience.

Global–Local Visual Processing in High Functioning Children with Autism: Structural vs. Implicit Task Biases

Global-local processing was examined in high-functioning children with autism and in groups of typically developing children. In experiment 1, the effects of structural bias were tested by comparing visual search that favored access to either local or global targets. The children with autism were not unusually sensitive to either level of visual structure. In experiment 2 a structural global bias was pitted against an implicit task bias favoring the local level. Children with autism were least sensitive to the structural global bias but showed greater sensitivity to the implicit task bias. This suggests that autism is associated with differences in the executive control processes used to guide attention to either the global or local level, and strategies may be more "data driven".

Local advantage in the visual processing of hierarchical stimuli following manipulations of stimulus size and element numerosity in monkeys (Cebus apella)

Previous studies suggest that monkeys process local elements of hierarchical visual patterns more quickly and more accurately than they process the global shape. These results could be indicative of differences between relatively high visual functions of humans and non-human primates. It is, however, important to rule out that relatively low-level factors can explain these differences. We addressed this issue with two experiments carried out on capuchin monkeys (Cebus apella) using matching-to-sample tasks featuring hierarchical stimuli. The first experiment assessed whether manipulations of stimulus size can affect the local advantage so far observed in this New World monkey species. An overall local versus global advantage still emerges in capuchins, irrespectively of the amplitude of the visual angle subtended by the hierarchical shapes. Moreover, a local-to-global interference, indicative of a strong local advantage, was observed for the first time. In the second experiment, we manipulated size and numerosity of the local elements of hierarchical patterns, mimicking procedures that in human perception relegate the local elements to texture and enhance a global advantage. Our results show that in capuchin monkeys, a local advantage emerges clearly even when these procedures are used. These results are of interest since extensive neurophysiological research is carried out on non-human primate vision, often taking for granted a similarity of visual skills in human and non-human primates. These behavioural results show that this assumption is not always warranted and that more research is needed to clarify the differences in the processes involved in basic visual skills among primates.

Microgenesis and ontogenesis of perceptual organization: evidence from global and local processing of hierarchical patterns

In two experiments, visual search and speeded classification were used to study perception of hierarchical patterns among participants aged 5 to 23 years. Perception of global configurations of few-element patterns and local elements of many-element patterns showed large age-related improvements. Only minor age-related changes were observed in perception of global configurations of many-element patterns and local elements of few-element patterns. These results are consistent with prior microgenetic analyses using hierarchical patterns. On the one hand, the rapid and effortless grouping of many small elements and the individuation of few large elements both mature by age 5. In contrast, the time-consuming and effortful grouping of few large elements and the individuation of many small elements improve substantially with age, primarily between ages 5 and 10. These findings support the view that perceptual organization involves multiple processes that vary in time course, attentional demands, and developmental trajectories.

Mental fatigue disturbs local processing more than global processing

Focusing of attention is influenced by external features such as the presence of global or local target stimuli, but also by motivation and mood states. In the current study, we examined whether working on cognitively demanding tasks for 2 h, which induces mental fatigue, subsequently had a differential effect on global and local processing. The results showed that, compared to non-fatigued participants, fatigued participants particularly displayed compromised local processing. This indicates that mental fatigue may also manifest itself as effects on attentional focusing. The findings of this study are in line with recent ideas about the nature of fatigue-related cognitive deficits, implying disturbances in the control over attention and behaviour.

Relative Weight of Local and Global Properties Depends on both the Position of Local Elements and the Saliency of Global Form

Abstract. Love, Rouder, and Wisniewski (1999 ) and Ripoll and Marty (2005 ) showed that subjects could process global properties very quickly in a same/different task on abstract visual scenes for which the conspicuity of local and global properties had been controlled. In this new experiment, two important new factors were manipulated: saliency of the global pattern and location of local similarity. The results showed that the saliency of the global form as well as the location of local similarity determines the strength of global and local effects. Global effects continue to manifest themselves even when the extraction of the global form is difficult. Finally, the whole pattern of results suggests that local and global processing proceeds simultaneously and involves two attentional systems whose spatial characteristics are very different.

The roles of distractor noise and target certainty in search: a signal detection model

Observers searched arrays of briefly presented near-isoluminant colored disks for a single disk of known color (feature search) or unknown color (oddity search). Speed and accuracy were converted to a single, model-based measure of performance (Perf), in units of (d')2 per second of latency. Perf decreased with set size in feature search and increased in oddity. In both types of search, grouping the distractors, making them homogeneous in color, and reducing their saturation, all increased Perf. These commonalities suggested an SDT-based model in which distractors increase noise in the same way in both types of search. However, in oddity, though not in feature search, distractors must be attended and so adding distractors also boosts the effective target contrast, overcoming the added noise. A model with two free parameters for noise and one for attention accounted for every combination except for oddity searches among heterogeneous grouped distractors.

Perceptual grouping and attention: Not all groupings are equal

We examined grouping under inattention using Driver, Davis, Russell, Turatto, & Freeman's (2001) method. On each trial, two successive displays were briefly presented, each comprising a central target square surrounded by elements. The task was to judge whether the two targets were the same or different. The organization of the background elements stayed the same or changed, independently of the targets. In different conditions, background elements grouped into columns/rows by color similarity, a shape (a triangle/arrow, a square/cross, or a vertical/horizontal line) by color similarity, and a shape with no other elements in the background. We measured the influence of the background on the target same-different judgments. The results imply that background elements grouped into columns/rows by color similarity and into a shape when no segregation from other elements was involved and the shape was relatively "good." In contrast, no background grouping was observed when resolving figure-ground relations for segregated units was required, as in grouping into a shape by color similarity. These results suggest that grouping is a multiplicity of processes that vary in their attentional demands. Regardless of attentional demands, the products of grouping are not available to awareness without attention.

What does a compound letter tell the psychologist's mind?

The paradigm based on using compound stimuli for studying global and local processing is revisited. Noting that not all researchers employ compound stimuli for the same purpose, the issue of its purpose is discussed. It is argued that the paradigm is pertinent for examining at least three notions--formation preference, global addressability, and within-object global precedence. It is suggested that findings in the paradigm are accommodated well by a disjunction of those three perceptual dispositions. A number of further issues associated with the interpretation of findings obtained with it are examined as well. An experimental study is reported that is meant to examine one such issue--a possible artifact putatively introduced by the special attribute of element homogeneity characteristic of compound stimuli. Seven experiments were used to examine to what extent, if at all, global advantage observed in compound stimulus paradigms depends on element heterogeneity. Across those experiments, heterogeneity did not have any effect that could be interpreted as suggesting that the paradigm is biased in favor of the global structure due to element homogeneity.

Locally oriented perception with intact global processing among adolescents with high-functioning autism: evidence from multiple paradigms

BACKGROUND

According to predictions from the Weak Central Coherence (WCC) theory for perceptual processing, persons with autism should display a tendency to focus on minute details rather than on a more general picture (Frith & Happé, 1994). However, the evidence for this theory is not consistent with findings of an enhanced detection of local targets (Plaisted, O'Riordan, & Baron-Cohen, 1998b; Plaisted, Swettenham, & Rees, 1999), but a typical global bias (Mottron, Burack, Stauder, & Robaey, 1999; Ozonoff, Strayer, McMahon, & Filloux, 1994).

METHOD

Adolescents with high-functioning autism and CA- (approximately 15 years) and IQ- (approximately 105-110) matched typically developing adolescents were administered a series of global-local visual tasks, including a traditional task of hierarchical processing, three tasks of configural processing, and a disembedding task that involved rapid perceptual processing.

RESULTS

No group differences were found on either the traditional task of hierarchical processing or on tasks of configural processing. However, group differences were found on the disembedding task as the search for embedded, in relation to isolated stimuli, was slower for the typically developing adolescents but similar for the participants with autism.

CONCLUSIONS

These findings are consistent with other reports of superior performance in detecting embedded figures (Jolliffe & Baron-Cohen, 1997; Shah & Frith, 1983), but typical performance in global and configural processing (Mottron, Burack et al., 1999; Ozonoff et al., 1994) among persons with high-functioning autism. Thus, the notions of local bias and global impairment that are part of WCC may need to be reexamined.

What does visual agnosia tell us about perceptual organization and its relationship to object perception?

The authors studied 2 patients, S.M. and R.N., to examine perceptual organization and its relationship to object recognition. Both patients had normal, low-level vision and performed simple grouping operations normally but were unable to apprehend a multielement stimulus as a whole. R.N. failed to derive global structure even under optimal stimulus conditions, was less sensitive to grouping by closure, and was more impaired in object recognition than S.M. These findings suggest that perceptual organization involves a multiplicity of processes, some of which are simpler and are instantiated in lower order areas of visual cortex (e.g., collinearity). Other processes are more complex and rely on higher order visual areas (e.g., closure and shape formation). The failure to exploit these latter configural processes adversely affects object recognition.

Segmentation of objects from backgrounds in visual search tasks

In most visual search experiments in the laboratory, objects are presented on an isolated, blank background. In most real world search tasks, however, the background is continuous and can be complex. In six experiments, we examine the ability of the visual system to separate search items from a background. The results support a view in which objects are separated from backgrounds in a single, preattentive step. This is followed by a limited-capacity search process that selects objects that might be targets for further identification. Identity information regarding the object's status (target or distractor) then accumulates through a limited capacity parallel process. The main effect of background complexity is to slow the accumulation of information in this later recognition stage. It may be that recognition is slowed because background noise causes the preattentive segmentation stage to deliver less effectively segmented objects to later stages. Only when backgrounds become nearly identical to the search objects does the background have the effect of slowing item-by-item selection.

The hole precedence in face but not figure discrimination and its neuronal correlates

The discriminative tasks of face and geometrical figure were trained in three rhesus monkeys. The hole feature speeds up learning of face discrimination, but hampers learning of figure discrimination. By reducing presentation duration of the stimuli, the detecting precedence appears to be the hole feature of the face component, while it appears to be shape feature of figures. The patterns of neuron firings in inferior temporal cortex (IT) are consistent with the context-dependent precedence of hole feature. The results might suggest that the neural correlates exist not only in IT neurons, but also in combination with executive mechanism.

Attentional capture by globally defined objects

The abrupt appearance of a new perceptual object in the visual field typically captures visual attention. However, if attention is focused in advance on a different location, onsets can fail to capture attention (Yantis & Jonides, 1990). In the present experiments, we investigated the extent to which the deployment of attention to the local level of a hierarchical scene may be affected by the abrupt appearance of a new object at the global level. Participants searched for a semi-disk target in an array of randomly oriented segmented disks ("pacmen"). On half the trials, a subset of the segmented disks induced a subjective square. On these critical trials, participants were significantly slower to respond to the presence of a local target even though the local features of the display were qualitatively identical across all conditions. This slowing was absent when outline pacmen were used (which do not induce subjective figures) and when the subjective square was perceptually old. When the participants' task was defined at the global level of the display, a new local element failed to capture attention, suggesting an asymmetry in the ability of objects at different levels of a hierarchical scene to capture attention. In a control experiment, a new local element captured attention, however, when the participants' task was defined at the local level, indicating that the local item was in principle capable of capturing attention. It is argued that global objects capture attention because they convey important information about the environment that is not available at the local level.

The perceptual organization of visual objects: a microgenetic analysis

Primed matching was used to examine the microgenesis of perceptual organization for line configurations that vary in the connectedness between their four line components, and for hierarchical patterns composed of four outline closed figures. The results for the line configurations showed that the configural organization of the disconnected line segments was available for priming very early, and its effect outweighed possible effects of the line components. An early relative dominance of the components was observed for the stimuli whose components were closed figures. These results suggest that uniform connectedness is not necessary for the designation of entry-level units. Disconnected line segments are rapidly organized into configurations, provided the presence of collinearity and/or closure. Closed figural elements are individuated early and are grouped into higher-level units with time.

Interactions between perceptual organization based on Gestalt laws and those based on hierarchical processing

Our recent research with compound stimuli (Han, Humphreys, & Chen, 1999a) suggests that grouping between local elements facilitates the perception of global structure, whereas encoding closure in local elements enhances their segmentation. The present study presents further evidence supporting this assertion. Experiment 1 first developed a new paradigm in which grouping between local elements was manipulated. Subjects responded to the orientations of perceptual groups consisting of local arrows or triangles embedded in background crosses. Responses to the orientations of the groups were slowed as a function of the increased contrast of the crosses, indicating that the strength of grouping between local arrows or triangles was gradually weakened by increasing the contrast of the crosses. Using a similar paradigm, Experiments 2 and 3 investigated the role of Gestalt factors in hierarchical analysis. Global arrows or triangles made up of local arrows, or triangles were embedded in background crosses. Subjects responded to global or local stimuli in terms of orientation or closure. Increasing the contrast of the background crosses produced stronger effects on global responses than on local responses and resulted in elimination of the global precedence effect and emerging of a local precedence effect, which was stronger for closure discrimination than for orientation discrimination. These results provide new evidence supporting our previous claim about the role of Gestalt factors in hierarchical analysis.

Hemispheric specialization in the detection of subjective objects

Three experiments were conducted to examine hemispheric specialization for the detection of subjective objects. In the first two experiments, observers searched for the presence of a square defined by subjective contours. The first experiment demonstrated that the left hemisphere made more errors for detecting these objects. The second experiment showed that the increased errors were due to the left hemisphere responding to the individual features of the objects and not the objects as a whole. In the second experiment, the right hemisphere was also faster for detecting the absence of a subjective object. A third experiment was conducted to determine if performance for the right hemisphere was due to object level processing. It was shown that the right hemisphere only makes illusory conjunctions for features within perceptual groups while the left hemisphere makes illusory conjunctions both within and across perceptual groups, providing converging evidence for object level processing in the right hemisphere. The results suggest that the right hemisphere conjoins feature information for the perception of objects.