Lana M. Trick

What enumeration studies can show us about spatial attention: evidence for limited capacity preattentive processing

Subitizing, the enumeration of 1-4 items, is rapid (40-120 ms/item) and accurate. Counting, the enumeration of 5 items or more, is slow (250-350 ms/item) and error-prone. Why are small numbers of items enumerated differently from large numbers of items? It is suggested that subitizing relies on a preattentive mechanism. Ss could subitize heterogeneously sized multicontour items but not concentric multicontour items, which require attentional processing because preattentive gestalt processes misgroup contours from different items to form units. Similarly, Ss could subitize target items among distractors but only if the targets and distractors differed by a feature, a property derived through preattentive analysis. Thus, subitizing must rely on a mechanism that can handle a few items at once, which operates before attention but after preattentive operations of feature detection and grouping.

LIFESPAN CHANGES IN ATTENTION: THE VISUAL SEARCH TASK

There are two popular frameworks for the study of visual attention. Treisman’s Feature Integration Theory focuses on the effortful process of binding together the multiple attributes of an object. Posner’s Visual Orienting Theory emphasizes the movement of an attentional spotlight across space. Although both aspects are undoubtedly important in any visual search task, it is not clear how each of these aspects changes with age. We tested observers aged 6, 8, 10, 22, and 72 years on visual search tasks designed to isolate these factors. No age-related differences were found in single- or double-feature discrimination, attention movement to a single item, or search for a singlefeature target among distracters. Two age-related changes were found: (1) young children were less able than either young adults or seniors to search for targets defined by a conjunction of features, and (2) both children and seniors were less able than young adults to move attention voluntarily from item to item. This implies that feature integration and voluntary movement of attention have different trajectories over the lifespan. The visual search task is an important tool in the study of visual-spatial attention. In this task, observers try to indicate as rapidly as possible whether a given target item is present in a display. The experimenter manipulates the number of other items that are present (called non-targets or distracters), usually using the slope of the response time function over display size (RT slope) as a convenient index of the involvement of attention (Duncan & Humphrey% 1989; Neisser, 1967; Treisman & Gelade, 1980; Wolfe, Cave, & Franzel, 1989).

Multiple parallel access in visual attention.

It is widely accepted that there exists a region or locus of maximal resource allocation in visual perception--sometimes referred to as the spotlight of attention. We have argued that even if there is a single locus of processing, there must be multiple loci of parallel access--several places in the visual field must be indexed at once and these indexes can be used to determine where attention is allocated. We have carried out a variety of studies to support these ideas, including experiments showing that subjects can track multiple independent moving targets in a field of identical distractors, that the enhanced ability to detect changes occurring on these targets does not accrue to nontargets nor to items lying inside the convex polygon that they form (so that a zoom-lens of attention does not fit the data). We have used a visual search paradigm to show that (serial or parallel) search can be confined to a subset of indexed items and the layout of these items is of little importance. We have also studied the phenomenon known as subitizing and have shown that subitizing occurs only when items can be preattentively individuated and in those cases location precuing has little effect, compared with when counting occurs, which suggests that subitizing may be carried out by counting active indexes rather than items in the visual field. And finally we have run studies showing that a certain motion effect that is sensitive to attention can occur at multiple precued loci. We believe that this evidence suggests that there is an early preattentive stage in vision where a small number of salient items in the visual field are indexed and thereby made readily accessible for a variety of visual tasks.

Paying attention behind the wheel: a framework for studying the role of attention in driving

Driver inattention is thought to cause many automobile crashes. However, the research on attention is fragmented, and the applied research on driving and attention is further split between three largely independent traditions: the experimental research, the differential crash rate research, and the automation research. The goal of this review is to provide a conceptual framework to unify the research—a framework based on the combination of two fundamental dimensions of attentional selection: selection with and without conscious awareness (controlled and automatic), and selection by innate and acquired cognitive mechanisms (exogenous and endogenous). When applied to studies chosen to represent a broad range within the experimental literature, it reveals links between a variety of factors, including inexperience, inebriation, distracting stimuli, heads-up displays, fatigue, rumination, and secondary tasks such as phone conversations. This framework also has clear implications for the differential crash literature and the study of automated systems that support or replace functions of the driver. We conclude that driving research and policy could benefit from consideration of the different modes of attentional selection insofar as they integrate literatures, reveal directions for future research, and predict the effectiveness of interventions for crash-prevention.

Clusters Precede Shapes in Perceptual Organization

Does perceptual grouping require attention? Recent controversy on this question may be caused by a conflation of two aspects of grouping: element clustering (determining which elements belong together) and shape formation (determining cluster boundaries). In Experiment I, observers enumerated diamonds that were drawn with either lines or dots. These two types of stimuli were subitized (enumerated rapidly and accurately in the range from one to three items) equally well, suggesting that clustering dots into countable entities did not demand attention. In contrast, when target diamonds were enumerated among distractor squares in Experiment 2, only line-drawn items could be subitized. We propose that clustering and shape formation not only involve different perceptual processes, but play different functional roles in vision.

Driving in fog: the effects of driving experience and visibility on speed compensation and hazard avoidance.

Inexperience is one of the strongest predictors for collisions, but it remains unclear how novice drivers differ from experienced drivers in terms of safety-related behavioural adaptations such as speed reduction in the presence of reduced visibility. To investigate the influence of driving experience on behavioural compensations to fog, average speed, speed variability, steering variability, collision rate, and hazard response time were measured in a driving simulator. Experienced drivers drove faster in clear visibility than novice drivers, yet they reduced their speed more in reduced visibility so that both groups drove at the same speed in simulated fog. Compared to experienced drivers, novice drivers had higher hazard response times, greater speed and steering variability, and were the only drivers to have collisions.

Life Span Changes in Visual Enumeration: The Number Discrimination Task

Participants from 5 groups with mean ages of 6, 8, 10, 22, and 72 years were tested on a series of speeded number discriminations: 1 vs. 2, 3 vs. 4, 6 vs. 7, and 8 vs. 9. The primary measure of interest-response time slope as a function of number size-decreased with age for numbers in the 1-4 range. However, a U-shaped age function emerged in the 6-9 range, with larger slopes for children and senior adults, and the smallest slopes for young adults. These data suggest that different processes are involved in enumerating small and large numbers of items. It is argued that subitizing, the process for small numbers, makes only minimal demands on spatial attention and thus shows developmental improvements without any decline in old age. In contrast, counting, the process for large numbers, requires sophisticated coordination of spatial attention, which has previously been shown to first improve and then decline over the life span.

More than superstition : Differential effects of featural heterogeneity and change on subitizing and counting

This study investigates the effects of item heterogeneity (differences in color and shape) and moment-to-moment feature change as it relates to the issue of whether subitizing and counting involve different processes. Participants enumerated displays of up to eight items that were either homogeneous or heterogeneous. In situations where the heterogeneous displays always had approximately half of the items of one type and half of the other, heterogeneity significantly sped enumeration in the counting range (6–8 items) and significantly slowed enumeration in the subitizing range (1–3 items), a dissociation that suggests that subitizing and counting involve different operations. Moment-to-moment feature change had no effect on subitizing. However, feature change slowed counting, but only when participants were enumerating heterogeneous items that were half of one type and half of the other, as might be expected if participants were using differences in features to select items by type.

The effects of visibility conditions, traffic density, and navigational challenge on speed compensation and driving performance in older adults.

Research on how older drivers react to natural challenges in the driving environment is relevant for both the research on mental workload and that on age-related compensation. Older adults (M age=70.8 years) were tested in a driving simulator to assess the impact of three driving challenges: a visibility challenge (clear day, fog), a traffic density challenge (low density, high density) and a navigational challenge (participants followed the road to arrive at their destination, participants had to use signs and landmarks). The three challenge manipulations induced different compensatory speed adjustments. This complicated interpretation of the other measures of driving performance. As a result, speed adjustment indices were calculated for each condition and participant and composite measures of performance were created to correct for speed compensation. (These speed adjustment indices correlated with vision test scores and subscales of the Useful Field of View.) When the composite measures of driving performance were analyzed, visibility x density x navigational challenge interactions emerged for hazard RT and SD of lane position. Effects were synergistic: the impact of the interaction of challenge variables was greater than the sum of independent effects. The directions of the effects varied depending on the performance measure in question though. For hazard RT, the combined effects of high-density traffic and navigational challenge were more deleterious in good visibility conditions than in fog. For or SD of lane position, the opposite pattern emerged: combined effects of high-density traffic and navigational challenge were more deleterious in fog than in clear weather. This suggests different aspects of driving performance tap different resources.

The relationship between postural stability and virtual environment adaptation

Currently little is known about how adaptive responses to virtual environments are different between individuals who experience sickness related symptoms and those who do not. It is believed that sensory interactions between visually perceived self-motion and static inertial cues from vestibular and/or proprioceptive sensory systems contribute to the development of adaptation symptoms. The aim of this study was to evaluate the relationship between adaptation symptoms and postural stability in a virtual environment (VE) driving simulator. In addition, the role of sensory interaction was assessed using direct electrical stimulation techniques of the vestibular and cutaneous sensory systems. Posture performance was measured using centre of pressure measures of single leg stance tests during eyes open and eyes closed conditions. Correlation analysis of postural measures and symptom scores were conducted, as well as analysis of variance of posture performance between SICK and WELL individuals. Results indicate that posture stability is negatively correlated to symptom reporting. WELL individuals displayed the greatest decrease in postural stability during eyes open single leg stance following VE simulation. Application of a secondary sensory stimulation (vestibular or cutaneous) resulted in increased visual dependency for postural control following simulation. Combined, these results suggest that sensory interactions drive postural changes that are observed following VE simulation and are related to how visual information is used to control posture.