Visual search in depth

Orienting attention across binocular disparity

The spatial distribution of covert visual attention following an exogenous cue is often described as a spotlight, which disregards depth. Here, we study the orienting of attention across binocular disparity, a key depth cue in primates. A small Gabor patch target was displayed at ±12-arcmin horizontal offset in each eye independently, resulting in four possible 3D locations. With some latency relative to target onset (0-300 ms), an attentional cue was displayed at one of five binocular locations, resulting in various combinations of relative azimuth (horizontal position) and disparity (depth). Observers' task was to discriminate the orientation of the target. Observers' performance decreased as the relative azimuth between the cue and the target increased. Performance also decreased with the difference in disparity, even when the azimuth remained constant. Performance varied with the delay between the cue and the target and was maximal between 100 and 150 ms. The orienting of attention in azimuth and depth followed the same time course. We mapped the 3D shape of attentional focus over time and found that the spatial envelope was approximately a Gaussian modulated in time. These results could not be explained by monocular confounds nor by eye movements. We conclude that exogenous cues direct attention not only to their visual direction but also to their depth and that binocular disparity is sufficient to define that depth. The identical time course and interaction between azimuth and depth suggest a shared mechanism, and therefore that visual attention to spatial location is an intrinsically 3D process.

Avoiding potential pitfalls in visual search and eye-movement experiments: A tutorial review

Examining eye-movement behavior during visual search is an increasingly popular approach for gaining insights into the moment-to-moment processing that takes place when we look for targets in our environment. In this tutorial review, we describe a set of pitfalls and considerations that are important for researchers – both experienced and new to the field – when engaging in eye-movement and visual search experiments. We walk the reader through the research cycle of a visual search and eye-movement experiment, from choosing the right predictions, through to data collection, reporting of methodology, analytic approaches, the different dependent variables to analyze, and drawing conclusions from patterns of results. Overall, our hope is that this review can serve as a guide, a talking point, a reflection on the practices and potential problems with the current literature on this topic, and ultimately a first step towards standardizing research practices in the field.

Experience with searching in displays containing depth improves search performance by training participants to search more exhaustively

In a typical visual search task, participants search for single targets amongst displays containing non-overlapping objects that are presented on a single depth plane. Recent work has begun to examine displays containing overlapping objects that are presented on different depth planes to one another. It has been found that searching displays containing depth improves response accuracy by making participants more likely to fixate targets and to identify targets after fixating them. Here we extended this previous research by seeking first of all to replicate the previous pattern of results, and then to determine whether extensive training using depth in search transfers to two-dimensional displays. We provided participants with sixteen sessions of training with displays containing transparent overlapping objects presented in depth, and found a similar pattern of results to our previous study. We also found evidence that some performance improvements from the depth training transferred to search of two-dimensional displays that did not contain depth. Further examinations revealed that participants learn to search more exhaustively (i.e., search for longer) in displays containing depth. We conclude that depth does influence search performance but the influences depend very much on the stimuli and the degree of overlap within them.

Evolutionary Relevance Facilitates Visual Information Processing

Visual search of the environment is a fundamental human behavior that perceptual load affects powerfully. Previously investigated means for overcoming the inhibitions of high perceptual load, however, generalize poorly to real-world human behavior. We hypothesized that humans would process evolutionarily relevant stimuli more efficiently than evolutionarily novel stimuli, and evolutionary relevance would mitigate the repercussions of high perceptual load during visual search. Animacy is a significant component to evolutionary relevance of visual stimuli because perceiving animate entities is time-sensitive in ways that pose significant evolutionary consequences. Participants completing a visual search task located evolutionarily relevant and animate objects fastest and with the least impact of high perceptual load. Evolutionarily novel and inanimate objects were located slowest and with the highest impact of perceptual load. Evolutionary relevance may importantly affect everyday visual information processing.

The effects of task difficulty on visual search strategy in virtual 3D displays

Analyzing the factors that determine our choice of visual search strategy may shed light on visual behavior in everyday situations. Previous results suggest that increasing task difficulty leads to more systematic search paths. Here we analyze observers' eye movements in an "easy" conjunction search task and a "difficult" shape search task to study visual search strategies in stereoscopic search displays with virtual depth induced by binocular disparity. Standard eye-movement variables, such as fixation duration and initial saccade latency, as well as new measures proposed here, such as saccadic step size, relative saccadic selectivity, and x-y target distance, revealed systematic effects on search dynamics in the horizontal-vertical plane throughout the search process. We found that in the "easy" task, observers start with the processing of display items in the display center immediately after stimulus onset and subsequently move their gaze outwards, guided by extrafoveally perceived stimulus color. In contrast, the "difficult" task induced an initial gaze shift to the upper-left display corner, followed by a systematic left-right and top-down search process. The only consistent depth effect was a trend of initial saccades in the easy task with smallest displays to the items closest to the observer. The results demonstrate the utility of eye-movement analysis for understanding search strategies and provide a first step toward studying search strategies in actual 3D scenarios.

Guidance of Eye Movements During Visual Conjunction Search: Local and Global Contextual Effects on Target Discriminability

The composition of a visual scene influences the ability of humans to select specific details within that scene for discrimination or foveation with saccadic eye movements. With the goal of establishing an animal model to investigate the neural mechanisms underlying the deployment of visual attention and the guidance of saccades during visual search, we studied the visual behavior of three monkeys while they performed a conjunction (color + form) search task similar to those used in human studies. We found that search performance declined when distractors adjacent to the target shared its color, thereby revealing that color was more discriminable than form in these displays and suggesting that monkeys perceptually grouped stimuli by proximity and similarity. Search performance also varied with the overall composition of the display. Most importantly, saccades were biased toward distractors sharing the target color when there were few of them within the display and away from those distractors when they were numerous. Last, the monkeys initiated saccades with a fixed latency, suggesting that their responses to the display were automatic and that search strategies did not involve attentional resources beyond those recruited for regulating saccades. We conclude that monkeys adapt their visual strategies, largely via bottom-up processes, to both the local and the global context of the search. These findings suggest that the visual behavior of monkeys is guided by strategies similar to those observed in humans.