Central capacity limits in consistent mapping, visual search tasks: four channels or more?

Pupillary dilation elicited by attending to two disks with different luminance

Pupils become smaller when people attend to a bright disk as compared to a dark disk. However, people can divide their attention into several distinct positions, which is referred to as divided attention, and pupillary responses under such conditions have not been investigated. In this study, we examined how pupils would respond when people attended to two disks presented at two distinct positions by conducting three experiments. We found that the pupillary response when attending to two disks with different luminance was larger than when attending to a single brighter disk and was comparable to that when attending to a single darker disk, whereas the pupillary response when attending to two disks with identical luminance was not larger than when attending to a single disk (irrespective of the disk luminance). Furthermore, we found that the magnitude of pupillary dilation was determined by the magnitude of the luminance difference between two disks. These results make a useful contribution to the literature on human pupillary responses.

Science of perception, decision making and fatigue in the maritime industry

In this paper, we investigate how perception, decision making and fatigue affect safety in the maritime industry. With maritime safety incidents often having disastrous effects on the lives of the seafarers, the natural environment, and the industry as a whole, a better understanding of the science behind the human errors is necessary to increase safety at sea. We conducted interviews and real time observations of seafarers, and we combine our findings with a review of relevant academic literature with the aim of using scientific knowledge to address a real-world problem. We conclude that the seafarers consistently work at the limits of their cognitive abilities and that better care for their wellbeing is crucial for safety at sea. It is hoped that this study will be beneficial to the practitioners in the field of the maritime industry as well as psychologists studying the effects of long term, high cognitive loads in the work environment.

Temporal Asynchrony but Not Total Energy Nor Duration Improves the Judgment of Numerosity in Electrotactile Stimulation

Stroke patients suffer from impairments of both motor and somatosensory functions. The functional recovery of upper extremities is one of the primary goals of rehabilitation programs. Additional somatosensory deficits limit sensorimotor function and significantly affect its recovery after the neuromotor injury. Sensory substitution systems, providing tactile feedback, might facilitate manipulation capability, and improve patient's dexterity during grasping movements. As a first step toward this aim, we evaluated the ability of healthy subjects in exploiting electrotactile feedback on the shoulder to determine the number of perceived stimuli in numerosity judgment tasks. During the experiment, we compared four different stimulation patterns (two simultaneous: short and long, intermittent and sequential) differing in total duration, total energy, or temporal synchrony. The experiment confirmed that the subject ability to enumerate electrotactile stimuli decreased with increasing the number of active electrodes. Furthermore, we found that, in electrotactile stimulation, the temporal coding schemes, and not total energy or duration modulated the accuracy in numerosity judgment. More precisely, the sequential condition resulted in significantly better numerosity discrimination than intermittent and simultaneous stimulation. These findings, together with the fact that the shoulder appeared to be a feasible stimulation site to communicate tactile information via electrotactile feedback, can serve as a guide to deliver tactile feedback to proximal areas in stroke survivors who lack sensory integrity in distal areas of their affected arm, but retain motor skills.

The channel capacity of visual awareness divided among multiple moving objects

If attention is distributed among multiple moving objects, how does this divided attention affect the temporal process for detecting a specific target motion? Well-trained observers in three experiments monitored ongoing random motions of multiple objects, trying to rapidly detect non-random target motions. Response time hazard rates revealed a simple lawful structure of the detection processes. Target detection rates (hazard rates, in bits /s) were inversely proportional to the number of observed objects. Detection rates at any response time and in any condition equaled a product of two parallel (functionally independent and concurrent) visual processes: visual awareness and motion integration. The rate of visual awareness was inversely proportional to Set Size (n = 1-12), constant over time, and invariant with integrated motion information. Thus, a single rate parameter, indicating a constant channel capacity of visual awareness, described detection rates over a wide range of conditions and response times. During an initial interval of roughly 0.5 s, detection rates increased proportionally with the duration and length of motion; but after this initial integration, detection rates were constant, independent of the time the target remained undetected. The relationship between the quantity of visual information and detection rates was simpler than anticipated by contemporary theories of attention, perception, and performance.

Contour Integration in Dynamic Scenes: Impaired Detection Performance in Extended Presentations

Since scenes in nature are highly dynamic, perception requires an on-going and robust integration of local information into global representations. In vision, contour integration (CI) is one of these tasks, and it is performed by our brain in a seemingly effortless manner. Following the rule of good continuation, oriented line segments are linked into contour percepts, thus supporting important visual computations such as the detection of object boundaries. This process has been studied almost exclusively using static stimuli, raising the question of whether the observed robustness and "pop-out" quality of CI carries over to dynamic scenes. We investigate contour detection in dynamic stimuli where targets appear at random times by Gabor elements aligning themselves to form contours. In briefly presented displays (230 ms), a situation comparable to classical paradigms in CI, performance is about 87%. Surprisingly, we find that detection performance decreases to 67% in extended presentations (about 1.9-3.8 s) for the same target stimuli. In order to observe the same reduction with briefly presented stimuli, presentation time has to be drastically decreased to intervals as short as 50 ms. Cueing a specific contour position or shape helps in partially compensating this deterioration, and only in extended presentations combining a location and a shape cue was more efficient than providing a single cue. Our findings challenge the notion of CI as a mainly stimulus-driven process leading to pop-out percepts, indicating that top-down processes play a much larger role in supporting fundamental integration processes in dynamic scenes than previously thought.

Optimizing the Set of Highlighted Options on Video Display Terminal Menus

Highlighting is typically used to draw individuals' attention to a particular area or word on a video display terminal menu. In many cases it serves a useful function. For example, highlighting can potentially speed the search for important information. However, highlighting can also slow the search for important information. Clearly, the function of highlighting is subverted when search times are slowed. The objective of the research to be reported is to develop a method for determining the optimal set of highlighted options in alphanumeric menus. This optimal set is defined as that set of options which produces the fastest search times for a given menu. The method can be used for any combination of highlighting attributes (e.g., reverse video, blinking, color).

Attending multiple items decreases the selectivity of population responses in human primary visual cortex

Multiple studies have documented an inverse relationship between the number of to-be-attended or remembered items in a display ("set size") and task performance. The neural source of this decline in cognitive performance is currently under debate. Here, we used a combination of fMRI and a forward encoding model of orientation selectivity to generate population tuning functions for each of two stimuli while human observers attended either one or both items. We observed (1) clear population tuning functions for the attended item(s) that peaked at the stimulus orientation and decreased monotonically as the angular distance from this orientation increased, (2) a set-size-dependent decline in the relative precision of orientation-specific population responses, such that attending two items yielded a decline in selectivity of the population tuning function for each item, and (3) that the magnitude of the loss of precision in population tuning functions predicted individual differences in the behavioral cost of attending an additional item. These findings demonstrate that attending multiple items degrades the precision of perceptual representations for the target items and provides a straightforward account for the associated impairments in visually guided behavior.

Dissociation between process-based and data-based limitations for conscious perception in the human brain

Successful performance of a cognitive task depends upon both the quality of the sensory information and the processing resources available to perform that task. Thus, task performance can either be data-limited or process-limited (D. A. Norman and D. G. Bobrow, 1975). Using fMRI, we show that these conceptual distinctions are neurally dissociable: A parieto-frontal network involved in conscious perception is modulated by target interference manipulations that strain attentional processing, but not by equally difficult manipulations that limit the quality of target information. These results suggest that limitations imposed by processing capacity have distinct neural effects from those arising from the quality of sensory input, and provide empirical support for an influential neurobiological theory of consciousness (S. Dehaene, J.-P. Changeux, L. Naccache, J. Sackur, and C. Sergent, 2006).

Intentional subitizing: exploring the role of automaticity in enumeration

Subitizing is traditionally described as the rapid, preattentive and automatic enumeration of up to four items. Counting, by contrast, describes the enumeration of larger sets of items and requires slower serial shifts of attention. Although recent research has called into question the preattentive nature of subitizing, whether or not numerosities in the subitizing range can be automatically accessed is yet to be empirically tested. In the current study, participants searched for two pre-defined digits in a circular visual-search array. Distractor dots of various set sizes were placed at the centre of the array. Despite the relevance of the distractor numerosities to the target detection task, the distractors did not influence target detection, thereby suggesting that their numerosities were not automatically accessed in Experiment 1. In Experiment 2, participants were explicitly instructed to enumerate the distractor dots. Here, congruent and incongruent distractor numerosities influenced the target detection task, thereby revealing that the distractor dots were capable of generating interference. Experiment 3 ensured that dots were attended by asking participants to detect the luminance of dots. Data confirmed that subitizing was not automatic. The present study also supported the alleged discontinuity between the subitizing and counting ranges because an examination of reaction time gradients in Experiment 2 found the counting gradient to be significantly steeper than the subitizing gradient. In sum, the results suggest that subitizing is a distinct but non-automatic style of enumeration.

Bottom-up guidance to grouped items in conjunction search: evidence for color grouping

Previous studies have demonstrated that observers can search through a subset of items carrying a minority feature to find a conjunction target (Sobel & Cave, 2002). We examined whether subset search takes place when participants have less specific foreknowledge of the target (when the target is one of two possible items), measuring eye movements as well as reaction times. When there were unequal ratios of distractors, fixations were initially directed to the small subset. These initial eye movements were often directed between items with the same feature, suggesting guidance from pooled feature values. There was stronger guidance within color- than orientation-defined groups, although the features were balanced for salience. The results suggest that grouping of items by color operates more globally than grouping in orientation.

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.

Modeling and Estimating Recall Processing Capacity: Sensitivity and Diagnostic Utility in Application to Mild Cognitive Impairment

We investigate the potential for using latency-based measures of retrieval processing capacity to assess changes in perfomance specific to individuals with mild cognitive impairment (MCI), a reliable precursor state to Alzheimer's Disease. Use of these capacity measures is motivated in part by exploration of the effects of atrophy on a computational model of a basic hippocampal circuit. We use this model to suggest that capacity may be a more sensitive indicator of undelying atrophy than speed of processing, and test this hypothesis by adapting a standard behavioral measure of memory (the free and cued selective reminding test, FCSRT) to allow for the collection of cued recall latencies. Participants were drawn from five groups: college-aged, middle-aged, healthy elderly, those with a diagnosis of MCI, and a sample of MCI control participants. The measure of capacity is shown to offer increased classificatory sensitivity relative to the standard behavioral measures, and is also shown to be the behavioral measure that correlated most strongly with hippocampal volume.

Multisensory numerosity judgments for visual and tactile stimuli

To date, numerosity judgments have been studied only under conditions of unimodal stimulus presentation. It is therefore unclear whether the same limitations on correctly reporting the number of unimodal visual or tactile stimuli presented in a display might be expected under conditions in which participants have to count stimuli presented simultaneously in two or more different sensory modalities. In Experiment 1, we investigated numerosity judgments using both unimodal and bimodal displays consisting of one to six vibrotactile stimuli (presented over the body surface) and one to six visual stimuli (seen on the body via mirror reflection). Participants had to count the number of stimuli regardless of their modality of presentation. Bimodal numerosity judgments were significantly less accurate than predicted on the basis of an independent modality-specific resources account, thus showing that numerosity judgments might rely on a unitary amodal system instead. The results of a second experiment demonstrated that divided attention costs could not account for the poor performance in the bimodal conditions of Experiment 1. We discuss these results in relation to current theories of cross-modal integration and to the cognitive resources and/or common higher order spatial representations possibly accessed by both visual and tactile stimuli.

Visual memory capacity in transsaccadic integration

How we perceive the visual world as stable and unified suggests the existence of transsaccadic integration that retains and integrates visual information from one eye fixation to another eye fixation across saccadic eye movements. However, the capacity of transsaccadic integration is still a subject of controversy. We tested our subjects' memory capacity of two basic visual features, i.e. luminance (Experiment 1) and orientation (Experiment 2), both within a single fixation (i.e. visual working memory) and between separate fixations (i.e. transsaccadic memory). Experiment 2 was repeated, but attention allocation was manipulated using attentional cues at either the target or distracter (Experiment 3). Subjects were able to retain 3-4 objects in transsaccadic memory for luminance and orientation; errors generally increased as saccade size increased; and, subjects were more accurate when attention was allocated to the same location as the impending target. These results were modelled by inputting a noisy extra-retinal signal into an eye-centered feature map. Our results suggest that transsaccadic memory has a similar capacity for storing simple visual features as basic visual memory, but this capacity is dependent both on the metrics of the saccade and allocation of attention.

A working memory workout: how to expand the focus of serial attention from one to four items in 10 hours or less

Five individuals participated in an extensive practice study (10 1-hr sessions, 11,000 trials total) on a self-paced identity-judgment (1)n-back task (n ranging from 1 to 5). Within Session 1, response time increased abruptly by about 300 ms in passing from n = 1 to n > 1, suggesting that the focus of attention can accommodate only a single item (H. Caravan, 1998; B. McElree, 2001). Within Session 10, response time was dramatically reduced and increased linearly with n for n < or = 4, with a slope of about 30 ms. The data suggest that working memory consists of a focus of attention governed by a limited-capacity search, expandable through practice, and a content-addressable region outside the focus of attention.

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.

Using hazard functions to assess changes in processing capacity in an attentional cuing paradigm

Processing capacity--defined as the relative ability to perform mental work in a unit of time--is a critical construct in cognitive psychology and is central to theories of visual attention. The unambiguous use of the construct, experimentally and theoretically, has been hindered by both conceptual confusions and the use of measures that are at best only coarsely mapped to the construct. However, more than 25 years ago, J. T. Townsend and F. G. Ashby (1978) suggested that the hazard function on the response time (RT) distribution offered a number of conceptual advantages as a measure of capacity. The present study suggests that a set of statistical techniques, well-known outside the cognitive and perceptual literatures, offers the ability to perform hypothesis tests on RT-distribution hazard functions. These techniques are introduced, and their use is illustrated in application to data from the contingent attentional capture paradigm.

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.

Visual-memory search: an integrative perspective

A large, single-frame, visual-memory search experiment is reported in which memory and display loads of 1, 2, and 4 alphanumeric characters were factorially combined. In addition to the usual Consistent Mapping and Varied Mapping conditions, the experiment also involved a Categorical Varied Mapping condition in which different sets of stimuli switched roles as targets and distractors over trials. The stimuli used in these various mapping conditions were either digits, letters, or digits and letters. Analyses of the response time means obtained early and late in training suggest that the presence of categorical distinctions among the stimuli is the most important determinant of search efficiency. Comparison of the load effects on the response time means and on their standard deviations revealed a fairly constant ratio throughout the experimental conditions, which suggests that similar search processes may have been involved. A feature-based comparison model is indeed shown to account for the response time means obtained after extensive training under just about all training conditions, as well as for the ratios of load effects on means and standard deviations. According to the model, improvement in search efficiency results from a reduction in the number of features considered. The model's performance questions the necessity to postulate qualitative differences between controlled and automatic processing, while the experiment forces a reassessment of the importance of the consistent mapping that underlies dual-process theories.

Relational Integration, Inhibition, and Analogical Reasoning in Older Adults

The difficulty of reasoning tasks depends on their relational complexity, which increases with the number of relations that must be considered simultaneously to make an inference, and on the number of irrelevant items that must be inhibited. The authors examined the ability of younger and older adults to integrate multiple relations and inhibit irrelevant stimuli. Young adults performed well at all but the highest level of relational complexity, whereas older adults performed poorly even at a medium level of relational complexity, especially when irrelevant information was presented. Simulations based on a neurocomputational model of analogical reasoning, Learning and Inference with Schemas and Analogies (LISA), suggest that the observed decline in reasoning performance may be explained by a decline in attention and inhibitory functions in older adults.

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.

Object-based selection operates on a grouped array of locations

In two experiments, space- and object-based selection effects were investigated, using variants of the ring-cuing paradigm of Egly and Homa (1984). The results revealed significant cuing modulation for nonring configurations of target locations spanning a range of retinal eccentricities, with the cuing effects independent of eccentricity and confined to the configuration of locations (rather than extending to locations within the space enclosed by the cued configuration). These results are consistent with object-based selection operating on a grouped spatial array (Vecera & Farah, 1994). Object selection may be based on a supradimensional saliency map representation of the field, modulated by feature-specific segmentation mechanisms (e.g., an object may be made salient on the basis of its color). Complex objects may be represented by grouped saliency signals. In this way, a two-dimensional spatial (saliency) representation may provide the common format for object-based selection, prior to full object definition.

Attentional capacity is undifferentiated: concurrent discrimination of form, color, and motion

We report a series of experiments on the concurrent discrimination of form, color, and motion attributes. All tasks involved joint discrimination of attributes, and positions and were highly demanding of attention. We quantified interference between concurrent discriminations by establishing the attention-operating characteristic. Interference was indistinguishable for similar and dissimilar task combinations (form-form, color-color, motion-motion, and color-form, color-motion, motion-color, and motion-form, respectively). These results suggest strongly that different visual discriminations draw on the same attentional capacity--in other words, that the capacity of visual attention is undifferentiated.

Visual selective attention: a theoretical analysis

The present paper outlines a framework which allows a consistent interpretation of data regarding visual selection in visual search tasks. It organizes and reviews visual search tasks in which the target is defined by primitive features, by conjunctions of features and when the target is categorically different from non-targets. The special role of spatial attention is reviewed and different theoretical accounts are discussed. Because visual selection depends principally on the outcome of the early parallel preattentive stage of processing, the main focus will be on this stage. It is concluded that visual selection is to a large extent determined by the physical characteristics of the stimuli present in the visual field. The early preattentive parallel process computes how different each object is from each of the other objects within a particular stimulus dimension. Attention is automatically drawn to the location having the highest activation, implying that the object at that location is automatically selected irrespective of the intentions of the subject. The model also assumes some top-down control. It is well known that attention can be voluntarily directed to nonfixated locations in visual space, varying from a uniform distribution over the visual field to a highly focused concentration. The model assumes that the endogenous direction of attention to an area in the visual field is the only top-down manner of affecting visual selection. Within the area of directed attention, no top-down control is possible: selection is completely determined by the physical properties of the stimuli.

Spatial cueing and the relation between the accuracy of "where" and "what" decisions in visual search

To study the processes underlying selective attention in visual search, the relation between the accuracy of "where" (location) and "what" (same/different orientation matching) decisions was analysed under various display conditions. Target-non-target discriminability was varied by contrasting single and multiple element displays; further, attention was directly manipulated by spatial cueing. In Experiment 1, analyses for both single and multiple displays showed that localization accuracy remained above chance when same/different matching failed; the inverse also obtained. It seems that accurate matching is not a prerequisite for target localization, nor is accurate localization a prerequisite for same/different matching. However, localization is a prerequisite for the accurate recognition of target orientation (Experiment 2). In this case, it seems that features critical for localization "call" attention to a particular candidate location. This facilitates further (shape) analysis of the stimulus that is found there. This orienting process is by-passed if attention is cued to the location in advance.