Eye Movement Trajectories and What They Tell Us
Stigchel, S. van der
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The present thesis is concerned with how our eyes move trough space. Since the pioneering work of Yarbus it is known that the trajectories of the eyes can tell a great deal about the underlying cognitive processes. The current thesis is not just concerned with eye movement trajectories in general but with one specific aspect of these trajectories. It was already Yarbus (1967) who noticed that our eyes do not travel in a straight line, but rather in a curved trajectory. These saccade curvatures turned out to be not just coincidental. In the last two decades, research has revealed the specific conditions that may generate deviations in saccade trajectories. In some conditions, the eyes deviate towards a particular visual stimulus; in other conditions, the eyes deviate away from it. Moreover, the mere observation that in some conditions eye movements deviate towards and sometimes away would not be so exciting if the source of the saccade trajectory deviations could not be attributed to competition occurring the superior colliculus (SC), a midbrain oculomotor structure involved in encoding stimuli as potential saccade targets. Saccade deviations give us a direct view on how the SC solves the competition between different possible target locations. Because the SC receives inputs from bottom-up visual areas as well as from classic top-down frontal areas, examining saccade deviations give us the opportunity to examine the time course of bottom-up and top-down activation and inhibition in the brain. It is generally agreed that the eyes deviate away from a visual stimulus when the location of the stimulus is actively inhibited in a top-down fashion. This occurs in conditions in which a saccade needs to be executed to a particular target goal while a nearby distractor is present. In these situations, eye movements typically deviate away from the distractor location. Deviation towards a visual stimulus is attributed to residual (exogenous) activation at the location of the distractor. For example, when executing a saccade to a target location, the exogenous activation at the location of the distractor may result in the merging of the populations of activity encoding the target and distractor location. The present thesis shows that when executing a saccade towards a target location, the exact location of a nearby distractor plays an important role (Chapter 2 and 3). When a distractor is presented close to the target, one typically observes deviations towards the distractor, whereas a distractor presented close to fixation evokes deviations away from the distractor. Because our results further show that distractors presented at smaller vertical distances from fixation evoke larger amounts of inhibition than at larger vertical distances, these results indicate that the inhibition is not coarsely coded, but fine-grained and sensitive to the vertical distance of the distractor from fixation. In another study, we used saccade trajectories to examine in great detail the relationship between attention and eye movements (Chapter 4 and 6). Even though previous research already established the close relationship between eye movements and attention (Corbetta, 1998; Rizzolatti et al., 1987; Rizzolatti et al., 1994), we show that the oculomotor system is also involved in tasks that basically only require the covert allocation of attention. While executing a classic Posner cueing task, covertly discriminating between two letters, we show that the eyes deviate away from validly and invalidly cued locations. It appears that the oculomotor system is activated wherever spatial attention is allocated. Even though previous research clearly demonstrated that the presence of a distractor can cause saccade deviations, in several studies we show that the mere expectation that a target or a distractor will be presented at a particular location may also influence saccade trajectories (Chapters 5 and 7). This finding is confirmed by an ERP study showing that cueing the distractor location in advance evokes active inhibition at the distractor location (Chapter 8). In the final part of this thesis, we used saccade trajectories to investigate neurological problems. In one study we examined whether distractors presented in the blind field of patients with visual hemifield defects still influence the oculomotor system (Chapter 9). These patients have a damaged retinogeniculostriate visual pathway but preserved retinotectal projections. Because the retinotectal pathway projects from the retina to the SC, deviations in saccade trajectories might reflect residual retinotectal processing. Indeed, we show that distractor effects can occur in these patients, although not consistently. Finally, we examined saccade trajectories in children with Attention-Deficit/Hyperactivity Disorder (ADHD) (Chapter 10). It is generally thought that deficits in response inhibition form an important area of dysfunction in patients with ADHD. Unlike common belief, our results show that children with ADHD do not have problems inhibiting task-irrelevant distractors. We speculate that patients with ADHD only have trouble inhibiting information when this inhibition requires the active involvement of working memory. In our task, no active inhibition was necessary and therefore patients with ADHD did not show any problems with inhibiting. To conclude, modifications of saccade deviations can inform us about the mechanisms that control and influence eye movements. They can therefore be applied to a wide variety of topics like response inhibition, attention, top-down expectancy and clinical investigations. Because they reflect the integration of visual and task related processes in the oculomotor system, they are a useful tool to obtain important information that is not easily obtained via other sources.