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Makoto Ichikawa, Yuko Masakura; The connection of visual stimulus with observer's voluntary motion affects the flash-lag effect.. Journal of Vision 2004;4(8):581. doi: 10.1167/4.8.581.
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The flash-lag effect has been assumed to be a consequence of a compensatory function for physiological delay of the visual processing. In order to examine how observer's voluntary control of the moving stimulus affect this compensatory function, we compared the extend of the flash-lag effects for three conditions that differ from each other in the degree of the observer's voluntary control of the stimulus movement. In the first condition, the position of the moving stimulus (19.1 × 19.0 arc min) was determined by the position of computer-mouse controlled by observers. In the second condition, the stimulus moved automatically without observers mouse control. In the third condition, until the-mid point of the moving distance, the stimulus position was determined by the computer-mouse, and then the stimulus moved automatically. The velocity of the automatic movement of the stimulus in the second and third conditions was determined by the average of the stimulus movement in the first condition. In these three conditions, the stimuli moved downward or upward by 28.8 arc deg. The flash stimulus (19.1 × 19.0 arc min) was presented beside the moving stimulus when the stimulus was at seven tenth of the moving distance. The vertical position lag between the flash and moving stimuli ranged from −76.0 to 76.0 arc min by 19.0 arc min step (negative value indicates that the position of the flash stimulus was behind of the moving stimulus). Observers judged whether the moving square was below or above the flash stimulus with fixating a red point at the center of the display in each trial. In the averaged data from eight observers, the extents of the flash-lag effect in the first and third conditions were significantly smaller than that of the second condition. There was no difference between the first and third conditions. These results suggest that the voluntary control of the stimulus movement has effects to inhibit the compensatory function that underlies the flash-lag effect.
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