Reversal of object-based benefits in visual attention

In divided-attention tasks, observers must make speeded (or near threshold accuracy) judgements concerning two target features in a display. Typically, when the two features belong to the same object they are more rapidly judged than when they belong to separate objects, a pattern of findings referred to here as a “same-object benefit”. However, we note here that many of these studies share common features, in particular the use of pre-exposed, outline, and/or overlapping objects, and their findings may not generalize to other types of display. Building substantially on previous work by Davis, Welch, Holmes, and Shepherd (2001), we show in four new studies that once these features are not present in a divided-attention task, no same-object benefits are reported. Rather we now find “same-object costs”, where features belonging to a single object are less rapidly judged than features belonging to separate objects.     

Crossmodal object-based attention: auditory objects affect visual processing

According to the object-based view, visual attention can be deployed to "objects" or perceptual units, regardless of spatial locations. Recently, however, the notion of object has also been extended to the auditory domain, with some authors suggesting possible interactions between visual and auditory objects. Here we show that task-irrelevant auditory objects may affect the deployment of visual attention, providing evidence that crossmodal links can also occur at an object-based level. Hence, in addition to the well documented control of visual objects over what we hear, our findings demonstrate that, in some cases, auditory objects can affect visual processing.     

Occlusion, symmetry, and object-based attention: reply to Saiki (2000)

J. Saiki (2000) argued that, because the stimuli used by M. Behrmann, R. S. Zemel, and M. C. Mozer (1998) were confounded by symmetry, conclusions about whether amodally completed objects can benefit from object-based attention are unwarranted. Here, the authors examine J. Saiki's claim further and expand on their view of the mechanisms underlying object-based attention, suggesting that perceptual organization is the process whereby features from a single object are selectively attended. In light of this, they claim that heuristics such as symmetry and collinearity play an important role in the facilitation of features from a single object. In support of this claim, they present data from a further experiment using displays that exploit common fate, another grouping heuristic, and show that, under these conditions, the hallmark of object-based attention, a single-object advantage, is obtained for the occluded (amodally completed) shapes.     

Interference between object-based attention and object-based memory

Research has shown that there are at least two kinds of visual selective attention: location based and object based. In the present study, we sought to determine the locus of spatially invariant object-based selection using a dual-task paradigm. In four experiments, observers performed an attention task (object feature report or visual search) with a concurrent memory task (object memory or spatial memory). Object memory was interfered with more by a concurrent object-based attention task than by a concurrent location-based attention task. However, this interference pattern was reversed for spatial memory, with greater interference by a location-based attention task than by an object-based attention task. These findings suggest that object-based attention and locationbased attention are functionally dissociable and that some forms of object-based selection operate within visual short-term memory.     

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

A new test was devised to avoid previous confounds in measures of object-based limits on divided visual attention. The distinction between objects was manipulated across a wide spatial extent. Target elements appeared on the same object only when far apart, and appeared close only when on different objects, so that object effects could not be reduced to spatial effects, nor vice versa. Subjects judged whether two odd elements within a display of two dashed lines were the same or different. They performed better when the target elements were far apart on a common line rather than on two distinct lines, even though the latter arrangement was more likely. Thus, nonstrategic object-based limits on divided attention can arise even across large distances. However, when subjects were precued to expect targets in a narrow region of the display, the object effect was eliminated, implying that object-based selection may only operate within spatially attended regions.     

Masked repetition and phonological priming within and across modalities

Lexical decision latencies to word targets presented either visually or auditorily were faster when directly preceded by a briefly presented (53-ms) pattern-masked visual prime that was the same word as the target (repetition primes), compared with different word primes. Primes that were pseudohomophones of target words did not significantly influence target processing compared with unrelated primes (Experiments 1-2) but did produce robust priming effects with slightly longer prime exposures (67 ms) in Experiment 3. Like repetition priming, these pseudohomophone priming effects did not interact with target modality. Experiments 4 and 5 replicated this general pattern of effects while introducing a different measure of prime visibility and an orthographic priming condition. Results are interpreted within the framework of a bimodal interactive activation model.     

Overt and covert object-based attention

To examine the role of perceptual object representations in the control of eye movements and attention, a pair of experiments adapted the object-cuing paradigm of Egly, Driver, and Rafal (1994) to require eye movements. Displays were pairs of adjacent rectangles, each containing two characters. Observers were asked to make a speeded judgment of a target character’s orientation, and a cue was provided prior to target/distractor onset to indicate the target’s likely location. Gaze-contingent presentation of target and distractors was used to demand overt scanning of displays. Eye movements during task performance evinced two forms of object-based effects. First, saccades following fixation on an invalidly cued item were more likely to be made within the cued rectangle than between rectangles. Second, saccades within the cued rectangle were preceded by shorter dwell times than saccades between rectangles. Extrafoveal processing of stimuli within the cued rectangle, however, was not facilitated, suggesting that covert attention was not allocated more densely within the cued than within the uncued object.     

Movements of attention across the visual field

Vocal reaction times were measured for targets presented at three distances from fixation. The targets were preceded by a cue, and the time interval between the cue and the target (stimulus-onset asynchrony; SOA) was varied. For each peripheral distance, the reaction time function initially declined as SOA was increased and then reached asymptote. The further the target from fixation, the longer the SOA at which the function reached asymptote. The asymptotic SOA values were taken as a measure of the time it takes attention to reach a given target. Comparisons of these values for the three peripheral distances permitted estimating the velocity of attention movements. These measurements suggest that when summoned by a peripheral cue, attention travels through space at a constant velocity of about 1 degree per 8 msec.     

The role of symmetry in determining perceived centers within shapes

This study was designed to assess the effects of symmetry and plane of presentation on the determination of the perceptual center of flat figures. Experiment 1 demonstrates the existence of effects in improving center determination, both in the number of sides of the shape and in rotational and reflective symmetry (confounded in the experiment). Experiment 2 shows that the presentation plane has no effect on center determination. In Experiment 3, we divide the effects of the two symmetry types, showing that rotational symmetry alone is as effective as the presence of both symmetry types—that is, the presence of symmetry axes is not very useful in finding perceived centers.     

Occlusion, symmetry, and object-based attention: comment on Behrmann, Zemel, and Mozer (1998)

The validity of M. Behrmann, R. Zemel, and M. Mozer's (1998) finding that object-based attention can be directed toward occluded objects is examined in 3 experiments. In M. Behrmann et al.'s (1998) original study, participants made speeded judgments of whether the numbers of bumps attached to 2 arms of an X shape were the same or different. The 2 sets of bumps belonged either to a single object, 2 different objects, or 2 separated parts of an occluded object. Unfortunately, this objecthood manipulation was confounded by the symmetry of the stimuli. Experiment 1 replicated M. Behrmann et al.'s main results using identical stimuli. Experiments 2a and 2b dissociated objecthood from symmetry. The results suggest that the effects of object-based attention found by M. Behrmann et al. are largely due to symmetry. The stimuli used in M. Behrmann et al. are not appropriate for examining the relation between object-based attention and occlusion.     

The spatial distribution of attention within and across objects

Attention operates to select both spatial locations and perceptual objects. However, the specific mechanism by which attention is oriented to objects is not well understood. We examined the means by which object structure constrains the distribution of spatial attention (i.e., a "grouped array"). Using a modified version of the Egly et al. object cuing task, we systematically manipulated within-object distance and object boundaries. Four major findings are reported: 1) spatial attention forms a gradient across the attended object; 2) object boundaries limit the distribution of this gradient, with the spread of attention constrained by a boundary; 3) boundaries within an object operate similarly to across-object boundaries: we observed object-based effects across a discontinuity within a single object, without the demand to divide or switch attention between discrete object representations; and 4) the gradient of spatial attention across an object directly modulates perceptual sensitivity, implicating a relatively early locus for the grouped array representation.     

The role of symmetry in determining perceived centers within shapes.

This study was designed to assess the effects of symmetry and plane of presentation on the determination of the perceptual center of flat figures. Experiment 1 demonstrates the existence of effects in improving center determination, both in the number of sides of the shape and in rotational and reflective symmetry (confounded in the experiment). Experiment 2 shows that the presentation plane has no effect on center determination. In Experiment 3, we divide the effects of the two symmetry types, showing that rotational symmetry alone is as effective as the presence of both symmetry types--that is, the presence of symmetry axes is not very useful in finding perceived centers.     

Perception of two-dimensional Markov constraints within visual displays

A method is described for constructing visual displays in which statistical constraints are encoded within two spatial dimensions without introducing one-dimensional linear constraints. Within each local group of four elements, the state of one element was determined with a given probability by the previously generated states of the other three. Ss rated such displays on a scale from “lumpy” (or crude texture) to “lacy” (or even texture). The consistency of classification obtained for displays with strong aggregated (“lumpy”) properties was substantially higher than that obtained for displays with strong distributed (“lacy”) properties. An incidental feature of the Ss’ behavior was their deliberate degrading of the visual quality of the displays. Comparison is made with one-dimensional displays concatenated in two dimensions.     

Neural dynamics of object-based multifocal visual spatial attention and priming: object cueing, useful-field-of-view, and crowding

How are spatial and object attention coordinated to achieve rapid object learning and recognition during eye movement search? How do prefrontal priming and parietal spatial mechanisms interact to determine the reaction time costs of intra-object attention shifts, inter-object attention shifts, and shifts between visible objects and covertly cued locations? What factors underlie individual differences in the timing and frequency of such attentional shifts? How do transient and sustained spatial attentional mechanisms work and interact? How can volition, mediated via the basal ganglia, influence the span of spatial attention? A neural model is developed of how spatial attention in the where cortical stream coordinates view-invariant object category learning in the what cortical stream under free viewing conditions. The model simulates psychological data about the dynamics of covert attention priming and switching requiring multifocal attention without eye movements. The model predicts how “attentional shrouds” are formed when surface representations in cortical area V4 resonate with spatial attention in posterior parietal cortex (PPC) and prefrontal cortex (PFC), while shrouds compete among themselves for dominance. Winning shrouds support invariant object category learning, and active surface-shroud resonances support conscious surface perception and recognition. Attentive competition between multiple objects and cues simulates reaction-time data from the two-object cueing paradigm. The relative strength of sustained surface-driven and fast-transient motion-driven spatial attention controls individual differences in reaction time for invalid cues. Competition between surface-driven attentional shrouds controls individual differences in detection rate of peripheral targets in useful-field-of-view tasks. The model proposes how the strength of competition can be mediated, though learning or momentary changes in volition, by the basal ganglia. A new explanation of crowding shows how the cortical magnification factor, among other variables, can cause multiple object surfaces to share a single surface-shroud resonance, thereby preventing recognition of the individual objects.     

Configural and contextual prioritization in object-based attention

When attention is directed to a location within an object, other locations within that object also enjoy an attentional advantage. Recently we demonstrated that this object-based advantage is mediated by increased attentional priority assigned to locations within an already attended object and not to early sensory enhancement due to the "spread" of attention within the attended object (Shomstein & Yantis, 2002). At least two factors might contribute to the assignment of attentional priority, one related to the configuration of objects in a scene and the other related to the probability of target appearance in each location imposed by task contingencies. We investigated the relative contribution of these factors by cuing one end of one of a pair of rectangles; a subsequent target appeared most often in the cued location. We manipulated attentional priority setting by varying (1) the probability that a target would appear in each of two uncued locations and (2) the cue to target stimulus onset asynchrony (SOA). On invalidly cued trials, the target appeared in the high-probability location (defined by an absolute spatial location, e.g., upper right) 83% of the time and in the low-probability location (e.g., lower left) 17% of the time. In both conditions, uncued targets appeared in the cued object half the time and in the uncued object half the time. At short SOAs, the same-object and probability effects were approximately additive. However, at longer SOAs, the same-object effects disappeared, and reaction times depended exclusively on location probability. These results suggest that observers adopt an implicit configural scanning strategy (in which unattended locations within an attended object have high priority) or an implicit contextual scanning strategy (in which objectively high-probability locations have high priority) depending on task contingencies and the amount of time that is available to deploy attention.     

Splitting attention across the two visual fields in visual short-term memory

Humans have the ability to attentionally select the most relevant visual information from their extrapersonal world and to retain it in a temporary buffer, known as visual short-term memory (VSTM). Research suggests that at least two non-contiguous items can be selected simultaneously when they are distributed across the two visual hemifields. In two experiments, we show that attention can also be split between the left and right sides of internal representations held in VSTM. Participants were asked to remember several colors, while cues presented during the delay instructed them to orient their attention to a subset of memorized colors. Experiment 1 revealed that orienting attention to one or two colors strengthened equally participants' memory for those colors, but only when they were from separate hemifields. Experiment 2 showed that in the absence of attentional cues the distribution of the items in the visual field per se had no effect on memory. These findings strongly suggest the existence of independent attentional resources in the two hemifields for selecting and/or consolidating information in VSTM.     

Time course of visual attention across perceptual levels and objects

Theories of shifts of visual attention based on attentional blink or dwell time do not directly address shifts of attention across different levels (global or local) involving multiple objects. Two experiments were conducted employing the attentional dwell time paradigm to investigate the shifts of visual attention between objects selected at same or different levels. Participants were instructed to identify two successive compound stimuli at a pre-specified level (global or local) presented at two different locations with variable SOA. The initial pair of locations in which the stimulus was presented was fixed in Experiment 1 but not in Experiment 2. Experiment 1 results showed very little impairment for second target identification when both the targets were at the global level. Attentional shift was better with both targets at the same level compared to different levels. Experiment 2 results showed that local followed by global target identification is difficult at short SOAs compared to other conditions. The results indicate that scope of attention affects the time course of visual attention. Global processing could be performed with very little capacity limitation simultaneously with distributed attention. The default mode of attention might be distributed and attention becomes focused for target identification. Different mechanisms may underlie shifts in focused attention between different locations and changes in attentional set required by changes in perceptual levels.     

Adults with dyslexia demonstrate space-based and object-based covert attention deficits: shifting attention to the periphery and shifting attention between objects in the left visual field

Performance on a covert visual attention task is compared between a group of adults with developmental dyslexia (specifically phonological difficulties) and a group of age and IQ matched controls. The group with dyslexia were generally slower to detect validly-cued targets. Costs of shifting attention toward the periphery when the target was invalidly cued were significantly higher for the group with dyslexia, while costs associated with shifts toward the fovea tended to be lower. Higher costs were also shown by the group with dyslexia for up-down shifts of attention in the periphery. A visual field processing difference was found, in that the group with dyslexia showed higher costs associated with shifting attention between objects in they LVF. These findings indicate that these adults with dyslexia have difficulty in both the space-based and the object-based components of covert visual attention, and more specifically to stimuli located in the periphery.