Quantifying the contributions of spatial attention and saccade preparation to perception: insights from evidence accumulation modelling

Human visual perception is the result of a complex interaction between eye movements and shifts in spatial attention. Despite a vast amount of research investigating the relationship between these two types of orienting, it remains unclear to what extent and in what circumstances spatial attention and saccade preparation are obligatorily coupled. The goal of this thesis was to use a novel methodological approach, evidence accumulation modelling, to address this longstanding question. Across five empirical chapters a saccadic dual-task was combined with evidence accumulation modelling. Using the drift rate parameter as a direct and comparable metric of orienting, we quantified the contributions of spatial attention and saccade preparation across blocked conditions and distinct tasks. Our goal was to shed new light on the relationship between each type of orienting and their underlying mechanisms. In Chapter 2 of this thesis we modified a typical saccadic dual-task to establish a paradigm that would be compatible with evidence accumulation modelling. In Chapter 3, drawing upon this methodology, we quantified the relative contributions of spatial attention and saccade preparation to the same dual-task. Spatial attention was found to influence performance away from the saccade goal, while the relative magnitude of this effect varied by cue type. Chapter 4 extended this computational approach by comparing the contributions of each type of orienting across two distinct tasks, cardinal and oblique orientation discrimination. The magnitude of both the saccade preparation and spatial attention effect was smaller for oblique relative to cardinal orientations. Chapter 5 investigated the possibility that there was not only a quantitative but also a qualitative dissociation between each type of orienting. Spatial attention and saccade preparation were measured across two tasks that differed in acuity demands and four conditions that modulated eye movement preparation and fixation. The results revealed a remarkably consistent pattern of dissociation between the effects of spatial attention and saccade preparation. The saccade preparation effect was found to vary by acuity, while the cueing effect did not. Furthermore, the cueing effect was found not to be affected by the preparation or fixation of eye movements. These findings challenge the notion that each type of orienting is obligatorily coupled and are consistent with the suggestion that spatial attention and saccade preparation are mediated by distinct underlying mechanisms. In Chapter 6 we sought to validate this conclusion with a model-independent research methodology. The texture segmentation task was used as a tool to investigate and compare the perceptual consequences of spatial attention and saccade preparation for the first time. In the final chapter of this thesis we summarise the key findings and discuss their implications for our understanding of visual orienting. Ultimately this thesis provides compelling and consistent evidence to suggest that spatial attention and saccade preparation are not only dissociable, but likely mediated by independent mechanisms.