Searching in dynamic displays: Effects of configural predictability and spatio-temporal continuity.

A visual search task was used to probe how well attention can operate over a dynamically changing visual display. Participants searched for a target item among an array of distractor items while the items either shifted location several times per second, or remained stationary. Not surprisingly, Experiment 1 showed that shifting display items slowed search. However, search was faster if the shift preserved the global, configural structure of the display. The results of Experiment 2 suggest that the benefit of maintaining configural structure comes from improved spatial predictability: knowing where the searchable items will be at any given moment enables faster search. Finally, Experiment 3 shows that, given spatio-temporal continuity, attention can operate just as efficiently over a dynamically changing display as it can over a stationary display. In the real world objects often move, but they do so in a predictable way. The current findings suggest that the mechanisms underlying search can capitalize on configural predictability and spatio-temporal continuity to enable efficient search in such dynamic situations.

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The representation of simple ensemble features outside the focus of attention.

The representation of visual information inside the focus of attention is precise relative to the representation of information outside the focus of attention. We find that the visual system can compensate for the cost of withdrawing attention by pooling noisy local features and computing summary statistics. The location of an individual object is a local feature, while the center of mass of several objects (centroid) is a summary feature representing the mean object location. Three Experiments showed that withdrawing attention degraded the representation of individual positions more than the representation of the centroid. It appears that information outside the focus of attention can be represented at an abstract level which lacks local detail, but nevertheless carries a precise statistical summary of the scene. The term "ensemble features" refers to a broad class of statistical summary features, which we propose collectively comprise the representation of information outside the focus of attention.

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Visual short-term memory operates more efficiently

on boundary featuresthan it does on the surface features.

A change detection task was used to estimate the visual short-term memory storage capacity for either the orientation or size of objects. On each trial, several objects were briefly presented, followed by a blank interval, and then by a second display of objects which was either identical to the first display or had a single object that was different (the object changed either orientation or size, in separate experiments). The task was to indicate whether the two displays were the same or different and the number of objects remembered was estimated from percent correct on this task. Storage capacity for a feature was nearly twice as large when that feature was defined by the object boundary rather than by the surface texture of the object. This dramatic difference in storage capacity suggests that a particular feature (e.g., "right tilted" or "small") is not stored in memory with an invariant abstract code. Instead there appear to be different codes for the boundary and surface features of objects, and memory operates on boundary features more efficiently than it operates on surface features.

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How many objects can you attentively track?: Evidence for a resource-limited tracking mechanism.

Much of our interaction with the visual world requires us to isolate some currently important objects from other less important objects. This task becomes more difficult when objects move, or when our field of view moves relative to the world, requiring us to track these objects over space and time. Previous experiments have shown that observers can track a maximum of about 4 moving objects. A natural explanation for this capacity limit is that the visual system is architecturally limited to handling a fixed number of objects at once, a so-called magical number 4 on visual attention. In contrast to this view, Experiment 1 shows that tracking capacity is not fixed. At slow speeds it is possible to track up to 8 objects, and yet there are fast speeds at which only a single object can be tracked. Experiment 2 suggests that that the limit on tracking is related to the spatial resolution of attention. These findings suggest that the number of objects that can be tracked is primarily set by a flexibly allocated resource, which has important implications for the mechanisms of object tracking and for the relationship between object tracking and other cognitive processes.

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Quadrantic deficit reveals anatomical constraints in attentional tracking.

Our conscious experience is of a seamless visual world, but many of the cortical areas that underlie our capacity for vision have a fragmented or asymmetrical representation of visual space. In fact, the representation of the visual field is fragmented into quadrants at the level of V2, V3, and possibly V4. In theory, this division could have no functional consequences and therefore no impact on behavior. Contrary to this expectation, we find robust quadrantlevel interference effects when attentively tracking two moving targets. Performance improves when target objects appear in separate quadrants (straddling either the horizontal or vertical meridian) compared with when they appear the same distance apart but within a single quadrant. These quadrant-level interference effects would not be predicted by cognitive theories of attention and tracking that do not take anatomical constraints into account. Quadrant-level interference strongly suggests that cortical areas containing a noncontiguous representation of the four quadrants of the visual field (i.e., V2, V3, and V4) impose an important constraint on attentional selection and attentive tracking.

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Space and time, not surface features, guide object persistence.

Successful visual perception relies on the ability to keep track of distinct entities as the same persisting objects from one moment to the next. This is a computationally difficult process and its underlying nature remains unclear. Here we use the object file framework to explore whether surface feature information (e.g., color, shape) can be used to compute such object persistence. From six experiments we find that spatiotemporal information (location as a function of time) easily determines object files, but surface features do not. The results suggest an unexpectedly strong constraint on the visual system's ability to compute online object persistence.

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How many locations can you select at once?

The visual system uses several tools to select only the most relevant visual information for further processing, including selection by location. In the present study we explore how many locations can be selected at once. Although past evidence from several visual tasks suggests that the visual system can operate on a fixed number of four objects or locations at once, we find that this capacity varies widely in response to the precision of selection required by the task. When we required precise selection regions, only 2-3 locations could be selected. But when the selection regions could be coarser, up to 6-7 locations could be selected.   We discuss potential mechanisms underlying the selection of multiple locations, and review the evidence for fixed limits in visual attention.

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Tracking unique objects.

Is the representation that subserves performance in multiple object tracking (MOT) experiments content-addressable? We devised an MOT variant featuring unique, nameable objects (cartoon animals) as stimuli. There were two possible response modes: standard, in which observers were asked to report the locations of all target items, and specific, in which observers had to report the location of a particular object (e.g., "Where is the zebra?"). A measure of capacity derived from accuracy allowed comparing results between conditions. We found that specific condition capacity (1.4 to 2.6 items across several experiments) was always reliably lower than standard condition capacity (2.3 to 3.4 items). Observers can locate specific objects, indicating a content-addressable representation. However, capacity differences between conditions, as well as differing responses to experimental manipulations, suggest that there may be two separate systems involved in tracking, one carrying only positional information and one carrying identity information as well.

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The role of global layout in visual short-term memory.

Recent research suggests that each object in memory is encoded relative to the other objects in memory. Specifically, the spatial configuration of the entire collection appears to play an important role in remembering any of the individual objects. However, the nature of the configural representation has not been explored. Here we identify a quantifiable dimension of spatial configuration, called regularity of connectedness, which captures an important component of the representation of spatial configuration in memory. Thus, the present study goes beyond demonstrating that spatial configuration is encoded, moving us towards understanding how and potentially why it is encoded in memory.

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How does attention select and track spatially extended objects?

New effects of attentional concentration and amplification.

Real-world situations involve attending to spatially extended objects, often under conditions of motion and high processing load.   We explored such processing by requiring observers to attentionally track a number of long, moving lines.   Concurrently, observers responded to sporadic probes, as a measure of the distribution of attention across lines. We found that attention is concentrated at the centers of lines during tracking, despite their uniformity.   Surprisingly, this center-advantage grew as the lines became longer: not only did observers get worse near the endpoints, but they became better at the lines' centers -- as if attention became more concentrated as the objects became more extended.   These results begin to show how attention is flexibly allocated in online visual processing to extended dynamic objects.

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Tracking Multiple Targets with Multifocal Attention.

Attention allows us to monitor objects or regions of visual space and select information from them for report or storage. Classical theories of attention assumed a single focus of selection but many everyday activities, such as video games, navigating busy intersections, or watching over children at a swimming pool, require attention to multiple regions of interest. Laboratory tracking tasks have indeed demonstrated the ability to track four or more targets simultaneously. Although the mechanisms by which attention maintains contact with several targets are not yet established, recent studies have identified several characteristics of the tracking process, including properties defining a 'trackable' target, the maximum number of targets that can be tracked, and the hemifield independence of the tracking process. This research also has implications for computer vision, where there is a growing demand for multiple-object tracking.

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Multielement visual tracking and visual search are 2 tasks that are held to require visual spatial attention. The authors used the attentional operating characteristic (AOC) method to determine whether both tasks draw continuously on the same attentional resource (i.e., whether the 2 tasks are mutually exclusive). The authors found that observers can search and track within the same trial significantly better than would be predicted if the 2 tasks were mutually exclusive. In fact, the AOC for tracking and search is similar to that for tracking and auditory monitoring. The results of additional experiments support an attention-switching account for this high level of dual-task performance in which a single attentional resource is efficiently switched between the tracking and search. Our results provide important constraints for architectures of visual selective attention and the mechanisms of multielement tracking.

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The ability to divide attention enables people to keep track of up to four independently moving objects. We now show that this tracking capacity is independently constrained in the left and right visual fields as if separate tracking systems were engaged, one in each field. Specifically, twice as many targets can be successfully tracked when they are divided between the left and right hemifields as when they are all presented within the same hemifield. This finding places broad constraints on the anatomy and mechanisms of attentive tracking, ruling out a single attentional focus, even one that moves quickly from target to target.

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The capacity of visual short-term memory is set both

by visual information load and by number of objects.

According to Luck and Vogel (1997), visual short-term memory has a fixed capacity of about 4 objects. However, we found that capacity varied substantially across the 5 stimulus classes we examined, ranging from 1.6 for shaded cubes to 4.4 for colors (estimated using a change detection task). We also estimated the information load per item in each class, using visual search rate. The changes we measured in memory capacity across classes were almost exactly mirrored by changes in the opposite direction in visual search rate (r2 = .992 between search rate and the reciprocal of memory capacity). The greater the information load of each item in a stimulus class (as indicated by the slower search rate), the fewer items from that class one can hold in memory. Extrapolating this linear relationship reveals that there is also an upper bound on capacity of approximately four or five objects. Thus, both the total information load and number of objects impose capacity limits on visual short-term memory.

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Limits on the resolution of visual short-term memory:

A tradeoff between number of items stored and the precision of storage.

Memory capacity can be characterized both in terms of the number of individual items that can be remembered (the item limit ), and by how precisely each individual item can be remembered (the resolution limit) . There is some debate over whether these two limits are independent of each other. To address this issue, we introduce a new method that enables us to estimate the resolution of memory as a function of the number of items stored. The results showed that as the number of items stored increased, the resolution of memory for the orientation of items decreased. These findings suggest that the item limit and resolution limit are not independent and that visual short-term memory capacity should therefore be expressed in terms of the number of objects that can be stored with a particular level of precision.

[e-mail me at alvarez@mit.edu for a copy of this manuscript]

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Primary role of the parietal lobe in

inter-hemispheric competition during sustained attention.

Inter-hemispheric competition between homologous areas in the human brain is believed to be involved in a wide variety of human behaviors from motor activity to visual perception. Inter-hemispheric competition is believed to be particularly important in attention. To directly assess the issue of reciprocal inhibition, we used fMRI to localize those brain regions active during attention-based visual tracking and then applied bilateral low- frequency repetitive transcranial magnetic stimulation (rTMS) over identified areas in the intraparietal sulcus to asses the behavioral effects on visual tracking. We induced a severe impairment in visual tracking that was selective for conditions of simultaneous tracking in both visual fields. Our data show for the first time that the parietal lobe is essential for visual tracking and that the two hemispheres compete for attentional resources during tracking. Our results provide a neurobiological basis for visual extinction in parietal patients.

[e-mail me at alvarez@mit.edu for a copy of this manuscript]

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Rapid enumeration is based on a segmented visual scene.

Results from visual search tasks suggest that the human visual system segments areas of the visual field into primitive objects broadly across the visual field. We suggest that one benefit of this broad segmentation is our ability to rapidly estimate the number of objects in a collection. Estimates of visual number could be generated from a non-segmented representation by using simpler cues such as the apparent area taken up by a collection. In contrast, we show that visual number estimation is strongly affected by subtle manipulations of segmentation cues, suggesting that number estimation relies on a visual representation that is segmented into discrete units. In Experiment 1, participants were asked to gauge which of two brief displays contained more squares. We found that participants greatly underestimated the relative number of squares in a display when pairs of squares were connected with irrelevant lines, relative to when the connecting lines were 'broken', suggesting that rapid number estimates were obtained over groupings of squares instead of the squares themselves. Experiment 2 presents further evidence that this segmentation is computed broadly.  

[e-mail me at alvarez@mit.edu for a copy of this manuscript]

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Remembering thousands of objects with high fidelity.

abstract coming soon.

[manuscript not yet available.]

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It takes attention to capture attention.

We have the ability to allocate visual attention to relevant stimuli in the world according to our current goals. But some visual stimuli, such as sudden motion, can capture our attention regardless of whether they are relevant to our current task. Presumably, the visual system monitors for such events because important changes in the environment might require an immediate response, even when we are focused on a difficult task. Across 12 visual search tasks,
we test whether the difficulty of the observer’s task modulates the ability of a stimulus to capture our attention. Surprisingly, when an observer’s task becomes more demanding, dynamic changes to the world fail to capture attention. We find the same effect across a review of several past studies. These results suggest that the system that controls these reflexive shifts of attention is not independent of other attentional processes, and may share resources with the processes involved in other visual tasks.

[e-mail me at alvarez@mit.edu for a copy of this manuscript]

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The speed of free will.

Do voluntary and task-driven shifts of attention have the same timecourse? In order to measure the time needed to voluntarily shift attention, we devised several novel visual search tasks which elicited multiple sequential attentional shifts. Observers could only respond correctly if they attended to the right place at the right time. In control conditions, search tasks were similar but observers were not required to shift attention in any order. Across five experiments, voluntary shifts of attention required 200 - 300 ms. Control conditions yielded estimates of 35 - 100 ms for task-driven shifts. We suggest that the slower speed of voluntary shifts reflects the "clock speed of free will" Wishing to attend to something takes more time than shifting attention in response to sensory input.

[e-mail me at alvarez@mit.edu for a copy of this manuscript]

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Capacity limit of visual working memory in parietal cortex

reflects the capacity limit of spatial selection.

The capacity limitation of human attention is best exemplified by attentive tracking: Our ability to track a subset of moving objects declines when more objects must be tracked. Recent neuroimaging studies show that activity in posterior parietal cortex (PPC) correlates with the number of objects tracked. However, tracking more objects increases not only the demand for indexing individual objects, but also general attentional effort. This study aims to dissociate attentional indexing from general attention in PPC using fMRI. Attentional indexing was varied by having subjects track 1 or 2 rotating pinwheels, whereas general attention was varied by increasing the pinwheels' rotation speed. Although tracking performance declined both when more pinwheels were tracked and when the pinwheels rotated faster, activity in PPC increased only when subjects tracked more wheels but not when the wheels rotated faster. We suggest that PPC is involved in attentional indexing rather than in general attention.

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