Testing and Training Lifeguard Visual Search
Total Page：16
File Type：pdf, Size：1020Kb
Testing and Training Lifeguard Visual Search By Victoria Alice Laxton A thesis submitted in partial fulfilment of the requirements of Nottingham Trent University for the degree of Doctor of Philosophy. September 2019 This work is the intellectual property of the author. You may copy up to 5% of this work for private study, or personal, non-commercial research. Any re-use of the information contained within this document should be fully referenced, quoting the author, title, university, degree level and pagination. Queries or requests for any other use, or if a more substantial copy is required, should be directed in the owner(s) of the Intellectual Property Rights. Abstract 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. Acknowledgements First and formost I would like to thank my supervisor Prof David Crundall for his support and guidance over the past four years. I thank you for getting me to the stage I am today, somewhere I did not believe possible four years ago. Thanks are also due to Christina Howard and Duncan Guest for their guidance and input. I am thankful to the technician team, Ben and Roy, for all their technical support. For all the times they have sorted out problems, from helping with eye-tracker issues over the phone to recoving files from broken memory sticks. Also, thanks to Josh, who has taught me the basics of coding and helped with the dynamic touch-screen experiments. I would like to say a special thank you to my parents, Lynne and David. I am here because of the unwavering support and guidance in life. There are too many people that I should also thank for their support throughout this process, but I cannot fit them all in. So, thank you to the friends who have supported me through conversation or just being there. It is with all these people that made this PhD experience successful and enjoyable. Finally, there are a number of leisure providers and pools that need to be acknowledged. These have been vital for data collection and without their interest and support, access to lifeguard participants would have been incredibly difficult. These were: • Waterfield Leisure Centre • RLSS lifesaving competitions and competitors • Melton Mowbray lifesaving club • Grantham Meres Leisure Centre • Larkfield Leisure Centre • Rutland Water Aqua Park • Gravesham Lifesaving Club • South Charnwood Leisure centre • Rushcliffe Areana • Nottingham City Council Pools 1 Table of contents Abstract........................................................................................................................... 0 Acknowledgements .......................................................................................................... 1 List of Figures ................................................................................................................... 6 List of Tables .................................................................................................................... 9 Chapter 1 ...........................................................................................................................10 1.1 Introduction...............................................................................................................11 1.1.1 The lifeguard role and qualification........................................................ 12 1.1.2 Drowning definitions ........................................................................... 13 1.1.3 Victim recognition ............................................................................... 16 1.2 Theoretical review of Visual Search ..............................................................................17 1.2.1 Saliency ............................................................................................. 20 1.2.2 Scene Context .................................................................................... 23 1.2.3 Templates and attentional set............................................................... 26 1.3 Why are some searches less successful than others?.......................................................30 1.3.1 Target Prevalence ............................................................................... 30 1.3.2 Target-distractor similarity ................................................................... 32 1.3.3 Multiple target search and dual target costs ........................................... 35 1.3.4 Crowding in visual search ..................................................................... 38 1.3.5 Dynamic visual search tasks .................................................................. 40 1.4 Factors that influence expert visual search ....................................................................43 1.4.1 Memory in search ............................................................................... 44 1.4.2 Chunking ........................................................................................... 46 1.4.3 Identification of the target ................................................................... 49 1.4.4 Situational awareness .......................................................................... 50 1.4.5 Multiple object tracking (MOT) and sustained attention ........................... 52 1.5 Research on lifeguards’ visual search skills ....................................................................53 1.5.2 Exploring lifeguard visual search with video stimuli .................................. 57 1.5.3 A new approach to assessing lifeguard skills............................................ 62 1.6 Rationale and Principle Aims of the Thesis.....................................................................63 1.7 Structure of this Thesis ................................................................................................65 Chapter 2 ...........................................................................................................................68 2. Introduction ................................................................................................................69 2.1 Experimental stimuli ...................................................................................................70 2.2 Eye-movements .........................................................................................................87 2.3 Apparatus..................................................................................................................90 2 2.4 Conclusion .................................................................................................................90 Chapter 3 ...........................................................................................................................92 3. Experiment