Control of attention around the fovea

Individual differences in crowding predict visual search performance

Visual search is an integral part of human behavior and has proven important to understanding mechanisms of perception, attention, memory, and oculomotor control. Thus far, the dominant theoretical framework posits that search is mainly limited by covert attentional mechanisms, comprising a central bottleneck in visual processing. A different class of theories seeks the cause in the inherent limitations of peripheral vision, with search being constrained by what is known as the functional viewing field (FVF). One of the major factors limiting peripheral vision, and thus the FVF, is crowding. We adopted an individual differences approach to test the prediction from FVF theories that visual search performance is determined by the efficacy of peripheral vision, in particular crowding. Forty-four participants were assessed with regard to their sensitivity to crowding (as measured by critical spacing) and their search efficiency (as indicated by manual responses and eye movements). This revealed substantial correlations between the two tasks, as stronger susceptibility to crowding was predictive of slower search, more eye movements, and longer fixation durations. Our results support FVF theories in showing that peripheral vision is an important determinant of visual search efficiency.

A Framework for Understanding the Allocation of Attention in Location-Precued Discrimination

The effects of attention on visual perception are assessed in the location-precuing paradigm. First, we present a review of some current metaphors for attention and relevant data. Then, a framework is suggested that provides an interpretation of the temporal sequence of external and assumed internal processes within a location-cuing trial. Cases when a precue correctly indicates the target location (valid trials) are compared to cases when the precue directs attention to the wrong location (invalid trials) with the cue location either at fixation or peripheral to the target location. Several specific hypotheses are suggested; these concern decrements in performance on invalid trials and effects of the location of a precue. For the most part, these hypotheses are supported by data in the literature and in some new studies. A gradient-filter metaphor for attention, which includes a synthesis of ideas from the gradient model and the attention gate model, is more consistent with the data than is a spotlight metaphor.

The distribution of spatial attention changes with task demands during goal-directed reaching

Goal-directed movements are commonly used to allow humans to interact with their environment. When making a goal-directed movement in a natural environment, there are many competing stimuli. It is therefore important to understand how making a goal-directed movement could be impacted by the need to divide attention between the movement and competing stimuli. We used a dual-task paradigm to investigate the sharing of attentional resources between a search task in central vision and a peripheral pointing task completed concurrently. Results suggest some degree of shared attentional resources between these two tasks with performance on both central and peripheral tasks degraded under dual-task conditions. Movement latency, but not movement time, was also affected by dual-task conditions. Altogether, the results suggest that there is a cost to reach performance if attention is engaged away from the movement goal. Interestingly, this cost is associated with movement planning rather than execution.

Overcoming Correlational Pitfalls

Do people need to explicitly encode conditioned stimuli–unconditioned stimuli (CS–US) pairings for evaluative conditioning (EC) effects to emerge? Despite the large number of studies that addressed this issue, no simple answer has emerged yet. In part, this is due to the relative lack of experimental evidence for the role of awareness of the CS–US contingency at encoding in EC. In the present experiment, participants’ encoding of the CS–US pairings was experimentally manipulated by relying on foveal and parafoveal presentations of the CSs. More specifically, spatial locations (i.e., foveal vs. parafoveal) of the CSs and US valence (i.e., positive vs. negative) were manipulated within participants, and CS–US pairings were counterbalanced across participants. Results reveal explicit encoding of the CSs and EC effects for the foveal CS presentations only. We discuss the implications of these experimental findings for the associative and propositional approach to EC.

Mapping the timecourse of goal-directed attention to location and colour in human vision

Goal-directed attention prioritises perception of task-relevant stimuli according to location, features, or onset time. In this study we compared the behavioural timecourse of goal-directed selection to locations and colours by varying the stimulus-onset asynchrony (SOA) between cue and target in a strategic cueing paradigm. Participants reported the presence or absence of a target following prior information regarding its location or colour. Results revealed that preparatory selection by colour is more effective at enhancing perceptual sensitivity than selection by location, even though both types of cue provided equivalent overall information. More detailed analysis revealed that this advantage arose due a limitation of spatial attention in maintaining a sufficiently broad focus (>2°) for target detection across multiple stimuli. In contrast, when target stimuli fell within 2° of the spatial attention spotlight, the strategic advantages and speed of spatial and colour attention were equated. Our findings are consistent with the conclusion that, under spatially optimal conditions, prior spatial and colour information are equally proficient at guiding top-down selection. When spatial locations are ambiguous, however, colour-based selection is the more efficient mechanism.

A demonstration of direct access to colored stimuli following cueing by color

To test whether cueing by color can affect orienting without first computing the location of the cued color, the impact of reorienting on the validity effect was examined. In Experiment 1 subjects were asked to detect a black dot target presented at random on either of two colored forms. The forms started being presented 750 ms before the onset of a central cue (either an arrow or a colored square). In some proportion of the trials the colors switched locations 150 ms after cue onset, simultaneously with target onset. The color switch was not found to retard responses following a color cue more than following a location cue. Furthermore, it did not reduce the validity effect of the color cue: Though the validity effect of the location cue was quite larger than the validity effect of the color cue, both effects were additive with the presence/absence of a color switch. In Experiment 2, subjects were rather asked to detect a change in shape of one of the colored forms. In this case, color switch was found to affect performance even less following a color cue. The fact that across experiments, color switch did not retard neither responding nor orienting selectively in the color cue condition, indicates that when attention is set to a certain color, reorienting to a new object following color switch does not require re-computing the address of the cued color. That finding is argued to embarrass a strong space-based view of visual attention.

Uncertainty compensation in human attention: evidence from response times and fixation durations

Background

Uncertainty and predictability have remained at the center of the study of human attention. Yet, studies have only examined whether response times (RT) or fixations were longer or shorter under levels of stimulus uncertainty. To date, no study has examined patterns of stimuli and responses through a unifying framework of uncertainty.

Methodology/Principal Findings

We asked 29 college students to generate repeated responses to a continuous series of visual stimuli presented on a computer monitor. Subjects produced these responses by pressing on a keypad as soon a target was detected (regardless of position) while the durations of their visual fixations were recorded. We manipulated the level of stimulus uncertainty in space and time by changing the number of potential stimulus locations and time intervals between stimulus presentations. To allow the analyses to be conducted using uncertainty as common description of stimulus and response we calculated the entropy of the RT and fixation durations. We tested the hypothesis of uncertainty compensation across space and time by fitting the RT and fixation duration entropy values to a quadratic surface. The quadratic surface accounted for 80% of the variance in the entropy values of both RT and fixation durations. RT entropy increased as a function of spatial and temporal uncertainty of the stimulus, alongside a symmetric, compensatory decrease in the entropy of fixation durations as the level of spatial and temporal uncertainty of the stimuli was increased.

Conclusions/Significance

Our results demonstrate that greater uncertainty in the stimulus leads to greater uncertainty in the response, and that the effects of spatial and temporal uncertainties are compensatory. We also observed compensatory relationship across the entropies of fixation duration and RT, suggesting that a more predictable visual search strategy leads to more uncertain response patterns and vice versa.

Limited influence of perceptual organization on the precision of attentional control

The role of perceptual organization in the precision of attentional control was assessed in three experiments. Observers viewed circular arrays of disks that varied in density. One disk was cued, directing attention to that disk. A series of tones then indicated shifts of attention to the next disk that was of the same color (Experiments 1 and 2) or on the same depth plane (Experiment 3). In the homogeneous condition, all of the disks were the same color (Experiments 1 and 2) or on the same depth plane (Experiment 3). In the heterogeneous condition, the disks alternated in color (Experiments 1 and 2) or stereoscopically defined depth (Experiment 3). If the observers were able to limit attention to disks within a group, the effective density of the displays in the heterogeneous conditions should have been one half that in the homogeneous conditions. There was little evidence that the observers could do this, indicating a limited role of perceptual organization in the precision of attentional control.

Are spatial and temporal attention independent?

Participants searched for one of two target letters in a rapid serial visual presentation (RSVP) sequence of 17 successive frames, each containing four letters arranged into a box around a central fixation point. In control trial blocks, the participants had no information about when or where one of the target letters would appear. In other trial blocks, visual cues were given to indicate with 100% validity either the spatial location of the target, the time at which it would be presented, or both where and when it would appear. The results showed that both types of cues were effective on their own in speeding target identification, and their effects combined additively when the cues were presented and used together. These results support a growing body of evidence indicating that early attentional selection of information in vision is independently attuned to spatial and temporal properties of the environment.

Working memory capacity and the top-down control of visual search: Exploring the boundaries of "executive attention"

The executive attention theory of working memory capacity (WMC) proposes that measures of WMC broadly predict higher order cognitive abilities because they tap important and general attention capabilities (R. W. Engle & M. J. Kane, 2004). Previous research demonstrated WMC-related differences in attention tasks that required restraint of habitual responses or constraint of conscious focus. To further specify the executive attention construct, the present experiments sought boundary conditions of the WMC-attention relation. Three experiments correlated individual differences in WMC, as measured by complex span tasks, and executive control of visual search. In feature-absence search, conjunction search, and spatial configuration search, WMC was unrelated to search slopes, although they were large and reliably measured. Even in a search task designed to require the volitional movement of attention (J. M. Wolfe, G. A. Alvarez, & T. S. Horowitz, 2000), WMC was irrelevant to performance. Thus, WMC is not associated with all demanding or controlled attention processes, which poses problems for some general theories of WMC.

Look here but ignore what you see: effects of distractors at fixation

Distractor interference effects were compared between distractors in the periphery and those placed at fixation. In 6 experiments, the authors show that fixation distractors produce larger interference effects than peripheral distractors. However, the fixation distractor effects are modulated by perceptual load to the same extent as are peripheral distractor effects (Experiments 1 and 2). Experiment 3 showed that fixation distractors are harder to filter out than peripheral distractors. The larger distractor effects at fixation are not due to the cortical magnification of foveal stimuli (Experiments 4 and 5), nor can they be attributed to cuing by the fixation point (Experiment 2), the lower predictability or greater location certainty of fixation distractors (Experiment 5), or their being in a central position (Experiment 6). The authors suggest that preferential access to attention renders fixation distractors harder to ignore than peripheral distractors.

Attentional selection of a peripheral ring overrules the central attentional bias

In visual search, distractors that fall between fixation and the target are more disruptive than distractors that fall at the same eccentricity as the target (Wolfe, O'Neill, & Bennett, 1998). This sensitivity to the relative eccentricity of target and distractor elements originates from a space-based bias favoring stimuli closer to the fovea. We show that this spatial bias can be overruled by cuing attention to a ring-shaped object. We rule out various space-based explanations of these findings, including (1) attention to fronto-parallel planes in depth and (2) serial attention to different portions of a ring. We suggest that attentional selection of a ring-shaped object operates independently of, and can overrule, spatial biases in selection.

Dynamic interaction of object- and space-based attention in retinotopic visual areas

We investigated the interaction between object- and space-based attention by measuring activity in early visual cortex. After central cueing, when subjects directed attention to a spatially defined part of an object, activity in early visual areas was enhanced at corresponding retinotopic representations but also at representations of other locations covered by the object. Different from the assumption of automatic attentional "spreading" within an object, however, activity was greater for representations of cued than of uncued locations on the same object. These findings support an interaction of object-based spatial selection with object-independent spatial mechanisms in directing attention. When the target stimulus did not appear at the expected location, we found higher activation in areas representing other locations on the same object than equidistant locations on other objects. Objects, hence, also guide spatial search, and this may account for the behaviorally observed delay in processing parts of an unattended object.

Surveying a Blank Field with the Attentional Spotlight

Spatial localization is a unique aspect of selective attention that precedes and facilitates other aspects of sensory processing. A common model of spatial attention is an attentional spotlight that enlarges to fit attentional demands. It is unclear if this attentional spotlight expands to survey a blank field or to scan the spatial environment with a smaller attentional spotlight. Two tachistoscopic experiments with young adults investigated how attention is surveyed in a blank field. Exp. 1 consisted of single letters at two concentric regions and under different conditions of attentional demand. Reaction times were proportionally slower for stimuli occurring in a combination of the two regions located at different distances from the fixation point than for stimuli occurring in each region alone. This finding is not consistent with attentional spread to encompass the entire attentional field. The addition of more stimulus locations within each single region yielded significantly slower reaction times. Together these findings suggest that a small attentional spotlight serially monitors different regions of the visual field when there are anticipated locations to be attended.

An ERP study of sustained spatial attention to stimulus eccentricity

Effects of attention on event-related brain potentials (ERPs) were measured when subjects kept sustained attention focused on ring-shaped regions of visual space to detect infrequently presented targets at a given eccentricity. In line with a previous study that employed a trial-by-trial cueing paradigm, no modulations of sensory-evoked P1 and N1 components were found. This suggests that attentional selectivity in complex spatial selection tasks is primarily located at post-perceptual processing levels. Enhanced negativities for attended as compared to unattended stimuli were present between 220 and 380 ms post-stimulus and were followed by an enlarged positivity for attended stimuli in the P3 time range. These effects reflected the distribution of attention in visual space, in part consistent with 'attentional gradient' and 'zoom-lens' models. However, ERPs also suggested the presence of selective mechanisms that exclude irrelevant stimuli located between two simultaneously attended areas.

Priming effects in attentional gating

Three experiments were conducted to determine the effects of amount of prior target information (Experiment 1) and semantic priming (Experiment 2) in an attentional gating task. The goal was to determine some causes of the processing deficits commonly observed in perceiving successive visual stimuli. Items in a rapid serial visual presentation (RSVP) stream are subject to processing deficits before they are processed to the level of recognition (early selection) and after they have been recognized (late selection). Deficits in the former case presumably are due to an early filter that prevents complete recognition and semantic analysis, whereas deficits in the latter case arise from interference or response competition, producing forgetting among a set of recognized items. The semantic-priming effects found between a cue and a target (Experiment 2) and between two successive targets (Experiment 3) indicate that top-down processes can increase the subjective availability of related items. The results are consistent with the idea that most processing deficits observed in search through an RSVP sequence are due to limited capacity in our ability to form episodic representations of all the items in the sequence.

Visuospatial attention: beyond a spotlight model

Much of the research in visual attention has been driven by the spotlight metaphor. This metaphor has been useful over many years for generating experimental questions in attention research. However, theories and models of visual selection have reached such a level of complexity that debate now centers around more specific questions about the nature of attention. In this review, the general question "Is visual attention like a spotlight?" is broken down into seven specific questions concerning the nature of visual attention, and the evidence relevant to each is examined. The answers to these specific questions provide important clues about why visual selection is necessary and what purpose attention plays in visual cognition.

Response times and eye movements in feature and conjunction search as a function of target eccentricity

In four experiments, saccadic eye movements, reaction times (RTs), and accuracy were measured as observers searched for feature or conjunction targets presented at several eccentricities. A conjunction search deficit, evidenced by a large eccentricity effect on RTs, accuracy, and number of saccades, was seen in Experiments 1A and 1B. Experiment 2 indicated that, when saccades were precluded, there was an even larger eccentricity effect for conjunction search targets. In Experiment 3, practice in a conjunction search task allowed both RT and number of saccades to become independent of eccentricity. Additionally, there was evidence of feature-based selectivity in that observers were more likely to fixate distractors that had the same contrast as the target. Results are consistent with the view that the oculomotor and attentional systems are functionally linked and provide constraints for models of visual attention and search.

Stroop can occur without Garner interference: strategic and mandatory influences in multidimensional stimuli

Stroop and Garner interference were studied in two experiments involving stimuli with several irrelevant features. Using these stimuli, which were more complex than those usually used in perceptual interference studies, a new phenomenon occurred: Stroop effects without a corresponding Garner interference were obtained in four out of six nontarget conditions, two with local and two with global targets. The effects with local targets were anomalous: on one dimension, incongruous Stroop stimuli were better than congruous ones, while on the other dimension, effects were restricted to a condition in which all nontargets were congruous. With global targets, more consistent cases of Stroop-without-Garner effects were obtained. All Stroop effects were replicated in Experiment 2, in which presentation time was varied. The effects showed a strong dependency on presentation time, in such a way as to suggest a dynamic growth of the percept. The results were interpreted in terms of an interaction between automatic and strategic components of perceptual processes, in agreement with a recently introduced perceptual-organization model, which yields a new interpretation of priming and interference phenomena.

Visual field tunneling in aviators induced by memory demands

Aviators are required rapidly and accurately to process enormous amounts of visual information located foveally and peripherally. The present study, expanding upon an earlier study (Williams, 1988), required young aviators to process within the framework of a single eye fixation a briefly displayed foveally presented memory load while simultaneously trying to identify common peripheral targets presented on the same display at locations up to 4.5 degrees of visual angle from the fixation point. This task, as well as a character classification task (Williams, 1985, 1988), has been shown to be very difficult for nonaviators: It results in a tendency toward tunnel vision. Limited preliminary measurements of peripheral accuracy suggested that aviators might be less susceptible than nonaviators to this visual tunneling. The present study demonstrated moderate susceptibility to cognitively induced tunneling in aviators when the foveal task was sufficiently difficult and reaction time was the principal dependent measure.

Tradeoffs between attentional effects of spatial cues and abrupt onsets

We determined the relative effectiveness and tradeoffs among central, peripheral, and abrupt onset cues in directing attention to a potential target character. Central cues were arrows located at the fixation point, whereas peripheral cues were arrows occurring about 3 degrees away from fixation, near the location of a potential target. These were contrasted with the abrupt onset of an ambiguous part of a character, which later was filled in to reveal a target or a distractor item. Each trial included an arrow cue and an abrupt onset cue, and both expected cue validities and cue-character SOAs were varied factorially. The results showed that, in general, abrupt onsets captured attention more effectively than either central or peripheral arrow cues. However, tradeoffs among separate cue effects indicated that the power of abrupt onsets to capture attention automatically could be overridden by a high-validity spatial cue presented in advance of the onset character. Tradeoffs between the effects of central and abrupt onset cues were additive, indicating that endogenous and exogenous cues have their main effects at different levels in the visual attention system. Peripheral cues and abrupt onsets showed mainly interactive effects, however, consistent with the idea that both types of cues have exogenous components that affect a common pool of attentional resources.