If Visual Saliency Predicts Search, Then Why? Evidence from Normal and Gaze-Contingent Search Tasks in Natural Scenes

The Itti and Koch (Vision Research 40: 1489?1506, 2000) saliency map model has inspired a wealth of research testing the claim that bottom-up saliency determines the placement of eye fixations in natural scenes. Although saliency seems to correlate with (although not necessarily cause) fixation in free-viewing or encoding tasks, it has been suggested that visual saliency can be overridden in a search task, with saccades being planned on the basis of target features, rather than being captured by saliency. Here, we find that target regions of a scene that are salient according to this model are found quicker than control regions (Experiment 1). However, this does not seem to be altered by filtering features in the periphery using a gaze-contingent display (Experiment 2), and a deeper analysis of the eye movements made suggests that the saliency effect is instead due to the meaning of the scene regions. Experiment 3 supports this interpretation, showing that scene inversion reduces the saliency effect. These results suggest that saliency effects on search may have nothing to do with bottom-up saccade guidance

Designing Better Symbols: An Attentional Approach to Match Symbols to Performance Goals

Effective displays require symbol sets that are customized to specific tasks and performance goals. In order to create such sets, designers must account for the effects of top-down and bottom-up attention. The current work presents a pair of experiments that examined the effects of salience and cueing in a change detection tasks within the flicker paradigm (Rensink, O’Regan and Clark, 1997). Each trial, participants either received no cue or a cue indicating which symbol would be the target. This cueing manipulation isolated top-down effects to the cued condition. Consistent with previous studies (Orchard, 2012; Steelman, Orchard, Fletcher, Cockshell, Williamson & McCarley, 2013), Study 1 found a response time benefit for low salience symbols in the cued condition. Study 2 served as a replication of Study 1, but included a background manipulation that preserved the layout of the symbols while manipulating the symbol’s contrast to the background color. Results indicated a benefit for low salience symbols in the cued condition only on the black background, consistent with Study 1. However, low salience symbols showed no benefit on the gray or the white background in the cued condition, failing to support the hypothesis that low salience symbol show a cueing benefit. Chapter 5 conducted an extended analysis of the data from Study 2 using a variety of multilevel models to investigate specific symbol characteristics that may drive response times. For both uncued and cued search, eccentricity and crowding effects predicted response times. For uncued search, response times decreased as salience increased and standard deviation increased. For cued search symbol discriminability and salience predicted response times. Implications for the design of symbols and symbol sets are discussed

Caractérisation des obstacles nuisant au travail des débroussailleurs et de leurs impacts sur la productivité par la modélisation et l'analyse du travail cognitif

RÉSUMÉ Les traitements sylvicoles sont généralement payés à forfait (taux unitaire en /ha) au Québec comme ailleurs dans le monde. Un taux unitaire mal ajusté peut induire des comportements risqués pour la santé et la sécurité. Les modèles de taux actuels ne prennent pas en compte l’abondance d’obstacles présents sur le terrain (résidus ligneux, amas de résidus, souches, pierres et boulders, chicots, rugosité du terrain), alors que les débroussailleurs considèrent que ceux-ci réduisent leur productivité. Lorsque le terrain est moins encombré, les débroussailleurs tendent à augmenter la cadence afin d’atteindre l’objectif salarial hebdomadaire qu’ils se sont fixés. Ce type de comportement augmente le risque de fatigue et de blessures (Bellemare & Shearer, 2010; Bender, Green, & Heywood, 2010; Johansson, Rask, & Stenberg, 2010; Sundström-Frisk, 1984). L'objectif de cette thèse était de modéliser la relation statistique entre la productivité des débroussailleurs et les obstacles du terrain lors du dégagement de la régénération naturelle et artificielle, dans le but de permettre un meilleur ajustement des taux unitaires pour ce traitement. Trois recherches ont été entreprises pour atteindre cet objectif. La première consistait à modéliser la productivité en fonction l'abondance de végétation compétitrice et d’obstacles à l'aide de la régression linéaire, et d'explorer les relations entre les types d’obstacles et la productivité grâce à une analyse en composantes principales. Des études du temps ont été menées dans des aires de traitement de 0,12 ha, avec la participation de 91 sujets représentant 22 entrepreneurs sylvicoles dans toutes les régions du Québec où des opérations de dégagement de la régénération avaient lieu. Le meilleur modèle de régression (R2 = 0,61, RMSE = 4,3 heures par hectare, n = 91) prédit la consommation de temps effectif à l'aide de trois variables: 1) le pourcentage de couverture (%) du terrain par trois espèces de broussailles, soient les framboisiers, les fougères et les épilobes à feuilles étroites; 2) la densité (nombre par hectare) des arbres et arbustes de hauteur supérieure à 1 m; et 3) la densité des obstacles qui entravent la progression des travailleurs sur le terrain (résidus ligneux laissés après la récolte, souches, boulders et rugosité du terrain). L'analyse en composantes principales (ACP) a révélé que deux composantes représentant les résidus ligneux et les souches contournées par le travailleur étaient significativement reliés au temps effectif requis pour traiter un hectare (R2 = 0,14, RMSE = 6,4 heures par hectare, n = 91).----------ABSTRACT Piece-rate payment systems (unit rate in /ha) are commonly used for silvicultural treatments in Quebec and elsewhere in the world. A piece-rate that is not properly adjusted can lead to risky behavior affecting health and safety. Current brushcutter productivity prediction models do not consider the abundance of obstacles on the ground, while the brushcutters believe they reduce their productivity and require them to increase the pace when possible, in order to achieve a desired weekly wage. This is a kind of behavior that increases the risk of fatigue and injury (Bellemare & Shearer, 2010; Bender, Green, & Heywood, 2010; Johansson, Rask, & Stenberg, 2010; Sundström-Frisk, 1984). The aim of this dissertation was to model the statistical relationship between brushcutter productivity and terrain obstacles (slash, slash piles, stumps, stones and boulders, snags, terrain roughness) in the regeneration release silvicultural treatment in order to allow better adjustment of piece-rates for this treatment. Three steps were taken to reach that goal. The first step was to model productivity based on competing vegetation and obstacles abundance using linear regression analyses, and to explore the relationship between individual obstacle types and productivity through principal component analysis. Work studies were conducted in 0.12 ha treatment plots, with the participation of 91 subjects representing 22 silvicultural contractors. The best regression model (R2=0.61, RMSE=4.3 hours per hectare, n=91) predicted effective time consumption (ETC) using three variables: 1) percent cover (%) of three brush species (raspberry, ferns, fireweed), 2) density of trees and shrubs higher than 1 m (number per hectare) and 3) density of obstacles hindering worker progression in the field (woody residue, stumps left after harvest, boulders and terrain roughness). Principal component analysis (PCA) was used to further revealed that two components representing woody residue and stumps bypassed by the worker were significant predictors of ETC (R2=0.14, RMSE=6.4 hours per hectare, n=91). Measurements of hindering obstacle abundance have not been found to be easily reproducible, suggesting that some cognitive processes may be involved in coping with changing conditions. Forty-one participants were engaged in observations and interviews. Hierarchical task analysis (HTA) was performed to describe release work, complemented by an analysis according to the Skills-Rules-Knowledge (SRK) framework. Cognitive content seemed higher when workers were faced with obstacles. Half of the observed time was spent in obstacle-free areas while 30% was spent coping with slash and slash piles. Results should help develop a valid and practical method to account for obstacles in the field and may also help improve brushcutter training programs

Testing and training lifeguard visual search

Lifeguards play a crucial role in drowning prevention. However, current U.K. lifeguard qualifications are limited in training and assessing visual surveillance skills, and little is known about how lifeguards successfully detect drowning swimmers. To improve our understanding of lifeguard visual search skill, and explore the potential for improving this skill through training, this thesis had the following aims: (a) to identify whether visual skills for drowning detection improve with lifeguard experience, (b) to understand why such differences occur, and (c) design and valid a visual training intervention to improve drowning detection on the basis of these results. The first two studies investigated drowning-detection skills of participants with differing levels of lifeguard experience in a dynamic search task with simulated drownings. Lifeguards were found to detect drownings faster and more often than non-lifeguards. In three follow-up studies these results were replicated with more naturalistic stimuli. Video footage from an American wave pool was extracted, which showed genuine instances of swimmer distress. Results again demonstrated lifeguard superiority in detecting the drowning targets. Eye tracking measures, recorded on both the simulated and naturalistic clips, failed to reveal any differences between lifeguards and non-lifeguards, suggesting that superior drowning detection for lifeguards did not result from better scanning strategies per se. Following this, two cognitive mechanisms that may underlie drowning-detection skill were investigated. Lifeguard and non-lifeguard performance on Multiple Object Avoidance (MOA) and Functional Field of View (FFOV) tests was assessed. Although lifeguards had better MOA task performance compared to non-lifeguards, only the lifeguards’ accuracy at detecting the central target in the FFOV task predicted performance on a subsequent drowning detection task. It was concluded that superior drowning detection was a result of better classification recognition of drowning swimmers (which was the central task in the FFOV test). Based on these findings the final experiment explored the effectiveness of an intense classification training task to improve drowning detection. An intervention was designed that required participants to differentiate between videos of isolated drowning and non-drowning swimmers. Non-lifeguards trained in this intervention showed greater improvement on a subsequent drowning-detection task compared to untrained control participants, who completed an active-control task. The results of this thesis suggest that drowning-detection skill can be reliably assessed, and that foveal processing of drowning characteristics is key to lifeguards' superior performance. Isolating and training this key sub-skill improves drowning-detection performance and offers a method for training future lifeguards

Role of foveal vision in static and dynamic environments

The visual field has multiple regions, with visual acuity being highest in the centre before declining rapidly outward toward the periphery. This central region, otherwise known as the fovea, is typically defined as the central 2.0o of vision. Although comparatively small with respect to other visual field regions, being able to discern objects of interest in fine detail is only possible in this region. Due to this, people make ballistic eye movements (saccades) towards the fine details and depending on the task, may stabilise their gaze in the form of a fixation to discriminate parts of this newly attended area. A typical everyday task that can exhibit such behaviours is visual search (scanning a visual environment for objects or features among distractors), and much of this thesis is concerned with the importance of foveal vision with respect to visual search tasks. Seven experiments are presented in this thesis, with chapters 1 – 4 containing a review of the literature, methodologies and a glossary. In Chapter 5, search experiments with targets of varying sizes were conducted to assess the role of foveal vision on search performance. This chapter builds on a debate on whether foveal vision was necessary for the successful completion of a visual search task. In addition, a novel algorithm was developed to embed targets at a specified change in local contrast to automate target placement. The presented results show that the time taken to find targets with and without foveal vision is nearly identical even when target size is small. Chapter 6 modulates search difficulty by manipulating salience to investigate the effect of target size and salience on search performance. Coined the Compensation Effect, the results of Chapter 6 show that the above-mentioned variables were able to compensate for one another, resulting in an improved search performance. This effect occurred regardless of visual field degradation. In the same chapter, another experiment revealed the stage in the search process where performance costs originated from, which was the final stage of search concerned with target verification. Finally, Chapter 7 transitions from static images to dynamic scenes which simulate self-motion. Additional algorithms were developed, including an extension to the existing gaze-based decomposition of search time (Malcolm & Henderson 2009). Chapter 7 investigated the role of foveal vision in visual search whilst optical flow was present. Unlike previous results, the final experiments of this thesis revealed the necessity of foveal vision for the attainment of a normal search performance. The results of this thesis demonstrate that the importance of foveal vision with respect to visual search is modulated by the stimulus environment, with it being relatively unimportant for static scenes, but important for dynamic scenes