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Daw-An Wu, Patrick Cavanagh; Where are you looking? Pseudogaze in afterimages. Journal of Vision 2010;10(7):560. doi: 10.1167/10.7.560.
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© ARVO (1962-2015); The Authors (2016-present)
The point in the visual scene that lands on the center of the fovea is assumed to define where we are looking – our direction of gaze. To test this, we asked subjects to “shift their gaze” to different locations in an afterimage. Once subjects had fixated a dim red laser point in a dark room, a strong flash illuminated a matte stimulus. The fixation point was then extinguished, leaving the afterimage as the only visual input. When subjects were asked to fixate points in the far periphery of the afterimage, they reported that the image jumped away in the direction of the attempted gaze shift. For points in the near periphery, however, subjects reported “fixating” them without causing any perceived motion of their afterimage. The region within which gaze could be shifted was generally limited to 2-4 degrees from true center, depending on the subject. Eye tracking data revealed constant movements of the eye, which the subjects were unaware of. During “fixation” of the central point of the afterimage, these drifts were random. When they set their gaze on a point within 2-4 degrees of the center, an additional, systematic component to the drift was often produced, in the same direction as the intended offset in gaze. Finally, when they fixated a point to the left of fovea and then attempted a saccade to a point directly below the fovea, some subjects' eyes moved diagonally in accordance with their subjective feeling, while other subjects' eyes moved vertically, in accordance with the target's actual position relative to fovea. These results suggest that the apparent direction of gaze can be flexibly assigned to an attended object near the fovea allowing visual coordinates to remain centered on a steady location in the world, despite the incessant small eye movements of fixation.
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