The role of attention in position encoding

Contrary to both our subjective experience of a stable visual space and to our knowledge about retinotopic organization in the brain, the perception of spatial position can be distorted by motion and eye movements, suggesting that positional representations also integrate sources of information other than retinal location. Despite its critical involvement in both motion perception and eye movements, the role of visual attention in positional encoding is poorly understood. In this thesis, by focusing on the phenomenon of position distortion, I examine the influence of attention on the visual representation of space. The thesis consists of three sets of studies, the results of which suggest that visual attention contributes to position distortion, independent of low-level motion and eye movement.; The first section shows that the high-level percept of motion can modulate the position shift, even when the low-level spatio-temporal properties of the motion stimulus remain constant, by using an ambiguous motion stimulus. As the perceptual organization of motion changed, the locations of the perceived illusory position shifts varied. The second section shows that the perceived position of the flashes is strongly modulated by attention, using an attentive tracking paradigm, which allows explicit control over the locus of attention. In terms of its characteristics, the position distortion induced by attentive tracking closely resembles those induced by motion and eye movements. The last section demonstrates bi-directional illusory position shifts toward the end point of apparent motion. Such position compression induced by a shift of attention from a start to an end point of apparent motion mirrors saccadic compression---the apparent shift of an object presented around the time of saccades toward the saccade target. This result suggests that saccadic compression may be driven by attention preceding the eyes to the target.; Collectively, these findings suggest that attention is an integral part of a common mechanism underlying the motion- and eye movement-induced position distortions. This discovery of an independent role of attention in position encoding contributes to a better understanding of how the brain represents the position of objects in visual space, and, in so doing, how it integrates information from multiple mechanisms.