Target template guidance of eye movements during real-world search
Malcolm, George Law
Humans must regularly locate task-relevant objects when interacting with the world around them. Previous research has identified different types of information that the visual system can use to help locate objects in real-world scenes, including low-level image features and scene context. However, previous research using object arrays suggest that there may be another type of information that can guide real-world search: target knowledge. When a participant knows what a target looks like they generate and store a visual representation, or template, of it. This template then facilitates the search process. A complete understanding of real-world search needs to identify how a target template guides search through scenes. Three experiments in Chapter 2 confirmed that a target template facilitates realworld search. By using an eye-tracker target knowledge was found to facilitate both scanning and verification behaviours during search, but not the search initiation process. Within the scanning epoch a target template facilitated gaze directing and shortened fixation durations. These results suggest that target knowledge affects both the activation map, which selects which regions of the scene to fixate, and the evaluation process that compares a fixated object to the internal representation of the target. With the exact behaviours that a target template facilitates now identified, Chapter 3 investigated the role that target colour played in template-guided search. Colour is one of the more interesting target features as it has been shown to be preferred by the visual system over other features when guiding search through object arrays. Two real-world search experiments in Chapter 3 found that colour information had its strongest effect on the gaze directing process, suggesting that the visual system relies heavily on colour information when searching for target-similar regions in the scene percept. Although colour was found to facilitate the evaluation process too, both when rejecting a fixated object as a distracter and accepting it as the target, this behaviour was found to be influenced comparatively less. This suggests that the two main search behaviours – gaze directing and region evaluation – rely on different sets of template features. The gaze directing process relies heavily on colour information, but knowledge of other target features will further facilitate the evaluation process. Chapter 4 investigated how target knowledge combined with other types of information to guide search. This is particularly relevant in real-world search where several sources of guidance information are simultaneously available. A single experiment investigated how target knowledge and scene context combined to facilitate search. Both information types were found to facilitate scanning and verification behaviours. During the scanning epoch both facilitated the eye guidance and object evaluation processes. When both information sources were available to the visual system simultaneously, each search behaviour was facilitated additively. This suggests that the visual system processes target template and scene context information independently. Collectively, the results indicate not only the manner in which a target template facilitates real-world search but also updates our understanding of real-world search and the visual system. These results can help increase the accuracy of future realworld search models by specifying the manner in which our visual system utilises target template information, which target features are predominantly relied upon and how target knowledge combines with other types of guidance information.