Top-down and exogenous effects on covert and overt orienting

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Due to numerous bottlenecks, the human brain is unable to consciously process all data \navailable at the retina. To overcome these constraints, evolution has developed a system \nthat breaks down retinal information into fragments and subsequently analyses them \naccording to current goals and expectations. This biasing system is frequently referred \nto as attention. Yet despite a long history of itself having been the focus of analysis, \nthere are a number of questions about attention that are clearly unanswered by the \nliterature. Therefore, we wanted to address three problems highlighted by our literature \nreview. Specifically, we wanted learn, (I) Are the effects of probabilistic expectations, \nwhen instructed either by spatial blocking of the target location or through a central cue, \non response latencies the product of a ballistic, attentional process, or the product of an \ninformation theoretical decision-making process? (II) Can the inhibitory aspects of \nspatial attention be pre-deployed by using a central cue to manipulate prior expectations \nof where a task-irrelevant distractor is likely to appear? (III) What is the relationship \nbetween attention and eye movements?We investigated this last question by way of testing healthy participants on \ncovert and overt versions of the behavioural paradigms designed to address questions I \nand II, and then in a neuropsychology patient who presented with hypometric saccades, \nwe investigated if eye movements and attention can be dissociated. Experiments 1-4, \nshowed that the effects of target probability - when either spatially manipulated or \ninstructed through a central cue - can neither be fully accounted for by attentional \naccounts or information theoretical accounts. Additionally, the outcome of target \nprobability is context dependent. That is, outcomes depend on how target probability \nwas instructed. Experiment 5 showed that spatial inhibition cannot be endogenously \ndeployed using central cues. Although we found that distractor suppression takes place \nwhen targets are invalidly cued, suggesting distractor suppression takes place during \nreorienting. Experiments 6-7 showed that attentional orienting can be preserved in the \npresence of oculomotor impairment, indicating eye movements and attention can be \nstructurally dissociated. Whereas the results of experiments 1-5 are consistent with \nclaims that covert and overt orienting are similarly affected by expectations due to a \ncommon attentional process. We conclude that expectations influence a mechanism \ncommon to overt and covert responses, but ultimately, both processes are distinct. \nIn the discussion chapter, we discuss a number of future avenues of research,including how electrophysiology could be used to further understand the phenomena \npresented here. Overall, the contribution of this body of research is to illustrate that the \nrelationship between top-down expectations and exogenous effects is extremely \ncomplicated, and are, currently, inadequately captured by present models of attention

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