Saliency-Driven Oculomotor Control

The research presented in this thesis unravels the effects of distractors on saliency-driven oculomotor control through a multidisciplinary approach using eye tracking and electrophysiology. The ability to respond within milliseconds to visual information is remarkable and even more so when multiple objects compete for our attention. The main question addressed in this thesis is what drives the eye to the next landing position. The competitive integration model is used as a framework to interpret these findings. In two parts, the spatial (i.e., oculomotor capture effect, global effect, saccade curvature) and temporal effects (i.e., remote distractor effect, minimal visual saccadic response time, fixation duration) of distractors on saliency-driven oculomotor control are described. Part I, ‘Exploring the Basics’, deals with the fundamental properties of distractors that influence oculomotor selection and how this is resolved when making an eye movement to a target. Part II, ‘When response times decrease’, deals with evoking and studying these extremely rapid visual responses known as express saccades. Although these eye movements form an integral part of our visual behavior, they are difficult to trigger intentionally. Notably, compared to regular saccades, these express saccades are less studied. Advancements in eye tracking technology have increased the accessibility of eye tracking immensely and open many opportunities as a non-invasive low-impact (and increasingly cheap) tool to investigate eye movement behavior, including express saccades, as the portal to understanding brain processes. The research in the eight chapters of this thesis sheds light on oculomotor selection and the underlying neural correlates, demonstrating that it is both complex and adaptive. It is demonstrated that oculomotor control processes change under the influence of bottom-up factors such as luminance contrast, color or through the integration of other senses. Critically, it is shown that top-down control can have an early effect on oculomotor selection. Moreover, it is shown that express saccades (which are often disregarded or considered irrelevant) are an integral part of eye movement behavior providing insight into neural processing mechanisms at a behavioral level. This notion is consistent with emerging research that finds strong associations between saccadic reaction times (including express saccades) and multiple other cognitive domains. Finally, in the laboratory, free viewing tasks are often used as an alternative for real world viewing. The research in this thesis can push human behavioral research into oculomotor selection in more dynamic and complex environments forward. The strong correlation between neural activation in the superior colliculus, fixation duration as its behavioral counterpart, and saliency at the saccade goal allows a cross-species comparison in order to understand oculomotor selection in the complex rich dynamic environment that we live in.Based on the cross-species comparison between monkey and human free viewing behavior, fixation duration can be a valid behavioral indicator of neural processing in the SC. The results in this thesis reveal a possible link between express saccades and (consecutive) short fixations.