Download or read book Motion and Position Interact at Both Early and Late Stages of the Human Visual System written by Peter J. Kohler and published by . This book was released on 2013 with total page 180 pages. Available in PDF, EPUB and Kindle. Book excerpt: The ability to correctly determine the position of objects in space is fundamental to any visual system. For an animal to successfully engage with a dynamic and complex spatial environment it must be able to encode not only the identity of the objects in a visual scene, but also where those objects are. In the human visual system, multiple regions are organized in topographic maps, where locations on cortex have direct correspondence with locations in the visual field. In principle, the position of an object could simply be encoded by the location of the neural activity in one or more of these cortical maps. The task turns out not to be so simple, however, because motion signals can cause the perceived position of stationary objects to deviate from their actual position in the world. A wide variety of these illusory motion-induced position shifts have been demonstrated over the years, beginning with a demonstration by Fröhlich (1923) that the starting position of a moving object appears to be shifted along its trajectory. The goal of this thesis is to investigate the stage in visual processing at which the interaction between motion and position encoding takes place, as well as the implications of this interaction for position encoding in general. To do this, we combined a version of the motion-induced position shift known as the flash grab effect (Anstis & Cavanagh, 2012; Anstis & Cavanagh, in press), with a bistable stimulus in which global motion percepts deviate from local motion signals measured at the moving edge. Specifically, we used psychophysics and functional neuroimaging (fMRI) to (a) measure the influence of local and global motion on motion-induced position shifts, (b) investigate the effects of local and global motion on position over time, and (c) identify regions in visual cortex that are responsible for coding perceived rather than physical position. Our psychophysical results demonstrate that both local and global motion influence motion-induce position shifts. This suggests that motion signals arising at both early and late stages in visual processing make independent contributions to these effects. Surprisingly, our fMRI data show that primary visual cortex encodes shifts in perceived position. Taken together, these studies present strong evidence for a multi-stage model of the interaction between motion and perceived position, and suggest that perceived position is encoded as early as primary visual cortex.