The whole is indeed more than the sum of its parts: perceptual averaging in the absence of individual item representation

We tested Ariely's (2001) proposal that the visual system represents the overall statistical properties of sets of objects against alternative accounts of rapid averaging involving sub-sampling strategies. In four experiments, observers could rapidly extract the mean size of a set of circles presented in an RSVP sequence, but could not reliably identify individual members. Experiment 1 contrasted performance on a member identification task with performance on a mean judgment task, and showed that the tasks could be dissociated based on whether the test probe was presented before or after the sequence, suggesting that member identification and mean judgment are subserved by different mechanisms. In Experiment 2, we confirmed that when given a choice between a probe corresponding to the mean size of the set and a foil corresponding to the mean of the smallest and largest items only, the former is preferred to the latter, even when observers are explicitly instructed to average only the smallest and largest items. Experiment 3 showed that a test item corresponding to the mean size of the set could be reliably discriminated from a foil but the largest item in the set, differing by an equivalent amount, could not. In Experiment 4, observers rejected test items dissimilar to the mean size of the set in a member identification task, favoring test items that corresponded to the mean of the set over items that were actually shown. These findings suggest that mean representation is accomplished without explicitly encoding individual items.     

Visual search for arbitrary objects in real scenes

How efficient is visual search in real scenes? In searches for targets among arrays of randomly placed distractors, efficiency is often indexed by the slope of the reaction time (RT) × Set Size function. However, it may be impossible to define set size for real scenes. As an approximation, we hand-labeled 100 indoor scenes and used the number of labeled regions as a surrogate for set size. In Experiment 1, observers searched for named objects (a chair, bowl, etc.). With set size defined as the number of labeled regions, search was very efficient (~5 ms/item). When we controlled for a possible guessing strategy in Experiment 2, slopes increased somewhat (~15 ms/item), but they were much shallower than search for a random object among other distinctive objects outside of a scene setting (Exp. 3: ~40 ms/item). In Experiments 4–6, observers searched repeatedly through the same scene for different objects. Increased familiarity with scenes had modest effects on RTs, while repetition of target items had large effects (>500 ms). We propose that visual search in scenes is efficient because scene-specific forms of attentional guidance can eliminate most regions from the “functional set size” of items that could possibly be the target.     

Working memory for relations among objects

Across many areas of study in cognition, the capacity of working memory (WM) is widely agreed to be roughly three to five items: three to five objects (i.e., bound collections of object features) in the literature on visual WM or three to five role bindings (i.e., objects in specific relational roles) in the literature on memory and reasoning. Three experiments investigated the capacity of observers’ WM for the spatial relations among objects in a visual display, and the results suggest that the “items” in WM are neither simply objects nor simply role bindings. The results of Experiment 1 are most consistent with a model that treats an “item” in visual WM as an object, along with the roles of all its relations to one other object. Experiment 2 compared observers’ WM for object size with their memory for relative size and provided evidence that observers compute and store objects’ relations per se (rather than just absolute size) in WM. Experiment 3 tested and confirmed several more nuanced predictions of the model supported by Experiment 1. Together, these findings suggest that objects are stored in visual WM in pairs (along with all the relations between the objects in a pair) and that, from the perspective of WM, a given object in one pair is not the same “item” as that same object in a different pair.     

How not to be seen: the contribution of similarity and selective ignoring to sustained inattentional blindness

When people attend to objects or events in a visual display, they often fail to notice an additional, unexpected, but fully visible object or event in the same display. This phenomenon is now known as inattentional blindness . We present a new approach to the study of sustained inattentional blindness for dynamic events in order to explore the roles of similarity, distinctiveness, and attentional set in the detection of unexpected objects. In Experiment 1, we found that the similarity of an unexpected object to other objects in the display influences attentional capture: The more similar an unexpected object is to the attended items, and the greater its difference from the ignored items, the more likely it is that people will notice it. Experiment 2 explored whether this effect of similarity is driven by selective ignoring of irrelevant items or by selective focusing on attended items. The results of Experiment 3 suggest that the distinctiveness of the unexpected object alone cannot entirely account for the similarity effects found in the first two experiments; when attending to black items or white items in a dynamic display, nearly 30% of observers failed to notice a bright red cross move across the display, even though it had a unique color, luminance, shape, and motion trajectory and was visible for 5 s. Together, the results suggest that inattentional blindness for ongoing dynamic events depends both on the similarity of the unexpected object to the other objects in the display and on the observer's attentional set.     

Ensemble representations: Effects of set size and item heterogeneity on average size perception

Observers can accurately perceive and evaluate the statistical properties of a set of objects, forming what is now known as an ensemble representation. The accuracy and speed with which people can judge the mean size of a set of objects have led to the proposal that ensemble representations of average size can be computed in parallel when attention is distributed across the display. Consistent with this idea, judgments of mean size show little or no decrement in accuracy when the number of objects in the set increases. However, the lack of a set size effect might result from the regularity of the item sizes used in previous studies. Here, we replicate these previous findings, but show that judgments of mean set size become less accurate when set size increases and the heterogeneity of the item sizes increases. This pattern can be explained by assuming that average size judgments are computed using a limited capacity sampling strategy, and it does not necessitate an ensemble representation computed in parallel across all items in a display.     

Objects on a collision path with the observer demand attention

ABSTRACT— How observers distribute limited processing resources to regions of a scene is based on a dynamic balance between current goals and reflexive tendencies. Past research showed that these reflexive tendencies include orienting toward objects that expand as if they were looming toward the observer, presumably because this signal indicates an impending collision. Here we report that during visual search, items that loom abruptly capture attention more strongly when they approach from the periphery rather than from near the center of gaze (Experiment 1), and target objects are more likely to be attended when they are on a collision path with the observer rather than on a near-miss path (Experiment 2). Both effects are exaggerated when search is performed in a large projection dome (Experiment 3). These findings suggest that the human visual system prioritizes events that are likely to require a behaviorally urgent response.     

Why don't we see changes?: The role of attentional bottlenecks and limited visual memory

Seven experiments explore the role of bottlenecks in selective attention and access to visual short-term memory in the failure of observers to identify clearly visible changes in otherwise stable visual displays. Experiment One shows that observers fail to register a color change in an object even if they are cued to the location of the object by a transient at that location as the change is occurring. Experiment Two shows the same for orientation change. In Experiments Three and Four, attention is directed to specific objects prior to making changes in those objects. Observers have only a very limited memory for the status of recently attended items. Experiment Five reveals that observers have no ability to detect changes that happen after attention has been directed to an object and before attention returns to that object. In Experiment Six, attention is cued at rates that more closely resemble natural rates and Experiment Seven uses natural images. Memory capacity remains very small (<4 items).     

奖赏预期调节局部注意干扰效应

局部注意干扰效应(Localized attentional interference, LAI)是指在视觉搜索任务中同时搜索在空间上分离的两个目标时, 或者目标周围存在一个无关奇异项时, 这两个关键刺激相距较近时产生的干扰现象。本实验采用视觉搜索范式, 探讨了奖赏预期对局部注意干扰效应的影响。实验1采用双目标搜索任务, 要求被试判断搜索画面中两个奇异项刺激的形状是否相同。结果显示奖赏条件和无奖赏条件都表现出显著的距离主效应, 随着目标间距离增大, 被试的正确率提高, 反应时下降。同时, 在远距离时, 奖赏条件下的正确率高于无奖赏条件, 而在近距离时, 奖赏条件下的正确率反而低于无奖赏条件, 即, 奖赏条件下局部注意干扰效应反而增大。高动机状态增强了两个目标的表征, 反而不利于解决局部注意干扰效应。实验2采用单目标搜索任务, 要求被试判断特定目标的方位并忽略另一无关奇异项刺激。与实验1不同的是, 实验2仅在无奖赏条件下观察到显著的距离主效应, 即被试行为表现在远距离时更好; 而在奖赏条件下, 被试在近距离和远距离的表现一样好, 即没有局部注意干扰效应。这说明在奖赏预期条件, 个体可以有效抑制分心物刺激的干扰, 将注意集中于目标刺激的加工, 从而对局部注意干扰效应产生调节。整个研究表明, 奖赏整体上能够提高对任务相关刺激的表征, 并抑制任务无关刺激, 虽然这并不总是能够提高任务表现。     

How do we track invisible objects?

We previously demonstrated that observers in multiple object tracking experiments can successfully track targets when all the objects simultaneously vanish for periods lasting several hundred milliseconds (Alvarez, Horowitz, Arsenio, Dimase, &; Wolfe, 2005). How do observers do this? Since observers can track objects that move behind occluders (e.g., Scholl &; Pylyshyn, 1999), they may treat a temporal gap as a case of complete occlusion. If so, performance should improve if occlusion cues (deletion and accretion) are provided and items disappear and reappear one by one (asynchronously), rather than simultaneously. However, we found better performance with simultaneous than with asynchronous disappearance (Experiment 1), whereas occlusion cues were detrimental (Experiment 2). We propose that observers tolerate a gap in tracking by storing the current task state when objects vanish and resuming tracking on the basis of that memory when the objects reappear (a task-switching account).     

The role of similarity in inattentional blindness: Selective enhancement,selective suppression,or both?†

When people selectively pay attention to one set of objects and ignore another, unexpected stimuli often go unnoticed. Noticing rates are higher when the unexpected object matches the features of the attended items and lower when it matches those of the ignored items. No prior studies have fully disentangled these aspects of similarity; in previous work, the unexpected object fell on a continuum between the attended and ignored objects, so increasing the similarity to one set of objects necessarily decreased it to the other. We designed an inattentional blindness task in which we held similarity to the attended set constant and varied it with respect only to the ignored set, or vice versa. In Experiment 1, objects that were more similar to the attended set were noticed more often and objects that were more similar to the ignored set were noticed less often. Experiment 2, using a different set of unexpected objects, replicated the result that similarity to the ignored set affected noticing, but showed no effect of similarity to the attended set. In both experiments, the pattern of noticing rates differed for similarity with respect to the attended and ignored items, suggesting that suppression of ignored objects functions independently of enhancement of attended objects.     

Why are there eccentricity effects in visual search? Visual and attentional hypotheses

In standard visual search experiments, observers search for a target item among distracting items. The locations of target items are generally random within the display and ignored as a factor in data analysis. Previous work has shown that targets presented near fixation are, in fact, found more efficiently than are targets presented at more peripheral locations. This paper proposes that the primary cause of this “eccentricity effect” (Carrasco, Evert, Chang, & Katz, 1995) is an attentional bias that allocates attention preferentially to central items. The first four experiments dealt with the possibility that visual, and not attentional, factors underlie the eccentricity effect. They showed that the eccentricity effect cannot be accounted for by the peripheral reduction in visual sensitivity, peripheral crowding, or cortical magnification. Experiment 5 tested the attention allocation model and also showed that RT X set size effects can be independent of eccentricity effects. Experiment 6 showed that the effective set size in a search task depends, in part, on the eccentricity of the target because observers search from fixation outward.     

Perceived size of familiar objects and the theory of off-sized perceptions

In three experiments, I examined the claim (Gogel, 1969; Gogel & Newton 1969)that familiar objects viewed under reduced stimulus conditions frequently appear to be off-sized (i.e., larger or smaller.than normal). In Experiments 1 and 2, I presented images ofdifferent familiar objects, one at a time, at distances of .1. and 2 m from the observers. The images were normal-, large-, or small-sized versions of familiar objects, and the observers judged the perceived size of each object rela.tive to its familiar normal size. In Experiment 3, I presented normal-, large-, and small-sized versions of thesame familiar object at physical distances of 1 and 2 m. The pattern of size results was similar across the three experiments. In general, normal-sized objects appeared normal to small-sized at the 1-mdistance and small-sized at the 2-mdistance; small-sized objects appeared small-sized at the 1-m distance and even smaller at the 2-m distance; and large-sized objects appeared normal- to large-sized at the 1-m distance and normal- to small-sized at the 2-m distance. The distance results of Experiment 3 indicated that familiar size was an effective determinant of reported distance. These results are consistent with Gogel’s theory of off-sized perceptions and, more generally, with the claim that familiar size is not an important determinant of perceived size.     

The horizon-ratio relation as information for relative size in pictures

The horizon-ratio relation was found to be an effective source of information for relative size in pictures under some conditions: when the difference in image size of depicted “same real size” objects was not too great (Experiment 1), and when the horizon line was not too high or too low in the picture (Experiment 2). The latter finding seemed to be linked to the observers’ identification of the horizontal line as the horizon (and not as the edge of a finite surface). In addition, individual patterns of response were remarkably systematic even in the absence of a horizon, or any other pictorial information, (Experiment 3). It is suggested that in this case observers imposed a horizon on the picture on which to base their relative size judgments, possibly based on the observer’s own eye level or on the content of the picture. It is concluded that although the horizon-ratio relation provides the same kind of information as that available in the optic arrays from real scenes, pictorial information requires the satisfaction of additional constraints in order to be fully effective.     

Judgments of relative time-to-contact of more than two approaching objects: Toward a method

Observers reported which of as many as eight computer-generated approaching objects would “hit” them first. Accuracy was above chance probability except when two-object displays contained pictorial relative size information that contradicted relative time-to-contact (TTC) information. Mean d’ and response time was greater, but mean efficiency (Barlow, 1978) was smaller with eight objects than with two. Performance was less effective when global expansion contradicted TTC information than when local expansion contradicted TTC. Results suggest that observers can judge relative TTC with as many as eight objects when certain sources of information are consistent with TTC and that observers rely on information other than, or in conjunction with, optical TTC. Also, the sources of visual information that affect performance may vary with set size, and identification (but not detection) judgments may be constrained by limited-capacity processing.     

Aging and the perceptual organization of 3-D scenes

In the current study, the authors investigated whether the ground dominance effect (the use of ground surface information for the perceptual organization of scenes) varied with age. In Experiment 1, a scene containing a ground, a ceiling, and 2 vertical posts was presented. The scene was either in its normal orientation or rotated to the side. In Experiment 2, a blue dot was attached to each post, with location varied from bottom to top of the posts. In Experiment 3, a scene similar to that in Experiment 1 was presented in different locations in visual field. Observers judged which of the 2 objects (posts in Experiments 1 and 3, blue dots in Experiment 2) appeared to be closer. The results indicated that both younger (mean age = 22 years) and older observers (mean age = 73 years) responded consistently with the ground dominance effect. However, the magnitude of the effect decreased for older observers. These results suggest a decreased use of ground surface information by older observers for the perceptual organization of scene layout.     

The effect of feedback training on distance estimation in virtual environments

It is often noted that distances are significantly underestimated in computer‐simulated (virtual) environments. Two experiments examine observers' ability to use error corrective feedback to improve the accuracy of judgments of egocentric and exocentric distances. In Experiment 1, observers viewed objects in an immersive virtual environment and estimated their distance through a blindfolded walking task. Different groups received feedback on either egocentric, exocentric or none of these judgments. Receiving feedback improved observers' ability to estimate only those distances for which feedback was provided. These effects persisted for at least 1 week. In Experiment 2, observers estimated egocentric distance by means of both a direct and indirect walking task. Receiving feedback on the direct walking task predominantly improved direct estimates and not indirect estimates. These findings suggest that although feedback training offers a relatively straightforward and immediate way of overcoming problems of distance estimation, its effects are specific to both the type of judgment and the type of response. Copyright © 2005 John Wiley & Sons, Ltd.     

Reversed better-than-average effect in direct comparisons of nonsocial stimuli depends on the set size

Studies on direct comparative judgments typically show that, for items that are positively evaluated, a single item randomly drawn from a larger set of similar items tends to be judged as better than average (the BTA effect). However, Windschitl, Conybeare, and Krizan (2008) demonstrated that, under timing conditions that do not favor focusing attention on the single item, the reversal of the BTA effect occurs. We report two experiments showing that the magnitude of the reversed BTA effect increases as a function of the size of a multiitem referent with which a single item target is compared. Specifically, in direct comparative judgments of the attractiveness of positively evaluated objects (nice-looking cloth buttons, attractive buildings, or cupcakes), underestimation of the attractiveness of singletons, as compared with a multiitem set (reversed BTA effect), increased with the increased set size. Analysis of absolute judgments obtained for singletons and for small and large multiitem sets suggests that, for attractive stimuli, both the reversed BTA effect in comparative judgments and its sensitivity to set size occur as a result of a positive relationship between set size and perceived attractiveness in absolute judgments.     

The role of context on boundary extension

Boundary extension (BE) is a memory error in which observers remember more of a scene than they actually viewed. This error reflects one’s prediction that a scene naturally continues and is driven by scene schema and contextual knowledge. In two separate experiments we investigated the necessity of context and scene schema in BE. In Experiment 1, observers viewed scenes that either contained semantically consistent or inconsistent objects as well as objects on white backgrounds. In both types of scenes and in the no-background condition there was a BE effect; critically, semantic inconsistency in scenes reduced the magnitude of BE. In Experiment 2 when we used abstract shapes instead of meaningful objects, there was no BE effect. We suggest that although scene schema is necessary to elicit BE, contextual consistency is not required.     

Capacity limit of ensemble perception of multiple spatially intermixed sets

The visual system is remarkably efficient at extracting summary statistics from the environment. Yet at any given time, the environment consists of many groups of objects distributed over space. Thus, the challenge for the visual system is to summarize over multiple groups. The current study investigates the capacity and computational efficiency of ensemble perception, in the context of perceiving mean sizes of multiple spatially intermixed groups of circles. In a series of experiments, participants viewed an array of one to eight sets of circles. Each set contained four circles in the same colors, but with different sizes. Participants estimated the mean size of a probed set. The set that would be probed was either known before onset of the array (pre-cue condition) or afterwards (post-cue condition). By comparing estimation error in the pre-cue and post-cue conditions, we found that participants could reliably estimate mean sizes for approximately two sets (Experiment 1). Importantly, this capacity was robust against attention bias toward individual objects in the sets (Experiment 2). Varying the exposure time to stimulus arrays did not increase the capacity limit, suggesting that ensemble perception could be limited by an internal resource constraint, rather than the speed of information encoding (Experiment 3). Moreover, we found that the visual system could not encode and hold more individual items than ensemble representations (Experiment 4). Taken together, these results suggest that ensemble perception provides an efficient way of information processing but with constraints.