Rate of information processing in visual perception: some results and methodological considerations

Experimental Studies of the Attention Processing Model in Multiple Object Tracking Task

(1) Background: Attention is an important cognitive process in daily life. However, limited cognitive resources have been allocated to attention, especially for multiple objects and its mechanism is still unclear. Most of the previous studies have been based on the static attention paradigms with relatively lower ecological validity. Thus, we aimed to explore the attention processing mechanism in a multiple object tracking (MOT) task by using a dynamic attention paradigm. Two experiments were conducted to assess whether there was a multi-focus attention processing model, and whether the processing model changes with the number of target balls. (2) Methods: During the experiments, 33 university students completed MOT combined with the simultaneous-sequential paradigm, with tracking accuracy and reaction time of correct reaction as indicators. (3) Results: (i) When there were two target balls, an obvious bilateral field advantage was apparent. (ii) When there were four target balls, participants' performance was significantly better when stimuli were presented simultaneously than when they were presented sequentially, showing a multi-focus attention processing model. (4) Conclusion: Attention processing is characterized by flexibility, providing strong evidence to support the multi-focus theory.

Hypothesis testing, attention, and 'Same'-'Different' judgments

Logic and common sense say that judging two stimuli as "same" is the converse of judging them as "different". Empirically, however, 'Same'-'Different' judgment data are anomalous in two major ways. The fast-'Same' effect violates the expectation that 'Same' reaction time (RT) should be predictable by extrapolating from 'Different' RT. The criterion effect violates the expectation that RTs measured when sameness is defined by a conjunction of matching attributes should predict RTs measured when sameness is defined by a disjunction of matching attributes. The two criteria are symmetrical, yet empirically they differ greatly, disjunctive judgments being by far the slower of the two. This study sought the sources of these two effects. With the aid of a cue, a selective-comparison method deconfounded the contributions of stimulus encoding and comparisons to the two effects. The results were paradoxical. Each additional irrelevant (uncued) letter in a random string incremented RT for conjunctive judgments as much as an additional relevant letter did. Yet irrelevant letters were not compared and relevant letters had to be compared. These results appeared again in a second experiment that used words as stimuli. Contrary to intuition, a distinct comparison mechanism-the heart of relative judgment models-is not necessary in judgments of sameness and difference. It is shown here that encoding can carry out the comparison function without the operation of a separate comparison mechanism. Attention mediates the process by selecting from the set of stimulus alternatives, thereby partitioning the set into the 'Same' and 'Different' subsets. The fast-'Same' and criterion effects result from a structural limitation on what attention can select at any one time. With attention mediating the task, 'Same'-'Different' judgments become, in effect, the outcome of a testing of a hypothesis, bridging the distinction between absolute stimulus identification and relative judgments.

Visual Search for Circumscribed Interests in Autism Is Similar to That of Neurotypical Individuals

Intense interests are a core symptom of autism spectrum disorders (ASD) and can be all-encompassing for affected individuals. This observation raises the hypothesis that intense interests in ASD are related to pervasive changes in visual processing for objects within that category, including visual search. We assayed visual processing with two novel tasks, targeting category search and exemplar search. For each task, three kinds of stimuli were used: faces, houses, and images personalized to each participant’s interest. 25 children and adults with ASD were compared to 25 neurotypical (NT) children and adults. We found no differences in either visual search task between ASD and NT controls for interests. Thus, pervasive alterations in perception are not likely to account for ASD behavioral symptoms.

Endogenous cueing effects for detection can be accounted for by a decision model of selective attention

Spatial cues help participants detect a visual target when it appears at the cued location. One hypothesis for this cueing effect, called selective perception, is that cueing a location enhances perceptual encoding at that location. Another hypothesis, called selective decision, is that the cue has no effect on perception, but instead provides prior information that facilitates decision-making. We distinguished these hypotheses by comparing a simultaneous display with two spatial locations to sequential displays with two temporal intervals. The simultaneous condition had a partially valid spatial cue, and the sequential condition had a partially valid temporal cue. Selective perception predicts no cueing effect for sequential displays given there is enough time to switch attention. In contrast, selective decision predicts cueing effects for sequential displays regardless of time. We used endogenous cueing of a detection-like coarse orientation discrimination task with clear displays (no external noise or postmasks). Results showed cueing effects for the sequential condition, supporting a decision account of selective attention for endogenous cueing of detection-like tasks.

Distinguishing between parallel and serial processing in visual attention from neurobiological data

Serial and parallel processing in visual search have been long debated in psychology, but the processing mechanism remains an open issue. Serial processing allows only one object at a time to be processed, whereas parallel processing assumes that various objects are processed simultaneously. Here, we present novel neural models for the two types of processing mechanisms based on analysis of simultaneously recorded spike trains using electrophysiological data from prefrontal cortex of rhesus monkeys while processing task-relevant visual displays. We combine mathematical models describing neuronal attention and point process models for spike trains. The same model can explain both serial and parallel processing by adopting different parameter regimes. We present statistical methods to distinguish between serial and parallel processing based on both maximum likelihood estimates and decoding the momentary focus of attention when two stimuli are presented simultaneously. Results show that both processing mechanisms are in play for the simultaneously recorded neurons, but neurons tend to follow parallel processing in the beginning after the onset of the stimulus pair, whereas they tend to serial processing later on.

Goal-directed unequal attention allocation during multiple object tracking

In standard multiple object tracking (MOT) tasks the relative importance of the targets being tracked is equal. This is atypical of everyday situations in which an individual may need to prioritize one target relative to another and so allocate attention unequally. We report three experiments that examined whether participants could unequally split attention using a modified MOT task in which target priority was manipulated. Specifically, we examined the effect of priority on participants' magnitude of error and used a distribution mixture analysis to investigate how priority affected both participants' probability of losing an item and tracking precision. Experiment 1 (trajectory tracking) revealed a higher magnitude of error and higher proportion of guessing for low- compared with high-priority targets. Experiments 2 (trajectory tracking) and 3 (position tracking) examined how fine-grained this ability is by manipulating target priority at finer increments. In line with Experiment 1, results from both these experiments indicated that participants could split attention unequally. There was some evidence that participants could allocate attention unequally at fine increments, but this was less conclusive. Taken together, these experiments demonstrate participants' ability to distribute attention unequally across multiple moving objects but suggest some limitation with the flexibility of attention allocation.

Charles "Erik" Eriksen (1923-2018)

A towering figure in experimental psychology, Charles W. Eriksen, passed away in February this year. "Erik" made extensive original and lasting contributions to both research methods and theories in several areas of psychology, especially involving visual information processing. His research exhibited consistent concerns with experimental methods for distinguishing among alternative explanations and distinguishing perception from behavior. Erik pioneered many research methods now in common use-including converging operations, visual search, rapid serial presentations, the stop-signal paradigm, temporal integration in form perception, spatial cues for guiding selective attention, and the flankers task. He also introduced and tested many theories of selective attention. Erik was the founding editor of Perception & Psychophysics, and served for 23 years as its principal editor. An impressive and unforgettable person, Erik was a compelling personification of "the greatest generation."

Division of Attention between Non-Verbal Signals: All-Or-None or Shared Processing?

Criteria proposed by Broadbent (1971) for deciding between “all-or-none” and “shared” models of divided attention are reviewed and modified. Of the modified criteria, the amount of the reduction in efficiency and the shape of the normalized ROC curve are identified as being diagnostic for deciding between the two models. When applied to the results of two experiments involving the recognition of difficult nonverbal signals, which showed performance on the signals presented together to be significantly worse than performance on the signals when presented alone, both the criteria favour the shared model rather than the all-or-none model. The results of a third experiment with easy signals indicated that artefacts unrelated to attention were unable to account for the reduction in efficiency with difficult signals. It is concluded that the shared model, which assumes some limit to processing capacity, has the widest application to these results and similar findings reported elsewhere. The role of the all-or-none model would appear to be restricted to conditions of extreme informational overload. Further, the movement of noncentral decision criteria and the unequal contribution of the individual dimensions to overall multi-dimension performance demonstrated in the first two experiments, suggest that a reduction in the quality of decision-making may accompany the division of attention, as well as the reduction of information derived from the signals.

Effect of emotions on temporal attention

Emotions play a significant role in guiding everyday actions and strongly interact with attention. The processing of emotional information over time and the influence of attention on such processing has been studied through the phenomenon of attentional blink using rapid serial visual presentations (RSVP) tasks. This chapter discusses the interaction between temporal attention and the type of emotional information (words, scenes, and facial expressions) presented during or before the RSVP stream. The findings show that the affective content and the arousal value of the emotional stimuli presented as first target, second target, or both affects the magnitude and the duration of the blink window. In addition, modulation of emotional context or presentation of emotions in the RSVP stream as task irrelevant distractors also influenced attentional blink. Further, this chapter discusses different models and theories of attentional blink and attempts to explain the emotional effects. The chapter concludes with possible scope for future studies.

Evidence for unlimited capacity processing of simple features in visual cortex

Performance in many visual tasks is impaired when observers attempt to divide spatial attention across multiple visual field locations. Correspondingly, neuronal response magnitudes in visual cortex are often reduced during divided compared with focused spatial attention. This suggests that early visual cortex is the site of capacity limits, where finite processing resources must be divided among attended stimuli. However, behavioral research demonstrates that not all visual tasks suffer such capacity limits: The costs of divided attention are minimal when the task and stimulus are simple, such as when searching for a target defined by orientation or contrast. To date, however, every neuroimaging study of divided attention has used more complex tasks and found large reductions in response magnitude. We bridged that gap by using functional magnetic resonance imaging to measure responses in the human visual cortex during simple feature detection. The first experiment used a visual search task: Observers detected a low-contrast Gabor patch within one or four potentially relevant locations. The second experiment used a dual-task design, in which observers made independent judgments of Gabor presence in patches of dynamic noise at two locations. In both experiments, blood-oxygen level-dependent (BOLD) signals in the retinotopic cortex were significantly lower for ignored than attended stimuli. However, when observers divided attention between multiple stimuli, BOLD signals were not reliably reduced and behavioral performance was unimpaired. These results suggest that processing of simple features in early visual cortex has unlimited capacity.

Serial vs. Parallel Processing: Sometimes They Look like Tweedledum and Tweedledee but they can (and Should) be Distinguished

A number of important models of information processing depend on whether processing is serial or parallel. However, many of the studies purporting to settle the case use weak experimental paradigms or results to draw conclusions. A brief history of the issue is given along with examples from the literature. Then a number of promising methods are presented from a variety of sources with some discussion of their potential. A brief discussion of the topic with regard to overall issues of model testing and applications concludes the paper.

Information processing capacity in psychopathy: Effects of anomalous attention

Hamilton and colleagues (2015) recently proposed that an integrative deficit in psychopathy restricts simultaneous processing, thereby leaving fewer resources available for information encoding, narrowing the scope of attention, and undermining associative processing. The current study evaluated this parallel processing deficit proposal using the Simultaneous-Sequential paradigm. This investigation marks the first a priori test of the Hamilton et al.'s theoretical framework. We predicted that psychopathy would be associated with inferior performance (as indexed by lower accuracy and longer response time) on trials requiring simultaneous processing of visual information relative to trials necessitating sequential processing. Results were consistent with these predictions, supporting the proposal that psychopathy is characterized by a reduced capacity to process multicomponent perceptual information concurrently. We discuss the potential implications of impaired simultaneous processing for the conceptualization of the psychopathic deficit. (PsycINFO Database Record

The attention-weighted sample-size model of visual short-term memory: Attention capture predicts resource allocation and memory load

We investigated the capacity of visual short-term memory (VSTM) in a phase discrimination task that required judgments about the configural relations between pairs of black and white features. Sewell et al. (2014) previously showed that VSTM capacity in an orientation discrimination task was well described by a sample-size model, which views VSTM as a resource comprised of a finite number of noisy stimulus samples. The model predicts the invariance of [Formula: see text] , the sum of squared sensitivities across items, for displays of different sizes. For phase discrimination, the set-size effect significantly exceeded that predicted by the sample-size model for both simultaneously and sequentially presented stimuli. Instead, the set-size effect and the serial position curves with sequential presentation were predicted by an attention-weighted version of the sample-size model, which assumes that one of the items in the display captures attention and receives a disproportionate share of resources. The choice probabilities and response time distributions from the task were well described by a diffusion decision model in which the drift rates embodied the assumptions of the attention-weighted sample-size model.

The effects of direction similarity in visual working memory: Behavioural and event-related potential studies

Object similarity can improve visual working memory (VWM) performance in the change-detection task, but impair the recognition performance when it occurs at retrieval of VWM in the recognition task. The effect of direction similarity is an issue that has not been well resolved. Furthermore, electrophysiological evidence in support of the mechanisms that underlie the effects of similarity is still scarce. In the current study, we conducted three behavioural experiments to examine the effects of direction similarity on memory performance with regard to both the encoding and retrieval phases of VWM and one event-related potential (ERP) experiment to explore the neural signatures of direction similarity in VWM. Our behavioural studies indicated that direction similarity improved performance when it occurred at the encoding phase but impaired performance when it occurred at the retrieval phase. Moreover, the ERP experiment showed that the amplitude of the contralateral delay activity (CDA) increased with the increasing set size for similar but not dissimilar directions. In addition, the CDA amplitude for similar directions was lower than that for dissimilar directions at set size 2. Taken together, these findings suggest that direction similarity at encoding has a positive effect on VWM performance and at retrieval has a negative effect. Given that VWM capacity depends on information load and the number of objects, the positive effect of similarity may be attributed to reduced information load of memory objects.

The capacity limitations of orientation summary statistics

The simultaneous-sequential method was used to test the processing capacity of establishing mean orientation summaries. Four clusters of oriented Gabor patches were presented in the peripheral visual field. One of the clusters had a mean orientation that was tilted either left or right, whereas the mean orientations of the other three clusters were roughly vertical. All four clusters were presented at the same time in the simultaneous condition, whereas the clusters appeared in temporal subsets of two in the sequential condition. Performance was lower when the means of all four clusters had to be processed concurrently than when only two had to be processed in the same amount of time. The advantage for establishing fewer summaries at a given time indicates that the processing of mean orientation engages limited-capacity processes (Exp. 1). This limitation cannot be attributed to crowding, low target-distractor discriminability, or a limited-capacity comparison process (Exps. 2 and 3). In contrast to the limitations of establishing multiple summary representations, establishing a single summary representation unfolds without interference (Exp. 4). When interpreted in the context of recent work on the capacity of summary statistics, these findings encourage a reevaluation of the view that early visual perception consists of creating summary statistic representations that unfold independently across multiple areas of the visual field.

Position tracking and identity tracking are separate systems: Evidence from eye movements

How do we track multiple moving objects in our visual environment? Some investigators argue that tracking is based on a parallel mechanism (e.g., Cavanagh & Alvarez, 2005; Pylyshyn, 1989), others argue that tracking contains a serial component (e.g. Holcombe & Chen, 2013; Oksama & Hyönä, 2008). In the present study, we put previous theories into a direct test by registering observers' eye movements when they tracked identical moving targets (the MOT task) or when they tracked distinct object identities (the MIT task). The eye movement technique is a useful tool to study whether overt focal attention is exploited during tracking. We found a qualitative difference between these tasks in terms of eye movements. When the participants tracked only position information (MOT), the observers had a clear preference for keeping their eyes fixed for a rather long time on the same screen position. In contrast, active eye behavior was observed when the observers tracked the identities of moving objects (MIT). The participants updated over four target identities with overt attention shifts. These data suggest that there are two separate systems involved in multiple object tracking. The position tracking system keeps track of the positions of the moving targets in parallel without the need of overt attention shifts in the form of eye movements. On the other hand, the identity tracking system maintains identity-location bindings in a serial fashion by utilizing overt attention shifts.

The perceptual processing capacity of summary statistics between and within feature dimensions

The simultaneous-sequential method was used to test the processing capacity of statistical summary representations both within and between feature dimensions. Sixteen gratings varied with respect to their size and orientation. In Experiment 1, the gratings were equally divided into four separate smaller sets, one of which with a mean size that was larger or smaller than the other three sets, and one of which with a mean orientation that was tilted more leftward or rightward. The task was to report the mean size and orientation of the oddball sets. This therefore required four summary representations for size and another four for orientation. The sets were presented at the same time in the simultaneous condition or across two temporal frames in the sequential condition. Experiment 1 showed evidence of a sequential advantage, suggesting that the system may be limited with respect to establishing multiple within-feature summaries. Experiment 2 eliminates the possibility that some aspect of the task, other than averaging, was contributing to this observed limitation. In Experiment 3, the same 16 gratings appeared as one large superset, and therefore the task only required one summary representation for size and another one for orientation. Equal simultaneous-sequential performance indicated that between-feature summaries are capacity free. These findings challenge the view that within-feature summaries drive a global sense of visual continuity across areas of the peripheral visual field, and suggest a shift in focus to seeking an understanding of how between-feature summaries in one area of the environment control behavior.

The Identity-Location Binding Problem

The binding problem is fundamental to visual perception. It is the problem of associating an object's visual properties with itself and not with some other object. The problem is made particular difficult because different properties of an object, such as its color, shape, size, and motion, are often processed independently, sometimes in different cortical areas. The results of these separate analyses have to be combined before the object can be seen as a single coherent entity as opposed to a collection of unconnected features. Visual bindings are typically initiated and updated in a serial fashion, one object at a time. Here, we show that one type of binding, location-identity bindings, can be updated in parallel. We do this by using two complementary techniques, the simultaneous-sequential paradigm and systems factorial technology. These techniques make different assumptions and rely on different behavioral measures, yet both came to the same conclusion.

On the cyclic nature of perception in vision versus audition

Does our perceptual awareness consist of a continuous stream, or a discrete sequence of perceptual cycles, possibly associated with the rhythmic structure of brain activity? This has been a long-standing question in neuroscience. We review recent psychophysical and electrophysiological studies indicating that part of our visual awareness proceeds in approximately 7-13 Hz cycles rather than continuously. On the other hand, experimental attempts at applying similar tools to demonstrate the discreteness of auditory awareness have been largely unsuccessful. We argue and demonstrate experimentally that visual and auditory perception are not equally affected by temporal subsampling of their respective input streams: video sequences remain intelligible at sampling rates of two to three frames per second, whereas audio inputs lose their fine temporal structure, and thus all significance, below 20-30 samples per second. This does not mean, however, that our auditory perception must proceed continuously. Instead, we propose that audition could still involve perceptual cycles, but the periodic sampling should happen only after the stage of auditory feature extraction. In addition, although visual perceptual cycles can follow one another at a spontaneous pace largely independent of the visual input, auditory cycles may need to sample the input stream more flexibly, by adapting to the temporal structure of the auditory inputs.

Summary statistics of size: fixed processing capacity for multiple ensembles but unlimited processing capacity for single ensembles

We assessed the processing capacity of establishing statistical summary representations (SSRs) of mean size in visual displays using the simultaneous-sequential method. Four clusters of stimuli, each composed of several circles with various diameters, were presented around fixation. Observers searched for the cluster with the largest or smallest mean size. In the simultaneous condition, all four clusters were presented concurrently; in the sequential condition, the clusters appeared two at a time. We found that the processing capacity of SSRs for multiple ensembles was as extreme as a fixed-rate bottleneck process (Experiment 1). A control experiment confirmed that this was not caused by having to compare the results of multiple averaging processes (Experiment 2). In contrast to computing SSRs across ensembles, computing SSRs for a single ensemble using the same stimuli was consistent with unlimited-capacity processing (Experiment 3). Contrary to existing claims, summary representations appear to be extracted independently for items within single ensembles but not multiple ensembles. A developing understanding of capacity limitations in perceptual processing is discussed.

Evidence of unlimited-capacity surface completion

Capacity limitations of perceptual surface completion were assessed using a simultaneous-sequential method. Observers searched among multiple surfaces requiring perceptual completion in front of other objects (modal completion) or behind other objects (amodal completion). In the simultaneous condition, all surfaces were presented at once, whereas in the sequential condition, they appeared in subsets of 2 at a time. For both modal and amodal surface completion, performance was as good in the simultaneous condition as in the sequential condition, indicating that surface completion unfolds independently for multiple surfaces across the visual field (i.e., has unlimited capacity). We confirmed this was due to the formation of surfaces defined by the pacmen inducers, and not simply to the detection of individual features of the pacmen inducers. These results provide evidence that surface-completion processes can be engaged and unfold independently for multiple surfaces across the visual field. In other words, surface completion can occur through unlimited-capacity processes. These results contribute to a developing understanding of capacity limitations in perceptual processing more generally.

Classification of imperfectly time-locked image RSVP events with EEG device

Classification based on EEG data in an RSVP experiment is considered. Although the latency in neural response relative to the stimulus onset time may be more realistically considered to vary across trials due to factors such as subject fatigue and environmental distractions, it is nevertheless assumed to be time-locked to the stimulus in most of the existing work as a means to alleviate the computational complexity. We consider here a more practical scenario that allows variation in response latency and develop a rigorous statistical formulation for modeling the uncertainty within the varying latency coupled with a likelihood ratio test (LRT) for classification. The new model not only improves the EEG classification performance, but also may predict the true stimulus onset time when this information is not precisely available. We test the proposed LRT algorithm on an EEG data set from an image RSVP experiment and show that, by admitting the latency variation, the proposed approach consistently outperforms a method that relies on perfect time-locking (AUC: 0.88 vs 0.86), especially when the stimulus onset time is not precisely available (AUC: 0.84 vs 0.71). Furthermore, the predicted stimulus onset times are highly enriched around the true onset time with p-value = 5.2 x 10(44).

Divided attention limits perception of 3-D object shapes

Can one perceive multiple object shapes at once? We tested two benchmark models of object shape perception under divided attention: an unlimited-capacity and a fixed-capacity model. Under unlimited-capacity models, shapes are analyzed independently and in parallel. Under fixed-capacity models, shapes are processed at a fixed rate (as in a serial model). To distinguish these models, we compared conditions in which observers were presented with simultaneous or sequential presentations of a fixed number of objects (The extended simultaneous-sequential method: Scharff, Palmer, & Moore, 2011a, 2011b). We used novel physical objects as stimuli, minimizing the role of semantic categorization in the task. Observers searched for a specific object among similar objects. We ensured that non-shape stimulus properties such as color and texture could not be used to complete the task. Unpredictable viewing angles were used to preclude image-matching strategies. The results rejected unlimited-capacity models for object shape perception and were consistent with the predictions of a fixed-capacity model. In contrast, a task that required observers to recognize 2-D shapes with predictable viewing angles yielded an unlimited capacity result. Further experiments ruled out alternative explanations for the capacity limit, leading us to conclude that there is a fixed-capacity limit on the ability to perceive 3-D object shapes.

The contents of visual working memory reduce uncertainty during visual search

Information held in visual working memory (VWM) influences the allocation of attention during visual search, with targets matching the contents of VWM receiving processing benefits over those that do not. Such an effect could arise from multiple mechanisms: First, it is possible that the contents of working memory enhance the perceptual representation of the target. Alternatively, it is possible that when a target is presented among distractor items, the contents of working memory operate postperceptually to reduce uncertainty about the location of the target. In both cases, a match between the contents of VWM and the target should lead to facilitated processing. However, each effect makes distinct predictions regarding set-size manipulations; whereas perceptual enhancement accounts predict processing benefits regardless of set size, uncertainty reduction accounts predict benefits only with set sizes larger than 1, when there is uncertainty regarding the target location. In the present study, in which briefly presented, masked targets were presented in isolation, there was a negligible effect of the information held in VWM on target discrimination. However, in displays containing multiple masked items, information held in VWM strongly affected target discrimination. These results argue that working memory representations act at a postperceptual level to reduce uncertainty during visual search.

The psychophysics of brain rhythms

It is becoming increasingly apparent that brain oscillations in various frequency bands play important roles in perceptual and attentional processes. Understandably, most of the associated experimental evidence comes from human or animal electrophysiological studies, allowing direct access to the oscillatory activities. However, such periodicities in perception and attention should, in theory, also be observable using the proper psychophysical tools. Here, we review a number of psychophysical techniques that have been used by us and other authors, in successful and sometimes unsuccessful attempts, to reveal the rhythmic nature of perceptual and attentional processes. We argue that the two existing and largely distinct debates about discrete vs. continuous perception and parallel vs. sequential attention should in fact be regarded as two facets of the same question: how do brain rhythms shape the psychological operations of perception and attention?

Extending the simultaneous-sequential paradigm to measure perceptual capacity for features and words

In perception, divided attention refers to conditions in which multiple stimuli are relevant to an observer. To measure the effect of divided attention in terms of perceptual capacity, we introduce an extension of the simultaneous-sequential paradigm. The extension makes predictions for fixed-capacity models as well as for unlimited-capacity models. We apply this paradigm to two example tasks, contrast discrimination and word categorization, and find dramatically different effects of divided attention. Contrast discrimination has unlimited capacity, consistent with independent, parallel processing. Word categorization has a nearly fixed capacity, consistent with either serial processing or fixed-capacity, parallel processing. We argue that these measures of perceptual capacity rely on relatively few assumptions compared to most alternative measures.

Direction information in multiple object tracking is limited by a graded resource

Is multiple object tracking (MOT) limited by a fixed set of structures (slots), a limited but divisible resource, or both? Here, we answer this question by measuring the precision of the direction representation for tracked targets. The signature of a limited resource is a decrease in precision as the square root of the tracking load. The signature of fixed slots is a fixed precision. Hybrid models predict a rapid decrease to asymptotic precision. In two experiments, observers tracked moving disks and reported target motion direction by adjusting a probe arrow. We derived the precision of representation of correctly tracked targets using a mixture distribution analysis. Precision declined with target load according to the square-root law up to six targets. This finding is inconsistent with both pure and hybrid slot models. Instead, directional information in MOT appears to be limited by a continuously divisible resource.

Distinguishing between parallel and serial accounts of multiple object tracking

Humans can track multiple moving objects. Is this accomplished by attending to all the objects at the same time or do we attend to each object in turn? We addressed this question using a novel application of the classic simultaneous-sequential paradigm. We considered a display in which objects moved for only part of the time. In one condition, the objects moved sequentially, whereas in the other condition they all moved and paused simultaneously. A parallel model would predict that the targets are tracked independently, so the tracking of one target should not be influenced by the movement of another target. Thus, one would expect equal performance in the two conditions. Conversely, a simple serial account of object tracking would predict that an observer's accuracy should be greater in the sequential condition because in that condition, at any one time, fewer targets are moving and thus need to be attended. In fact, in our experiments we observed performance in the simultaneous condition to be equal to or greater than the performance in the sequential condition. This occurred regardless of the number of targets or how the targets were positioned in the visual field. These results are more directly in line with a parallel account of multiple object tracking.

Selective visual attention ensures constancy of sensory representations: Testing the influence of perceptual load and spatial competition

We report findings from several variants of a psychophysical experiment using an acceleration detection task in which we tested predictions derived from recent neurophysiological data obtained from monkey area MT. The task was designed as a Posner paradigm and required subjects to detect the speed-up of a moving bar, cued with 75% validity. Displays varied according to number of simultaneously presented objects, spatial distance, and difficulty of the task. All data obtained under different levels of competition with multiple objects were compared to a corresponding condition, in which only a single moving bar was presented in the absence of any interfering distracter object. For attended objects, subjects did not show any difference in their ability to detect accelerations, regardless of the strength of inter-object competition or spatial distance. This finding was consistent in all of the experiments, and was even obtained when the acceleration was made hardly detectable. In contrast, increasing competitive interactions either by enhancing number of objects or spatial proximity resulted in strong reduction of performance for non-attended objects. The findings support current noise reduction models and suggest that attention adjusts neuronal processing to ensure a constant sensory representation of the attended object as if this object was the only one in the scene.

Mirror-image symmetry and search asymmetry: a comparison of their effects on visual search and a possible unifying explanation

Visual search may be affected by mirror-image symmetry between target and non-targets and also by switching the roles of target and non-target. Do different attention mechanisms underlie these two phenomena? Can a unifying explanation account for both? We conducted two experiments to decompose processing into component parts, and compared results to competing models' predictions. Mirror-image search was unimpaired after target discrimination had been balanced across search conditions-results were consistent with an unlimited-capacity, decision noise model. Search asymmetry affected higher-level processing, however, resulting in capacity limitations that necessitated serial processing. A unifying explanation can account for these two seemingly unrelated phenomena.

Attention capacity and task difficulty in visual search

When a visual search task is very difficult (as when a small feature difference defines the target), even detection of a unique element may be substantially slowed by increases in display set size. This has been attributed to the influence of attentional capacity limits. We examined the influence of attentional capacity limits on three kinds of search task: difficult feature search (with a subtle featural difference), difficult conjunction search, and spatial-configuration search. In all 3 tasks, each trial contained sixteen items, divided into two eight-item sets. The two sets were presented either successively or simultaneously. Comparison of accuracy in successive versus simultaneous presentations revealed that attentional capacity limitations are present only in the case of spatial-configuration search. While the other two types of task were inefficient (as reflected in steep search slopes), no capacity limitations were evident. We conclude that the difficulty of a visual search task affects search efficiency but does not necessarily introduce attentional capacity limits.

The serial-parallel dilemma: a case study in a linkage of theory and method

The question as to whether humans perceive, remember, or cognize psychological items simultaneously (i.e., in parallel) or sequentially (i.e., serially) has been of interest to philosophers and psychologists since at least the 19th century. The advent of the information-processing approach to cognition in the 1960s reopened the inquiry, initiating a flood of experiments and models in the literature. Surprisingly for so elemental an issue, persuasive experimental tests have, until recently, proven rather elusive. Several decades of theoretical, methodological, and experimental effort, propelled and shaped by a meta-theoretical perspective, are leading to powerful strategies for assessing this and related cognitive issues. The present article reviews the theoretical and empirical history of these inquiries and details situations in which decisive experimental tests are possible.

Are there capacity limitations in symmetry perception?

Previous researchers have proposed that there are two types of symmetry detection: one based on crude preattentive symmetry judgments and another based on detailed scrutiny of individual parts (Barlow & Reeves, 1979; S. E. Palmer & Hemenway, 1978; Royer, 1981). Four experiments were conducted to examine capacity limits in different symmetry judgments. Observers were required to discriminate between random patterns and approximate symmetry (Experiments 1 and 3) or between perfect and approximate symmetry (Experiments 2 and 4). The patterns were divided into two sets of dots, presented either simultaneously or successively. A comparison of accuracy under these two presentation conditions suggested that symmetry detection involves an analysis that is spatially parallel but coarse, regardless of either task difficulty or task type (detecting symmetry vs. detecting asymmetry).

A Theory of Interactive Parallel Processing: New Capacity Measures and Predictions for a Response Time Inequality Series

The authors present a theory of stochastic interactive parallel processing with special emphasis on channel interactions and their relation to system capacity. The approach is based both on linear systems theory augmented with stochastic elements and decisional operators and on a metatheory of parallel channels' dependencies that incorporates standard independent and coactive parallel models as special cases. The metatheory is applied to OR and AND experimental paradigms, and the authors establish new theorems relating response time performance in these designs to earlier and novel issues. One notable outcome is the remarkable processing efficiency associated with linear parallel-channel systems that include mutually positive interactions. The results may offer insight into perceptual and cognitive configural-holistic processing systems.

Divided attention and visual search for simple versus complex features

Under what search conditions does attention affect perceptual processes, resulting in capacity limitations, rather than affecting noisy decision-making processes? Does parallel or serial processing cause the capacity limitations? To address these issues, we varied stimulus complexity, set size, and whether distractors were mirror images of the target. Both target detection and localization produced similar patterns of results. Capacity limitations only occurred for complex stimuli used in within-object conjunction searches. Parallel processing, rather than serial processing, probably caused these capacity limitations. Moreover, although mirror-image symmetry adversely affected early visual processing, it did not place additional demands on attention.

Preattentive perception of multiple illusory line-motion: a formal model of parallel independent-detection in visual search

The phenomenon referred to as illusory line-motion (ILM; O. Hikosaka, S. Miyauchi, & S. Shimojo, 1993a) has been described as a measure of the local facilitation of attention gradient. However, J. Kawahara, K. Yokosawa, S. Nishida, and T. Sato (1996) have demonstrated a spatially parallel search for an "odd man out" in the ILM direction. Apart from showing preattentive ILM perception in terms of an analogy between line-motion and apparent motion, the authors examined whether ILM perception is possible without attention from another point of view. Four experiments revealed that the ILM target can be detected in parallel without invoking attentional facilitation and invalidated the possible contribution of attentional set in parallel ILM search. Participants were able to correctly detect the ILM target among multiple nontargets, even when the line orientation was changed from trial to trial. The authors' independent-detection model predicted ILM search performance well on several occasions. These findings strongly support a preattentive and stimulus-driven explanation of ILM perception.

Is evidence for late selection due to automatic or attentional processing of stimulus identities?

This study determined whether evidence for late selection is due to attention processing or to processing by an automatic system that is separate from attention (two systems framework; Eriksen, Webb, & Fournier, 1990). The task was a two-choice discrimination of a target that appeared in one of two sequentially cued locations in an eight-letter visual display. Attention was directed to the first cued location (cue 1), and whether identification processing occurred at a different location before the second cue (cue 2) directed attention there was determined. Cue validity varied across two experiments, and critical trials were those in which the target appeared at cue 2. For these trials, the target was preceded by a letter (either identical, neutral, or incompatible) that changed to the target at various time intervals following cue 2. Automatic identification was assumed if the incompatible letter interfered with response to the target when it appeared only before cue 2 onset and independent of cue validity. The incompatible letter appearing only before cue 2 onset interfered with the target when the target occurred equally often at cue 1 and cue 2, but not when the target occurred at cue 1 70% and at cue 2 30% of the time. This disconfirms the two systems framework and suggests that attention is required for spatial form processing and response competition.

Perceptual awareness and its loss in unilateral neglect and extinction

We review recent evidence from studies of patients with unilateral neglect and/or extinction, who suffer from a loss of awareness for stimuli towards the affected side of space. We contrast their deficit with the effects of damage to primary sensory areas, noting that such areas can remain structurally intact in neglect, with lesions typically centred on the right inferior parietal lobe. In keeping with preservation of initial sensory pathways, many recent studies have shown that considerable residual processing can still take place for neglected or extinguished stimuli, yet without reaching the patient's awareness. This ranges from preserved visual grouping processes through to activation of identity, semantics and emotional significance. Similarly to 'preattentive' processing in normals, such residual processing can modulate what will enter the patient's awareness. Recent studies have used measures such as ERPs and fMRI to determine the neural correlates of conscious versus unconscious perception in the patients, which in turn can be related to the anatomy of their lesions. We relate the patient findings to neurophysiological data from areas in the monkey parietal lobe, which indicate that these serve as cross-modal and sensorimotor interfaces highlighting currently relevant locations as targets for intentional action. We speculate on the special role such brain regions may play in perceptual awareness, seeking to explain how damage to a system which appears primarily to code space could eliminate awareness even for non-spatial stimulus properties at affected locations. This may relate to the extreme nature of 'winner-takes-all' functions within the parietal lobe, and their correspondingly strong influence on other brain areas.

New evidence for the zoom lens model using the RSVP technique

A main prediction from the zoom lens model for visual attention is that performance is an inverse function of the size of the attended area. The "attention shift paradigm" developed by Sperling and Reeves (1980) was adapted here to study predictions from the zoom lens model. In two experiments two lists of items were simultaneously presented using the rapid serial visual presentation technique. Subjects were to report the first item he/she was able to identify in the series that did not include the target (the letter T) after he/she saw the target. In one condition, subjects knew in which list the target would appear, in another condition, they did not have this knowledge, having to attend to both positions in order to detect the target. The zoom lens model predicts an interaction between this variable and the distance separating the two positions where the lists are presented. In both experiments, this interaction was observed. The results are also discussed as a solution to the apparently contradictory results with regard to the analog movement model.

Attentional competition between modalities: extinction between touch and vision after right hemisphere damage

The phenomenon of extinction, which occurs frequently after unilateral brain damage, involves a failure to detect the more contralesional of two simultaneously presented stimuli, but with preserved detection of single ipsilesional or contralesional stimuli. Current accounts suggest that the disorder reflects a bias of selective attention, in which ipsilesional stimuli have a competitive advantage. Extinction may be manifested within any one of the major sensory modalities (vision, audition, touch), or it may occur within several modalities in a given individual. Given recent evidence in normals for attentional links between separate sensory modalities, we examined whether extinction can also occur cross-modally, i.e. for double-simultaneous stimuli in separate sensory modalities. We tested whether an ipsilesional event sufficient to extinguish a contralesional stimulus within the same modality may also extinguish a contralesional stimulus in a different modality. Our three patients had right hemisphere damage, and reliable within-modality extinction for visual and tactile stimuli. They also showed significant cross-modal extinction, such that an ipsilesional tactile (or visual) event extinguished awareness of a simultaneous visual (or tactile) event on the contralesional side. These results, which provide the first quantitative evidence for cross-modal extinction, were replicated in a second experiment in which visual and tactile stimuli in the cross-modal conditions were presented at non-homologous elevations within each hemispace. We conclude that after unilateral damage, ipsilesional stimuli have a competitive advantage over contralesional stimuli, and that this affects competition between stimuli from different modalities as well as stimuli within the same modality. These findings are consistent with recent evidence for competitive interactions between tactile and visual events in the control of spatial attention in normals.

The effect of attention on phenomenal length

The effect of attention of the perceived length of briefly presented peripheral lines was investigated. Attention was manipulated by engaging observers in a second concurrent task (letter identification). Observers used the method of adjustment to indicate the length of the stimulus lines. In two experiments it was found that the primary effect of attention was to reduce the variability of line length adjustments. Previously investigators had reported that attention reduces perceived line length. A third experiment suggested that these previously reported results might have been the outcome of a spatial interaction with the cue used to manipulate attention, but not the result of visual attention.