Role of object semantics in guiding overt attention of younger and older adults, and AD patients
For decades, vision research has investigated the influence of object semantic information on visual attention by examining eye movements. However, it has provided contrasting evidence about the extent to which object semantics can be processed in the extra-fovea and guide gaze, as well as about the temporal dynamics of such processing. Moreover, it is still unclear whether and how healthy and pathological ageing impairs the ability to process semantic information and to use it for attentional guidance. Finally, how the high-level semantic features of objects interact with their low-level visual features, e.g., visual saliency, in guiding eye movements and how their interplay changes according to the type of task performed are still debated. Six experiments in this thesis contribute novel findings to this discussion by examining the influence of object-to-object semantic relatedness, as well as visual saliency, on behavioural and eye-movement responses of different populations of participants, i.e., young adults, healthy older adults, and people with Alzheimer’s disease (AD), while they inspect objects arrays during a visual search task and a memorization task. In Chapter 1, I review the relevant research literature, whereas in Chapter 2, I present the methodology used throughout the experiments of this thesis. Chapter 3 describes three visual search experiments carried out with younger (Experiments 1 and 2) and healthy older participants (Experiment 3). I found that younger and older adults looked earlier and for longer at a critical object when it was semantically unrelated than related to the other objects in the array, both when it was the search target (target-present trials) and when it was a target’s semantically related competitor (target-absent trials). For the younger adults only, semantic relatedness effects manifested already during the very first fixation after array onset. I conclude that the semantic information of objects can be processed in extra-foveal vision so early to guide initial eye movements. Older adults process object semantics in the extra-fovea to orient gaze as younger adults do, although there might be age-related differences in the time-course of such processing. In Chapter 4, I used the same visual search task to compare the eye-movement behaviour of AD patients and healthy age-matched controls (Experiment 4). I found that both groups looked at the critical object earlier when unrelated than related to the other objects, with no difference among groups. In contrast to previous findings from the literature pointing at a complete disruption of semantic memory in AD, I argue that object semantic processing can be preserved in AD patients in specific circumstances. Visual saliency did not affect eye movements during any of the visual search experiments. This finding is consistent with previous studies showing that saliency plays a marginal role in attentional control during search, while exerting a stronger influence during non-cued visual tasks, such as memorization. In Chapter 5, two experiments monitored the eye movements of younger (Experiment 5) and older adults (Experiment 6) who were asked to freely inspect the same object arrays, in preparation for immediate verbal recall. I found that, in both younger and older adults, the critical object was more likely to be recalled when semantically related than unrelated to the distractors. The effect of semantic relatedness on memory was explained by differences in the allocation of overt attention, with semantically related critical objects being more likely to be fixated than semantically unrelated ones, although in older adults only, other cognitive mechanisms seemed to be involved, e.g., semantic priming. Visual saliency had a significant effect neither on memory nor on eye movements, presumably because of methodological limitations of the experimental design. All in all, the findings of this thesis pose a challenge to current models of visual attention by highlighting the primary role of extra-foveal semantic processing in guiding visual attention, and provide evidence that such processing is preserved in healthy ageing and AD. By influencing oculomotor behaviour, semantic information strongly affects our ability to search for objects and/or to store them in memory.