Dokument

Summary:
This dissertation is devoted to the question of how the healthy human brain generates visual experience of its environment, in its homogenous and coherent nature, given the stream of heterogeneous and incoherent information available to the visual system. Heterogeneity refers to the varying spatial resolution of visual information processing across the visual field (fovea to periphery) and incoherence to disruptions of visual information by fast jerk-like eye movements called saccades. Both of these aspects and their implications are described in the Introduction. Individual approaches and outcomes of four studies, each contributing to the understanding of the issue of visual stability stated above, are outlined subsequently. To gain understanding on whether and how information from before and after a disruptive saccade is integrated into perception, the first study (Study I) investigated whether perception of a stimulus observed across a saccade can be predicted by a statistically optimal integration of pre- and postsaccadic signals. Results revealed that perceptual performance was close to predictions for optimal transsaccadic integration. Integration even seemed to occur when the presented stimulus changed some visual properties during the saccade. As the result of the first study implied that integration of pre- and postsaccadic information is a phenomenon that is robust against visual discrepancies, the question emerged as to what would lead to transsaccadic segregation i.e., a percept of discrepancy between the pre- and postsaccadic information. Driven by the idea that the ability to integrate or segregate information develops over the lifespan, the second study (Study II) aimed to investigate transsaccadic segregation in children compared to young adults. The study showed that children detect stimulus displacements across a saccade less precisely than adults, indicating less transsaccadic segregation at childhood. However, children’s segregation abilities showed a stronger improvement due to the implementation of a perceptual aid (postsaccadic blank) compared to adults. In addition, children made less accurate and less precise saccades than adults but were also faster to correct their saccade landing errors. These results suggest that saccadic uncertainty (expectations about self-induced position errors) play a role in transsaccadic perception. To further determine the principles guiding transsaccadic segregation, the third study (Study III) investigated perception of intrasaccadic shape changes (circularity increase or decrease), and its relationship with shape appearance across the visual field. Results revealed that shape changes where we increased circularity across saccades were more likely to be perceived by participants (than circularity-decrease changes). In addition, shape appeared more circular before a saccade in the peripheral visual field compared to after a saccade in the fovea. These results suggest the existence of a predisposition to detect shape changes opposite (circularity increase) to the typical transsaccadic experience (circularity decrease). This gives further support to the assumption that expectations regarding transsaccadic contingencies play a key role in the ability to detect intrasaccadic changes. The fourth study (Study IV) turned towards the issue of how presaccadic visual stimulation affects postsaccadic perception and investigated the effect of short-term luminance adaptation before a saccade on contrast perception after the saccade. Results revealed that postsaccadic perception can be altered by presaccadic adaptation during very short durations corresponding to natural fixation durations. To conclude, transsaccadic perception is determined by the integration or segregation of pre- and postsaccadic information. Study I revealed that transsaccadic integration can occur despite large intrasaccadic stimulus changes. Studies II and III suggest that transsaccadic segregation depends on typical transsaccadic experience. Study IV showed that transsaccadic perception is likely to be affected by basic aspects of visual information processing such as adaptation. Taken together, this dissertation suggests that the visual system has developed statistically optimal and predictive mechanisms for heterogeneous and incoherent information to result in a coherent and adaptable perception of the environment.

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