Publications

In Press
Cohen MC, Alvarez GA, Nakayama K. The gist can be missed: Natural scene perception requires attention. Psychological Science. In Press.Abstract

Is visual attention required for visual consciousness? In the past decade, many have claimed that awareness can arise in the absence of attention. This claim is largely based on the notion that natural scene (or �gist�) perception occurs without attention. Against this, we first show that when observers perform a variety of demanding, sustained attention tasks, inattentional blindness occurs for natural scenes. In addition, scene perception is impaired under dual-task conditions, but only when using sufficiently demanding tasks. This suggests that previous studies claiming to have demonstrated scene perception without attention failed to fully engage attention and that natural scene perception does indeed require attention. Thus, natural scene perception is not a �preattentive� process and cannot be used to support the idea of awareness without attention.

Brady TF, Konkle T, Alvarez GA. A review of visual memory capacity: beyond individual items and towards structured representations. Journal of Vision. In Press.
Alvarez GA. Attention and Action. In: Ochsner K, Kosslyn S Oxford Handbook of Cognitive Neuroscience . Oxford University Press ; In Press.Abstract

At every moment, we face choices: Is it time to work, or to play? Should I listen to this lecture, or check my e-mail? Should I pay attention to what my significant other is saying, or do a mental inventory of what work I need to accomplish today? Should I keep my hands on the wheel, or change the radio station? Without any change to the external environment, it is possible to select a subset of these possibilities for further action. The process of selection is called attention, and it operates in many domains, from selecting our higher-level goals, to selecting the sensory information on which we focus, to selecting what actions we perform. This chapter focuses on the relationship between visual attention (selecting visual inputs), and action (selecting and executing movements of the body). As a case study, we focus on visual-spatial attention, the act of choosing to attend to a particular location in the visual field, and its relationship to eye-movement control. Visual attention appears to select the targets for eye-movements, as attention to a location necessarily precedes an eye-movement to that location. Moreover, there is a great deal of overlap in the neural mechanisms that control spatial attention and eye-movements, and the neural mechanisms that are specialized for spatial attention or eye-movements are highly intertwined. This link between spatial attention and eye-movements strongly supports the idea that a computational goal of the visual attention system is to select targets for action, and suggests that many of the design properties of the spatial attention system might be optimized for the control of eye-movements. Whether this relationship will hold broadly between other forms of attention (e.g., goal selection, auditory selection, tactile selection), and other forms of action (e.g., hand movements and locomotion) is an important topic of contemporary and future research.

2011
Brady TF, Alvarez GA. Hierarchical encoding in visual working memory: ensemble statistics bias memory for individual items. Psychological Science. 2011;22 (3) :384-392.Abstract

Influential models of visual working memory treat each item to be stored as an independent unit and assume that there are no interactions between items. However, real-world displays have structure that provides higher-order constraints on the items to be remembered. Even a display with simple colored circles contains statistics, such as the mean circle size, that can be computed by observers to provide an overall summary of the display. We examined the influence of such an ensemble statistic on visual working memory. We report evidence that the remembered size of each individual item in a display is biased toward the mean size of the set of items in the same color and the mean size of all items in the display. This suggests that visual working memory is constructive, encoding displays at multiple levels of abstraction and integrating across these levels, rather than maintaining a veridical representation of each item independently.

bradyalvarez-psychsci.pdf
Suchow JW, Alvarez GA. Motion silences awareness of visual change. Current Biology. 2011;21 (2) :1-4. Publisher's VersionAbstract

Loud bangs, bright flashes, and intense shocks capture attention, but other changes—even those of similar magnitude—can go unnoticed. Demonstrations of change blindness have shown that observers fail to detect substantial alterations to a scene when distracted by an irrelevant flash, or when the alterations happen gradually [1–5]. Here, we show that objects changing in hue, luminance, size, or shape appear to stop changing when they move. This motion-induced failure to detect change, silencing, persists even though the observer attends to the objects, knows that they are changing, and can make veridical judgments about their current state. Silencing demonstrates the tight coupling of motion and object appearance.

suchowalvarez-2011-currbio.pdf
Alvarez GA. Representing multiple objects as an ensemble enhances visual cognition. Trends in Cognitive Sciences. 2011;15 (3) :122-131.Abstract

The visual system can only accurately represent a handful of objects at once. How do we cope with this severe capacity limitation? One possibility is to use selective attention to process only the most relevant incoming information. A complementary strategy is to represent sets of objects as a group or ensemble (e.g., represent the average size of items). Recent studies have established that the visual system computes accurate ensemble representations across a variety of feature domains, and current research aims to determine how these representations are computed, why they are computed, and where they are coded in the brain. Ensemble representations enhance visual cognition in many ways, making ensemble coding a crucial mechanism for coping with the limitations on visual processing.

alvarez-2011-tics-ensemble.pdf
2010
Shim W, Vickery T, Alvarez GA, Jiang Y. The number of attentional foci and their precision are dissociated in the posterior parietal cortex. Cerebral Cortex. 2010;20 :1341-1349.Abstract

Many everyday tasks require us to track moving objects with attention. The demand for attention increases both when more targets are tracked and when the targets move faster. These 2 aspects of attention—assigning multiple attentional foci (or indices) to targets and monitoring each focus with precision—may tap into different cognitive and brain mechanisms. In this study, we used functional magnetic resonance imaging to quantify the response profile of dorsal attentional areas to variations in the number of attentional foci and their spatiotemporal precision. Subjects were asked to track a specific spoke of either 1 or 2 pinwheels that rotated at various speeds. Their tracking performance declined both when more pinwheels were tracked and when the tracked pinwheels rotated faster. However, posterior parietal activity increased only when subjects tracked more pinwheels but remained flat when they tracked faster moving pinwheels. The frontal eye fields and early visual areas increased activity when there were more targets and when the targets rotated faster. These results suggest that the posterior parietal cortex is specifically involved in indexing independently moving targets with attention but not in monitoring each focus with precision.

Konkle T, Brady TF, Alvarez GA, Oliva A. Conceptual distinctiveness supports detailed visual long-term memory for real-world objects. Journal of Experimental Psychology: General. 2010;139 (3) :558-578.
Konkle T, Brady TF, Alvarez GA, Oliva A. Scene memory is more detailed than you think: the role of categories in visual long-term memory. Psychological Science. 2010;21 (11) :1551-1556.Abstract

Observers can store thousands of object images in visual long-term memory with high fidelity, but the fidelity of scene representations in long-term memory is not known. Here, we probed scene-representation fidelity by varying the number of studied exemplars in different scene categories and testing memory using exemplar-level foils. Observers viewed thousands of scenes over 5.5 hr and then completed a series of forced-choice tests. Memory performance was high, even with up to 64 scenes from the same category in memory. Moreover, there was only a 2% decrease in accuracy for each doubling of the number of studied scene exemplars. Surprisingly, this degree of categorical interference was similar to the degree previously demonstrated for object memory.Thus, although scenes have often been defined as a superset of objects, our results suggest that scenes and objects may be entities at a similar level of abstraction in visual long-term memory.

Carlson TC, Alvarez GA, Wu D, Verstraten F. Rapid assimilation of external objects into the body schema. Psychological Science. 2010;21 (7) :1000-1005. Publisher's Version
2009
Horowitz TS, Wolfe JM, Alvarez GA, Cohen MA, Kuzmova YI. The speed of free will. The Quarterly Journal of Experimental Psychology. 2009.Abstract

Do voluntary and task-driven shifts of attention have the same time course? In order to measure the time needed to voluntarily shift attention, we devised several novel visual search tasks that elicited multiple sequential attentional shifts. Participants could only respond correctly if they attended to the right place at the right time. In control conditions, search tasks were similar but participants 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.

horowitz-etal-qjep-2009.pdf
Franconeri SL, Bemis D, Alvarez GA. Number estimation relies on a set of segmented objects. Cognition. 2009;113 :1-13.Abstract

How do we estimate the number of objects in a set? Two types of visual representations might underlie this ability–an unsegmented visual image or a segmented collection of discrete objects. We manipulated whether individual objects were isolated from each other or grouped into pairs by irrelevant lines. If number estimation operates over an unsegmented image, then this manipulation should not affect estimates. But if number estimation relies on a segmented image, then grouping pairs of objects into single units should lead to lower estimates. In Experiment 1 participants underestimated the number of grouped objects, relative to disconnected objects in which the connecting lines were ‘broken’. Experiment 2 presents evidence that this segmentation process occurred broadly across the entire set of objects. In Experiment 3, a staircase procedure provides a quantitative measure of the underestimation effect. Experiment 4 shows that the strength of the grouping effect was equally strong for a single thin line, and the effect can be eliminated by a small break in the line. These results provide direct evidence that number estimation relies on a segmented input.

Brady TF, Konkle T, Oliva A, Alvarez GA. Detecting changes in real-world objects: The relationship between visual long-term memory and change blindness. Communicative & Integrative Biology. 2009;2 (1) :1-3.Abstract

A large body of literature has shown that observers often fail to notice significant changes in visual scenes, even when these changes happen right in front of their eyes. For instance, people often fail to notice if their conversation partner is switched to another person, or if large background objects suddenly disappear (1,2). These "change blindness" studies have led to the inference that the amount of information we remember about each item in a visual scene may be quite low (1). However, in recent work we have demonstrated that long-term memory is capable of storing a massive number of visual objects with significant detail about each item (3). In the present paper we attempt to reconcile these findings by demonstrating that observers do not experience "change blindness" with the real world objects used in our previous experiment if they are given sufficient time to encode each item. The results reported here suggest that one of the major causes of change blindness for real-world objects is a lack of encoding time or attention to each object (see also refs. 4 and 5).

brady-etal-cib-2009.pdf
Brady TF, Konkle T, Alvarez GA. Compression in visual working memory: Using statistical regularities to form more efficient memory representations. Journal of Experimental Psychology: General. 2009;138 (4) :487-502.Abstract

The information we can hold in working memory is quite limited, but this capacity has typically been studied using simple objects or letter strings with no associations between them. However, in the real world there are strong associations and regularities in the input. In an information theoretic sense, regularities introduce redundancies that make the input more compressible. Here we show that observers can take advantage of these redundancies, enabling them to remember more items in working memory. In two experiments, we introduced covariance between colors in a display so that over trials some color pairs were more likely than other color pairs. Observers remembered more items from these displays than when the colors were paired randomly. The improved memory performance cannot be explained by simply guessing the high probability color pair, suggesting that observers formed more efficient representations to remember more items. Further, as observers learned the regularities their working memory performance improved in a way that is quantitatively predicted by a Bayesian learning model and optimal encoding scheme. We therefore suggest that the underlying capacity of their working memory is unchanged, but the information they have to remember can be encoded in a more compressed fashion.

bradykonklealvarez-2009-jepg.pdf
Battelli L, Alvarez GA, Carlson TA, Pascual-Leone A. The role of the parietal lobe in visual extinction studied with transcranial magnetic stimulation. Journal of Cognitive Neuroscience. 2009;21 (10) :1946-1955.Abstract

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 and particularly attention. For example, patients with lesions in the posterior parietal cortex are unable to selectively track objects in the contralesional side of visual space when targets are simultaneously present in the ipsilesional visual field, a form of visual extinction. Visual extinction may arise due to an imbalance in the normal inter-hemispheric competition. 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 low-frequency repetitive transcranial magnetic stimulation (rTMS) over identified areas in the left and right 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 that the parietal lobe is essential for visual tracking and that the two hemispheres compete for attentional resources during tracking. Our results provide a neuronal basis for visual extinction in patients with parietal-lobe damage.

Alvarez GA, Thompson TW. Overwriting and rebinding: why feature-switch detection tasks underestimate the binding capacity of visual working memory. Visual Cognition. 2009;17 (1-2) :141-159.Abstract

In these two experiments, we explored the ability to store bound representations of colour and location information in visual working memory using three different tasks. In the location-cue task, we probed how well colour information could be recalled when observers are given a location cue. In the feature-cue task, we probed how well location information could be recalled when observers are given a colour cue. Finally, in the feature-switch detection task, we tested how well observers could detect a recombination of features (e.g., switching the locations of the red and green items). We hypothesized that these tasks might reveal differences in binding capacity limits between switching and nonswitching tests of visual working memory. We also hoped the tasks could provide an explanation for those differences in terms of the component processes of working memory do failures occur in the encoding, maintenance, or retrieval stages of the task? Experiment 1 showed that performance in the two cued-recall tasks was equally high, and was significantly better than performance in the feature-switch detection task. Thus, the feature- switch detection task underestimates the number of colour location bindings that can be remembered, but is a useful task for examining the fragile nature of feature binding in working memory. Experiment 2 explored why feature-switch detection underestimates the binding capacity of visual working memory by examining whether the feature switch errors occur at the level of encoding, maintaining, or retrieving binding information from visual working memory. The results suggest that feature switch errors reflect failures to maintain bound objects in working memory, perhaps due to the automatic rewriting and rebinding of information in the face of new perceptual input.

brockmole-franconeri-specialissueintro-viscog.pdf
Alvarez GA, Oliva A. Spatial ensemble statistics are efficient codes that can be represented with reduced attention. Proceedings of the National Academy of Sciences, USA. 2009;106 :7345-7350.Abstract

There is a great deal of structural regularity in the natural environment, and such regularities confer an opportunity to form compressed, efficient representations. While this concept has been extensively studied within the domain of low-level sensory coding, there has been limited focus on efficient coding in the field of visual attention. Here we show that spatial patterns of orientation information ("spatial ensemble statistics") can be efficiently encoded under conditions of reduced attention. In our task, observers monitored for changes to the spatial pattern of background elements while they were attentively tracking moving objects in the foreground. Using stimuli that enable us to dissociate changes in local structure from changes in the ensemble structure, we found that observers were more sensitive to changes to the background that altered the ensemble structure than to changes that did not alter the ensemble structure. We propose that reducing attention to the background increases the amount of noise in local feature representations, but that spatial ensemble statistics capitalize on structural regularities to overcome this noise by pooling across local measurements, gaining precision in the representation of the ensemble.

alvarez-oliva-pnas09.pdf
2008
Shim W, Alvarez GA, Jiang Y. Spatial separation between targets constrains maintenance of attention on multiple objects. Psychonomic Bulletin & Review. 2008;15 (2) :390-397.Abstract

Humans are limited in their ability to maintain multiple attentional foci. In attentive tracking of moving objects, performance declines as the number of tracked targets increases. Previous studies have interpreted such reduction in terms of a limit in the number of attentional foci. However, increasing the number of targets usually reduces spatial separation among different targets. In this study, we examine the role of target spatial separation in maintaining multiple attentional foci. Results from a multiple-object tracking task show that tracking accuracy deteriorates as the spatial separation between targets decreases. We propose that local interaction between nearby attentional foci modulates the resolution of attention, and that capacity limitation from attentive tracking originates in part from limitations in maintaining critical spacing among multiple attentional foci. These findings are consistent with the hypothesis that tracking performance is limited not primarily by a number of locations, but by factors such as the spacing and speed of the targets and distractors.

shim-etal-2008-pbr.pdf
Brady TF, Konkle T, Alvarez GA, Oliva A. Visual long-term memory has a massive storage capacity for object details. Proceedings of the National Academy of Sciences, USA. 2008;105 (38) :14325-14329. Publisher's VersionAbstract

One of the major lessons of memory research has been that human memory is fallible, imprecise, and subject to interference. Thus, although observers can remember thousands of images, it is widely assumed that these memories lack detail. Contrary to this assumption, here we show that long-term memory is capable of storing a massive number of objects with details from the image. Participants viewed pictures of 2,500 objects over the course of 5.5 h. Afterward, they were shown pairs of images and indicated which of the two they had seen. The previously viewed item could be paired with either an object from a novel category, an object of the same basic-level category, or the same object in a different state or pose. Performance in each of these conditions was remarkably high (92%, 88%, and 87%, respectively), suggesting that participants successfully maintained detailed representations of thousands of images. These results have implications for cognitive models, in which capacity limitations impose a primary computational constraint (e.g., models of object recognition), and pose a challenge to neural models of memory storage and retrieval, which must be able to account for such a large and detailed storage capacity.

bradykonklealvarezoliva2008_combined1.pdf
Alvarez GA, Oliva A. The representation of simple ensemble features outside the focus of attention. Psychological Science. 2008;19 (4) :392-398.Abstract

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.

alvarez-oliva-2008-psychsci.pdf

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