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Junghyun Park, Shinsuke Shimojo, John Schlag; Distorting visual space without motion signal. Journal of Vision 2005;5(8):201. doi: 10.1167/5.8.201.
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Studies have shown that the perceived position of flashed objects is distorted in the presence of stimulus or eye motion: e.g., flash-lag effects (Nijhawan 1994; Schlag et al. 2000), presaccadic mislocalization and compression (Cai et al. 1997; Burr et al. 1997), and position capture (Whitney & Cavanagh 2000). Here we demonstrate that, even in the absence of such motions, the perceived position of flashed objects can be systematically distorted depending on the timing of their presentation with respect to other objects in visual field. When a target object is flashed at the offset of another object (here called ‘inducer’), the perceived position of the target object shifts toward the center of gaze, regardless of the number of inducers and their location.
Observers judged the relative alignment of two vertical lines separated by a short gap. One line was continuously present at 8 deg right to the central fixation point, the other was briefly flashed at least 1s after the onset and 1s before the offset of the continuous line. At various times around the line flash, four small disks grouped around the lines were presented: either 1) continuously, 2) from the trial onset to line flash, 3) from the line flash to the end of the trial, or 4) flashed simultaneously with the line.
We found that when the line was flashed at the extinction of the disks (i.e., conditions 2 & 4), the perceived position of the line shifted toward the center of gaze. The perceived shift was several times larger than the baseline tendency of compression (Mateeff & Gourevich 1983) obtained in control trials with no disk. In other experiments, the stimulus offset asynchrony (SOA) between the flashed line and disks, the number of disks and their location relative to the line were varied. The perceived shift was strongest at SOA = 0ms and disappeared with SOA > 100ms. Although the perceived shift was affected to some extent by the spatial configuration of the disks, its direction was always toward the center of gaze.
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