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Eric Gamphe, Paul Gamlin; The Duncker illusion affects the perception of targets moving in depth. Journal of Vision 2003;3(9):860. doi: 10.1167/3.9.860.
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The trajectory of a target moving across a wide-field, coherent motion stimulus is incorrectly perceived as including a directional component opposite to that of the background flow field (Duncker, 1929). This induced motion illusion has been shown to perturb both saccades and hand movements to remembered targets (Zivotofsk et al., 1996; Soechting et al., 2001), and to affect the perceived initial position of a target (Sheth and Shimojo, 2000). Since, to our knowledge, all previous studies had investigated the Duncker illusion for stimuli during frontoparallel motion, we investigated the effect of this illusion on targets moving in depth.
Experiments were conducted to quantify the illusion strength, and to determine the effect of flow field speed. Using stereo shutter glasses five subjects viewed dichoptic images on a monitor. A vertically moving flow field was presented in the plane of the screen while targets moved in depth over a range of ±2 of disparity, with a vertical component that ranged between ±10 . Control trials consisted of either no flow field motion or non-coherent motion. Flow field direction reversed half way through each trial. Subjects reported the perceived tilt of the target trajectory as up or down. Responses at the different trajectories and flow field speeds were graphed, and the strength of the illusion was determined by comparing the offsets between curves obtained under experimental and control conditions.
The results demonstrate that the Duncker illusion influences target motion-in-depth. The illusion created a significant shift in perceived target trajectory ranging from 2.5 to 12.5 of vertical tilt for the subjects. Speed of the flow field affected the strength of illusion in a non-linear fashion, with reduced influence at both low and high speeds.
We conclude that the trajectory of a target moving in depth can influenced by background flow field motion.
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