On the spatial extent of attention in object-based visual selection

Selective attention as tuning: the case of stroke weight

It has long been demonstrated that when grouping occurs, attention transfer between grouped elements is facilitated, as compared with attention transfer between elements-similarly distant-that are not grouped. This has been shown for grouping by connectedness, by orientation, and by color. The present article extends these findings to the case of similarity in coarseness. By using spatial cuing to elements drawn with different strokes, it is shown that the visual processing of elements that sharestroke heaviness with the cued element is more efficient than that of elements that do not. Three experiments, in which cue validity regarding the target's location and/or its stroke is manipulated, show that the facilitation has both an endogenous and an exogenous component. The findings are discussed in terms of visual tuning to the features of a stimulus, with tuning being the initial stage of visual processing required for identification and discrimination. It is proposed that grouping, rather than explaining the facilitation observed, can be explained by the notion of visual tuning to features. The findings also point to potential methodological pitfalls when different stroke weights are used, unintentionally, in visual displays.

Grouping does not require attention

Many theories of visual perception stipulate that Gestalt grouping occurs preattentively. Subjects' failure to report perceiving even salient grouping patterns under conditions of inattention challenges this assumption (see, e.g., Mack, Tang, Tuma, Kahn, & Rock, 1992), but Moore and Egeth (1997) showed that although subjects are indeed unable to identify grouping patterns outside the focus of attention, effects of these patterns on visual perception can be observed when they are assessed using implicit, rather than explicit, measures. However, this finding, which is the only one to date demonstrating grouping effects without attention, is open to an alternative account. In the present study, we eliminated this confound and replicated Moore and Egeth's findings, using the Müller-Lyer illusion (Experiments 1 and 2). Moreover, we found converging evidence for these findings with a variant of the flanker task (Experiment 3), when the amount of available attentional resources was varied (Experiments 4 and 5). The results reinforce the idea that, although grouping outside the focus of attention cannot be the object of overt report, grouping processes can occur without attention.

The Effect of Concurrent Task Load on Stimulus Over-Selectivity

Stimulus over-selectivity is a phenomenon displayed by individuals with autism, and has been implicated as a basis for many autistic-spectrum symptoms. In four experiments, non-autistic adult participants were required to learn a simple discrimination using picture cards, and then were tested for the emergence of stimulus over-selectivity, both with and without a concurrent task. Greater stimulus over-selectivity was noted when participants completed the concurrent task. The results are discussed in relation to the implications for the development of a model of memory deficits in autism.

The role of local and global properties in comparison of analogical visual scenes

Love, Rouder, and Wisniewski (1999) obtained interesting results showing that, in a same/different task on abstract visual scenes, subjects were able to process global properties quickly, even before local properties were identified. Our aim in this work is to explore more fully the complex relationships that exist between local processing and global processing. In our first experiment, we tested the robustness and generality of these global and local effects by using another, very different kind of local element. We showed that the global effects remain strong even when the local elements are neither conventional nor easily discriminable. In the second experiment, we showed that there exists an intermediate level of similarity between purely local and purely global similarity. Furthermore, we found that even when a stronger form of local dissimilarity is manipulated (through the introduction of different local elements), global effects were still observed. We conclude with a discussion of the respective roles of global and local properties in light of our findings.

Object-based attention and cognitive tunneling

Simulator-based research has shown that pilots cognitively tunnel their attention on head-up displays (HUDs). Cognitive tunneling has been linked to object-based visual attention on the assumption that HUD symbology is perceptually grouped into an object that is perceived and attended separately from the external scene. The present research strengthens the link between cognitive tunneling and object-based attention by showing that (a) elements of a visual display that share a common fate are grouped into a perceptual object and that this grouping is sufficient to sustain object-based attention, (b) object-based attention and thereby cognitive tunneling is affected by strategic focusing of attention, and (c) object-based attention is primarily inhibitory in nature.

Characteristics of attention and visual short-term memory: implications for visual interface design

Although the human retina can code many object images simultaneously, observers are often aware of only a tiny fraction of this information. These processing limitations of the visual brain have evolved to prioritize particularly relevant features of a scene while ignoring other irrelevant features. However, such selectivity has its drawbacks. In information-rich environments, such as driving a car or landing an aircraft, vision can fail to cope, and accidents can result. Accordingly, much recent research in psychophysics and ergonomics has examined how display characteristics affect our ability to process multiple features of the visual environment simultaneously. The majority of these experiments has found that performance can be optimized by combining several features into one visual 'object'. In contrast, several recent studies from my own laboratory have found the opposite pattern, indicating that information sources can often be more efficiently processed when they belong to separate objects. Indeed, these data suggest that the number of objects has no general effect on our perceptual performance. Instead, I argue for a two-pathway approach to understanding human visual capacities, and suggest that this approach may have important implications for a diverse range of display technologies, including cockpit displays.

THE HUMAN VISUAL CORTEX

The discovery and analysis of cortical visual areas is a major accomplishment of visual neuroscience. In the past decade the use of noninvasive functional imaging, particularly functional magnetic resonance imaging (fMRI), has dramatically increased our detailed knowledge of the functional organization of the human visual cortex and its relation to visual perception. The fMRI method offers a major advantage over other techniques applied in neuroscience by providing a large-scale neuroanatomical perspective that stems from its ability to image the entire brain essentially at once. This bird's eye view has the potential to reveal large-scale principles within the very complex plethora of visual areas. Thus, it could arrange the entire constellation of human visual areas in a unified functional organizational framework. Here we review recent findings and methods employed to uncover the functional properties of the human visual cortex focusing on two themes: functional specialization and hierarchical processing.

Modulation of object-based attention by spatial focus under endogenous and exogenous orienting

In R. Egly, J. Driver, and R. D. Rafal's (1994) influential double-rectangle spatial-cuing paradigm, exogenous cues consistently induce object-based attention, whereas endogenous cues generally induce space-based attention. This difference suggests an interdependency between mode of orienting (endogenous vs exogenous) and mode of selection (object based vs space based). However, mode of orienting is generally confounded with initial focus of attention: Endogenous orienting begins with attention focused on a central cue, whereas exogenous orienting begins with attention widely spread. In this study, an attentional-focusing hypothesis is examined and supported by experiments showing that for both endogenous and exogenous cuing, object-based effects are obtained under conditions that encourage spread attention, but they are attenuated under conditions that encourage focused attention. General implications for object-based attention are discussed. ((c) 2003 APA, all rights reserved)

Perceptual grouping in change detection

Detection of an item's changing of its location from one instance to another is typically unaffected by changes in the shape or color of contextual items. However, we demonstrate here that such location change detection is severely impaired if the elongated axes of contextual items change orientation, even though individual locations remain constant and even though the orientation was irrelevant to the task. Changing the orientations of the elongated stimuli altered the perceptual organization of the display, which had an important influence on change detection. In detecting location changes, subjects were unable to ignore changes in orientation unless additional, invariant grouping cues were provided or unless the items changing orientation could be actively ignored using feature-based attention (color cues). Our results suggest that some relational grouping cues are represented in change detection even when they are task irrelevant.

Dynamic interaction of object- and space-based attention in retinotopic visual areas

We investigated the interaction between object- and space-based attention by measuring activity in early visual cortex. After central cueing, when subjects directed attention to a spatially defined part of an object, activity in early visual areas was enhanced at corresponding retinotopic representations but also at representations of other locations covered by the object. Different from the assumption of automatic attentional "spreading" within an object, however, activity was greater for representations of cued than of uncued locations on the same object. These findings support an interaction of object-based spatial selection with object-independent spatial mechanisms in directing attention. When the target stimulus did not appear at the expected location, we found higher activation in areas representing other locations on the same object than equidistant locations on other objects. Objects, hence, also guide spatial search, and this may account for the behaviorally observed delay in processing parts of an unattended object.

Attentional capture within and between objects

The present study addressed the question whether attentional capture by abrupt onsets is affected by object-like properties of the stimulus field. Observers searched for a target circle at one of four ends of two solid rectangles. In the focused attention condition the location of the upcoming target was cued by means of a central arrowhead, whereas in the divided attention condition, the target location was not cued. Irrelevant abrupt onsets could appear either within the attended or within the non-attended object. The results showed that in the focused attention condition, onsets ceased to capture attention irrespective of whether the onset appeared within an attended object or within a non-attended object.

The modulation of inhibition of return by object-internal structure: implications for theories of object-based attentional selection

Recently, Vecera, Behrmann, and McGoldrick (2000), using a divided-attention task, reported that targets are detected more accurately when they occur on the same structural part of an object, suggesting that attention can be directed toward object-internal features. We present converging evidence using the object-based inhibition of return (IOR) paradigm as an implicit measure of selection. The results show that IOR is attenuated when cues and targets appear on the same part of an object relative to when they are separated by a part boundary. These findings suggest that object-based mechanisms of selection can operate over shape representations that make explicit information about object-internal structure.

Inhibition of return for objects and locations in static displays

When orienting attention, inhibition mechanisms prevent the return of attention to previously examined stimuli. This inhibition of the return of attention (IOR) has been shown to be associated additively with location- and object-based representations. That is, when static objects are attended, IOR is associated with both the object and the location cued, and hence IOR is larger than when only spatial location is attended. Recently McAuliffe, Pratt, and O'Donnell (2001) failed to observe such additive effects except under a narrow set of conditions (at short cue-target intervals and using mixed blocks in which object- and pure location-based effects were probed in the same display). The present study shows that additive IOR effects are observed under conditions that violate all of these boundary conditions. The results also show that IOR is modulated by internal structural properties of objects. These findings are cosistent with the hypothesis that IOR operates over functionally independent object- and location-based frames of reference.

Object-based selection within and beyond the focus of spatial attention

In a series of experiments, we examined the effect of perceptual objects on visual attentional processing in the presence of spatially cued attentional selection. Subjects made speeded judgments about two visual elements that were either both on the same object or on two different objects. Judgments were faster when the elements were on the same object than when they were on different objects, revealing an object advantage. Importantly, the object advantage remained even when either exogenous or endogenous spatial cues were used to direct the subjects' attention to a part of the display, contrary to earlier findings of other researchers. The object advantage, however, did disappear when the stimulus duration was shortened substantially. The results show that object-based selection is pervasive and is not diminished by the act of selective attention. The results are discussed in terms of their implications for the mechanisms that underlie attentional selection.

The attentional blink in space and time

We report the results of two experiments addressing spatiotemporal variations in the "attentional blink" (AB). In the first experiment, six streams of letters were presented simultaneously around a circle on a screen. The identity of the letters changed every 140 ms. The task was to identify two target digits (T1 and T2) that could appear in any of the streams with a variable time lag between the two. The results show that the AB is not constant across space and that following the allocation of attention to a certain location (the location of T1), discrimination can be better at locations quite far away from T1, than at locations closest to T1. Furthermore, performance at the farthest locations seemed to recover sooner from the AB than locations closer to where T1 appeared. Similar results were obtained in a second experiment where observers performed a cued discrimination task. The results accord well with the proposal that there is a region around the attended site (the center of attention) where attentional resolution is particularly poor, worse than at sites further away from the attended one. We propose that this reflects lateral inhibition of neurons responsive to the region around the attended site, with the goal of suppressing potentially distracting or interfering information.

The neural basis of biased competition in human visual cortex

A typical scene contains many different objects that compete for neural representation due to the limited processing capacity of the visual system. At the neural level, competition among multiple stimuli is evidenced by the mutual suppression of their visually evoked responses and occurs most strongly at the level of the receptive field. The competition among multiple objects can be biased by both bottom-up sensory-driven mechanisms and top-down influences, such as selective attention. Functional brain imaging studies reveal that biasing signals due to selective attention can modulate neural activity in visual cortex not only in the presence, but also in the absence of visual stimulation. Although the competition among stimuli for representation is ultimately resolved within visual cortex, the source of top-down biasing signals likely derives from a distributed network of areas in frontal and parietal cortex. Attention-related activity in frontal and parietal areas does not reflect attentional modulation of visually evoked responses, but rather the attentional operations themselves.

Can attention select only a fixed number of objects at a time?

Several previous studies have suggested that we may attend only a fixed number of 'objects' at a time. However, whereas findings from two-target experiments suggest that we can attend only one object at a time, other results from object-tracking and enumeration paradigms point instead to a four-object limit. Here, we note that in these previous studies the number of objects covaried with the overall size and complexity of the stimulus, such that apparent one-object or four-object limits in those tasks may reflect changes in the complexity of attended stimuli, rather than the number of objects per se. Accordingly, in the current experiments we employ stimuli in which the number of objects varies, while overall size and complexity are held constant. Using these refined measures of object-based effects, we find no evidence for a one-object or four-object limit on attention. Indeed, we conclude that the number of attended objects does not affect how efficiently we can attend a given stimulus. We propose and test an alternative approach to object-based attention limitations based on within-object and between-object feature-binding mechanisms in human vision.

Object-based selection: the role of attentional shifts

The objective of this paper was to investigate under what conditions object-based effects are observed. Recently, Watson and Kramer (1999) used a divided-attention task and showed that unless top-down factors induce a bias toward selection at a higher level, object-based effects are obtained when same-object targets belong to the same uniformly connected (single-UC) region, but not when they belong to different single-UC regions grouped into a higher order object (grouped-UC regions). We refine this claim by proposing that a critical factor in determining whether or not object-based effects with grouped-UC regions are observed is the need to shift attention. The results of four experiments support this hypothesis. Stimuli and displays were similar to those used by Egly, Driver, and Rafal (1994). Subjects had to make size judgments. Using different paradigms, we obtained object-based effects when the task required shifts of attention (spatial cuing, same vs. different judgment with asynchronous target onsets), but not when attention remained either broadly distributed (same vs. different judgment with simultaneous targets) or tightly focused (response competition paradigm).

Attending to objects: costs or benefits?

The single-object advantage is said to occur when performance is faster and/or more accurate when the two targets to be compared appear on one object than when they appear on two different objects. The single-object advantage has been interpreted to suggest that attention can select objects rather than unparsed regions of visual space. In five experiments we explored whether directing attention to one object rather than two objects produces a benefit or a minor cost. Participants were required to compare two target features that belonged to one object, to two objects, or did not belong to any object. In addition, we varied the relevance to the task of object-related global information, such as symmetry of the object and perceptual cluttering of the background. Results showed that attending to one object in comparison to attending to no object produced a benefit only when object-related global information was relevant to the task. In contrast, when object-related global information was irrelevant to the task, attending to one object produced a cost. Thus, it can be concluded that attending to an object does not produce an absolute benefit, but rather produces a smaller cost than attending to two objects.

Object-based attention: sensory modulation or priority setting?

The detection of an invalidly cued target is faster when it appears within a cued object than when it appears in an uncued object equally distant from the cued location; this is a manifestation of object based attention. Five experiments are reported in which it was investigated whether early sensory enhancement (in which attention "spreads" within an attended object but stops at its borders) or a later attentional prioritization mechanism best accounts for these effects. In Experiments 1-4, subjects identified a centrally located target with a buttonpress while attempting to ignore flanking distractors that were mapped to either a compatible or an incompatible response. The flankers appeared either within the object occupied by the target or in a different object but at the same distance from the target. The well-known effect of distance between the target and the flankers on the magnitude of the compatibility effect was replicated. However, whether the target and the flankers were in the same or different objects had no effect on the magnitude of the compatibility effect. In Experiment 5, when attention could not be narrowly focused in advance, object-based modulation of the flanker effect was observed. These results suggest that object-based selection may reflect an object-specific attentional prioritization strategy, rather than object-based attentional modulation of an early sensory representation.

The spatial resolution of visual attention

Two tasks were used to evaluate the grain of visual attention, the minimum spacing at which attention can select individual items. First, observers performed a tracking task at many viewing distances. When the display subtended less than 1 degrees in size, tracking was no longer possible even though observers could resolve the items and their motions: The items were visible but could not be individuated one from the other. The limiting size for selection was roughly the same whether tracking one or three targets, suggesting that the resolution limit acts independently of the capacity limit of attention. Second, the closest spacing that still allowed individuation of single items in dense, static displays was examined. This critical spacing was about 50% coarser in the radial direction compared to the tangential direction and was coarser in the upper as opposed to the lower visual field. The results suggest that no more than about 60 items can be arrayed in the central 30 degrees of the visual field while still allowing attentional access to each individually. Our data show that selection has a coarse grain, much coarser than visual resolution. These measures of the resolution of attention are based solely on the selection of location and are not confounded with preattentive feature interactions that may contribute to measures from flanker and crowding tasks. The results suggest that the parietal area is the most likely locus of this selection mechanism and that it acts by pointing to the spatial coordinates (or cortical coordinates) of items of interest rather than by holding a representation of the items themselves.

Processing demand modulates the effects of spatial attention on the judged duration of a brief stimulus

How does attention influence the judged duration of a brief stimulus? In the four experiments reported here, we show that the effect of spatial attention on duration judgment depends on the processing demand of the concurrent nontemporal task. When participants had to perform a speeded letter discrimination task in addition to duration rating, the judged duration was longer at a cued location than at an uncued location, regardless of whether the cue was exogenous or endogenous. However, when the same stimuli were presented but no concurrent nontemporal task was required, duration was judged to be shorter at the cued location, as compared with the uncued locations. Furthermore, although spatial attention influenced duration judgment, no object-based attentional effects were found. These findings suggest that, although spatial attention plays an important role in the judged duration of a briefly presented stimulus, its effect is mediated by the processing demand of the task.

Cued visual attention does not distinguish between occluded and occluding objects

Does visual attention spread from the cued end of an occluded object to locations occupied by inferred portions of that object? We investigated this question by using a probe detection paradigm with two-dimensional (2-D) displays of occluded objects. Probes could appear in occluded or nonoccluded locations on either a cued or noncued object. Participants responded faster to probes appearing within the region of space occupied by the cued object. This was true not only when the probe appeared in positions separated from the cued location by an occluder (as demonstrated by Moore, Yantis, & Vaughan, 1998), but also when it appeared in positions on the occluder itself. Thus, results suggest that cued facilitation spreads to regions of noncued occluding objects that overlap cued occluded objects in 2-D space.

The spatial profile of visual attention in mental curve tracing

In a curve-tracing task, subjects have to judge whether items are located on a single, continuous curve. Spatially separate segments of such a curve are related to each other through grouping criteria, like collinearity and connectedness. These grouping cues need to be exploited during curve tracing, but it is still an open issue how grouping of contour segments is achieved by the visual system. Many contemporary theories of visual perception assume that grouping operations are carried out pre-attentively, with unlimited capacity. The present study examines this assumption by investigating the involvement of attention in curve tracing. The results show that attention is directed to contour segments that need to be grouped together. The distribution of attention is guided by grouping criteria, such as connectedness. Apparently, attention is required to group spatially separate contour segments into a coherent representation of a curve.

Objects and attention: the state of the art

What are the units of attention? In addition to standard models holding that attention can select spatial regions and visual features, recent work suggests that in some cases attention can directly select discrete objects. This paper reviews the state of the art with regard to such 'object-based' attention, and explores how objects of attention relate to locations, reference frames, perceptual groups, surfaces, parts, and features. Also discussed are the dynamic aspects of objecthood, including the question of how attended objects are individuated in time, and the possibility of attending to simple dynamic motions and events. The final sections of this review generalize these issues beyond vision science, to other modalities and fields such as auditory objects of attention and the infant's 'object concept'.

A neurodynamical model of visual attention: feedback enhancement of spatial resolution in a hierarchical system

Human beings have the capacity to recognize objects in natural visual scenes with high efficiency despite the complexity of such scenes, which usually contain multiple objects. One possible mechanism for dealing with this problem is selective attention. Psychophysical evidence strongly suggests that selective attention can enhance the spatial resolution in the input region corresponding to the focus of attention. In this work we adopt a computational neuroscience perspective to analyze the attentional enhancement of spatial resolution in the area containing the objects of interest. We extend and apply the computational model of Deco and Schürmann (2000), which consists of several modules with feedforward and feedback interconnections describing the mutual links between different areas of the visual cortex. Each module analyses the visual input with different spatial resolution and can be thought of as a hierarchical predictor at a given level of resolution. Moreover, each hierarchical predictor has a submodule that consists of a group of neurons performing a biologically based 2D Gabor wavelet transformation at a given resolution level. The attention control decides in which local regions the spatial resolution should be enhanced in a serial fashion. In this sense, the scene is first analyzed at a coarse resolution level, and the focus of attention enhances iteratively the resolution at the location of an object until the object is identified. We propose and simulate new psychophysical experiments where the effect of the attentional enhancement of spatial resolution can be demonstrated by predicting different reaction time profiles in visual search experiments where the target and distractors are defined at different levels of resolution.

The spatial spread of attentional modulation of the motion aftereffect

The spatial spread of attentional modulation of selective adaptation was investigated in four experiments in which the duration of the movement aftereffect (MAE) was measured with and without processing of intermittently changing digits at the fixation point. In the first experiment, the effects of diverting attention on MAE duration were found to reduce as the distance between the fixation digits and the inner edge of the surrounding adapt/test grating was increased. A second experiment suggested that eye movements were unlikely to underlie the attentional effects. In experiment 3, the attentional effect stayed constant as the outer diameter of the adapt/test gratings was increased. In experiment 4 (as in experiment 1) the modulatory effects of attention were larger the closer the adapt/test gratings were to the locus of attention, when the area of the grating was held constant but its eccentricity varied. In experiments 1 and 4, an intermittently changing fixation digit was found to reduce MAE durations more than an unchanging digit, even when subjects were not required to process it, suggesting that exogenous as well as endogenous attentional processes modulate early motion processing.

Perceptual grouping via spatial selection in a focused-attention task

Theories of attention can be separated into those that select by location, and those that select by location-invariant representation. Experiments demonstrating stronger interference or facilitation from distractors grouped by nonspatial features with the target than ungrouped distractors have been considered as evidence for the selection of location-invariant representations. However, few studies have measured spatial attention directly at the locations of the grouped or ungrouped objects. In these experiments subjects responded to spatial probes (dots) while also identifying a cued target letter among distractors. Probe responses were faster for distractor locations with the target color than for those with the nontarget color, implying that target-color locations receive more attention. This pattern of spatial attention may explain why target-color distractors interfere more with target identification than nontarget-color distractors. These results suggest that although attention can be directed by nonspatial properties such as grouping by color or organization of the scene into objects, selection may ultimately be based on location.

Attending to the parts of a single object: part-based selection limitations

Studies of object-based attention have demonstrated poorer performance in dividing attention between two objects in a scene than in focusing attention on a single object. However, objects often are composed of several parts, and parts are central to theories of object recognition. Are parts also important for visual attention? That is, can attention be limited in the number of parts processed simultaneously? We addressed this question in four experiments. In Experiments 1 and 2, participants reported two attributes that appeared on the same part or on different parts of a single multipart object. Participants were more accurate in reporting the attributes on the same part than attributes on different parts. This part-based effect was not influenced by the spatial distance between the parts, ruling out a simple spatial attention interpretation of our results. A control study demonstrated that our spatial manipulation was sufficient to observe shifts of spatial attention. This study revealed an effect of spatial distance, indicating that our spatial manipulation was adequate for observing spatial attention. The absence of a distance effect in Experiments 1 and 2 suggests that part-based attention may not rely entirely on simple shifts of spatial attention. Finally, in Experiment 4 we found evidence for part-based attention, using stimuli controlled for the distance between the parts of an object. The results of these experiments indicate that visual attention can selectively process the parts of an object. We discuss the relationship between parts and objects and the locus of part-based attentional selection.

A hierarchical neural system with attentional top-down enhancement of the spatial resolution for object recognition

We present a hierarchical neurodynamical system for object recognition based on attentional control of the spatial resolution with which an object is analyzed during an iterative hypothesis testing cycle. Psychophysical evidence strongly suggests that attentional processing results in the enhancement of the spatial resolution in the input region corresponding to the focus of attention. We adopt a computational neuroscience approach in order to analyze this attentional enhancement of the spatial resolution for object recognition. The system consists of a where- and a what-module which include networks with feedforward and feedback interconnections describing the mutual links between different areas of the visual cortex.

Mechanisms of visual attention in the human cortex

A typical scene contains many different objects that, because of the limited processing capacity of the visual system, compete for neural representation. The competition among multiple objects in visual cortex can be biased by both bottom-up sensory-driven mechanisms and top-down influences, such as selective attention. Functional brain imaging studies reveal that, both in the absence and in the presence of visual stimulation, biasing signals due to selective attention can modulate neural activity in visual cortex in several ways. Although the competition among stimuli for representation is ultimately resolved within visual cortex, the source of top-down biasing signals derives from a network of areas in frontal and parietal cortex.

Reappraising the apparent costs of attending to two separate visual objects

Support for object-based accounts of visual attention has been drawn from several different types of effect. One effect is found when observers try to restrict their attention to a particular region of a display. Other regions belonging to the same object are often selected as well, suggesting that attention spreads spatially over entire objects. Another effect is found when judging two visual attributes; performance is often less efficient when the attributes belong to separate objects rather than both belonging to a single object. This latter effect has been taken to imply that only one segmented object can be attended at a time. However, it may instead merely be a variant of the first effect. If, as we assume here, attention spreads to task-irrelevant regions of relevant objects, it will encompass a larger spatial region and more information when judging attributes of two objects rather than one. Here we compared judging one versus two objects, while manipulating whether the two objects occupied a wider extent than the single object condition (as in previous work), or not. Costs were found for judging two objects versus one only when together they occupied a wider spatial extent. We conclude that reported difficulties in attending two objects may be due to attention spreading across the entire spatial extent of objects when judging their parts, rather than a fixed inability to process more than object at a time.

Object-based selection under focused attention: a failure to replicate

In a recent study, Lavie and Driver (1996) reported that object-based effects found with distributed attention disappear when attention is focused on a narrow area of the display. This finding stands in contrast with previous reports of object-based effects under conditions of focused attention (e.g., Atchley & Kramer, 1998; Egly, Driver, & Rafal, 1994). The present study was an attempt to replicate Lavie and Driver's finding, using similar task and stimuli. While Lavie and Driver's object-based effect in the distributed attention condition was replicated, its absence in the focused attention condition was not. In the two experiments reported in this paper, object-based effects were found under conditions of both distributed and focused attention, with no difference in the magnitude of the object-based effects in the two conditions. It is concluded that, in contrast with Lavie and Driver's claim, the initial spatial setting of attention does not influence object-based constraints on the distribution of attention.

Selective attention to the parts of an object

Recent empirical results suggest that there is a decrement in dividing attention between two objects in a scene compared with focusing attention on a single object. However, objects can be made of individual parts. Is there a decrement for dividing attention across different parts of a single object? We addressed this question in two experiments. In Experiment 1, we demonstrated that attention can exhibit part-based selection--that is, the subjects were more accurate in reporting two attributes from the same part of an object than they were in reporting attributes from different parts of an object. In Experiment 2, we demonstrated that part-based attentional decrements occurred simultaneously with object-based attentional decrements. The results from Experiment 2 demonstrated that part-based attention is evident at the same time as objects are processed as coherent whole. Our results imply that there is an attentional mechanism that can select either objects or their parts.

Coding space within but not between objects: evidence from Balint's syndrome

The ability to make spatial judgements was examined in a patient demonstrating poor perception of multiple objects following bilateral parietal lesions, under conditions in which the presence of the stimuli to which judgements were made could be detected. The tasks required judgements of spatial length or the position of coloured parts of stimuli. We manipulated the degree to which two uprights in a display could be encoded into a single perceptual object using either stored knowledge or bottom-up cues based on 2D or 3D image relations. Performance was dependent on the presence of both bottom-up grouping and familiarity. However, connectedness in the image was not sufficient to benefit performance, when stimuli were separate objects in 3D space. This deficit in spatial judgements, arising following detection of the relevant stimulus elements, is attributed to an impairment in coding the spatial relations between separate perceptual objects. This deficit could be overcome if stimuli could be grouped in 3D, using bottom-up cues and top-down knowledge.

Object-based visual attention with endogenous orienting

In a series of experiments, we examined covert orienting using endogenous cuing, in which attention is voluntarily directed toward a peripheral location. In one experiment, subjects were cued to attend to one end of an oblong object. They then detected targets on the cued object or elsewhere. In another experiment, subjects provided judgments of the relative temporal order of two flashes after their attention had moved endogenously. In a third experiment, subjects were directed to attend to an empty spatial location and subsequently discriminated features of objects that appeared at or near the locus of attention. In each of these situations, attentional orienting was object based, in the sense that nonattended locations that were on the cued object had an advantage over nonattended locations that were not on the object. The results are discussed in terms of their implications for object-based representations and the differences between exogenous and endogenous orienting of attention.

Driving experience and the functional field of view

Research has suggested that novice drivers have different search strategies compared with their more experienced counterparts, and that this may contribute to their increased accident liability. One issue of concern is whether experienced drivers have a wider field of peripheral vision than less experienced drivers. This study attempted to distinguish between people of varying driving experience on the basis of their functional fields of view. Participants searched video clips taken from a moving driver's perspective for potential hazards while responding to peripheral target lights. Hit rates for peripheral targets decreased for all participant groups as processing demands increased (i.e. when hazards occurred) and as the eccentricity of the target increased, though there was no interaction. An effect of experience was also found which suggests that this paradigm measures a perceptual skill or strategy that develops with driving experience.

An object-based cost of visual filtering

Although evidence for object-based attention has been reported in a variety of paradigms, few studies have examined directly the relationship between efficiency in the processing of targets and the number of intervening distractors. In five experiments, observers judged whether the vertices of two relevant shapes were of the same height. Experiments 1 and 2 manipulated observers' perceptual set so that identical stimulus displays were perceived as containing either intervening or flanking distractors. The observers were faster when the distractors were flanking rather than intervening between the targets. Experiments 3-5 varied the number of intervening distractors directly. The observers' response latencies correlated positively with the distractor set-size. Because the distractors were highly discriminable from the targets and the spatial separation between the targets and their interactions with the adjacent distractors were held constant, it was unlikely that the differential reaction times across the conditions were caused by lateral inhibition or response competitions from the distractors. The results suggest the existence of an object-based filtering cost. The implications of the present data for attentional selection over noncontiguous regions are also discussed.

Integrating top-down and bottom-up sensory processing by somato-dendritic interactions

The classical view of cortical information processing is that of a bottom-up process in a feedforward hierarchy. However, psychophysical, anatomical, and physiological evidence suggests that top-down effects play a crucial role in the processing of input stimuli. Not much is known about the neural mechanisms underlying these effects. Here we investigate a physiologically inspired model of two reciprocally connected cortical areas. Each area receives bottom-up as well as top-down information. This information is integrated by a mechanism that exploits recent findings on somato-dendritic interactions. (1) This results in a burst signal that is robust in the context of noise in bottom-up signals. (2) Investigating the influence of additional top-down information, priming-like effects on the processing of bottom-up input can be demonstrated. (3) In accordance with recent physiological findings, interareal coupling in low-frequency ranges is characteristically enhanced by top-down mechanisms. The proposed scheme combines a qualitative influence of top-down directed signals on the temporal dynamics of neuronal activity with a limited effect on the mean firing rate of the targeted neurons. As it gives an account of the system properties on the cellular level, it is possible to derive several experimentally testable predictions.

Strategic effects on object-based attentional selection

The same-object benefit, that is faster and/or more accurate performance when two target properties to be identified appear on one object than when each of the properties appear on different objects, has been a robust and theoretically important finding in the study of attentional selection. Indeed, the same-object benefit has been interpreted to suggest that attention can be used to select objects and perceptual groups rather than unparsed regions of visual space. In the present studies we report and explore a different-object benefit, that is faster identification performance when two target properties appear on different objects than when they appear on a single object. The results from the three experiments suggest that the different-object benefit was the result of mental rotation and translation strategies that subjects performed on objects in an effort to determine whether two target properties matched or mismatched. These image manipulation strategies appear to be performed with similar but not with dissimilar target properties. The results are discussed in terms of their implications for the study of object-based attentional selection.