Perceptual Salience and Its Consequences on Attentional Object Selection

This thesis investigated the effects of salience on visual object selection, and focused on whether salience impacts the visual system’s ability to process multiple items at the same time (parallel selection) or one-at-a-time (serial selection). Chapter 2 consists of two experiments that used an ERP component, the N2pc, to track the deployment of attention in a visual search task with highly dissimilar distractors. Although the time to find the target increased with the number of distractors, observers were able to select the target at the same time, regardless of the size of the search display. This suggests that task relevance can make otherwise non-salient items “pop out” at the level of attentional selection. Chapter 3 comprises a single experiment in which attentional selection was measured overtly, using eye tracking, as observers inspected and compared two singletons of differing salience. Discreet eye movements were made from one singleton to the other and the order of inspection was strongly biased by target salience, with the initial saccade being made to the more salient singleton on the vast majority of trials. This suggests that, in the absence of top-down control, the order of attentional selection is dictated by salience. Finally, Chapter 4 consists of three variants of the same two-singleton search task used in Chapter 3. The first experiment replicated Chapter 3 but with attentional selection tracked covertly via the N2pc. In the next two experiments, task parameters were manipulated to encourage slower shifts of attention from one singleton to the other and to encourage the initial inspection of the less salient singleton. Attentional object selection was purely serial in some cases and partially parallel in others. The biasing effect of salience could also be subverted, such that the less salient item was selected first, however that item was not selected as rapidly as the more salient item. Chapter 4 thus reveals that (i) the earliest time at which an item becomes available for attentional selection depends on its relative salience, and (ii) the speed of attentional redeployment varies with the nature of the response required.