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Robust Size Illusion Produced by Expanding and Contracting Flow Fields

34.26, Sunday, 17-May, 2:30 pm - 4:15 pm, Talk Room 2
Session: Motion Perception: Biological motion and motion in depth

Xue Dong1, Min Bao1; 1Key Laboratory of Behavioral Science, Institute of Psychology, Chinese Academy of Sciences, Beijing, P.R. China

When observing a moving object, its position appears shifted in the direction of motion. This illusion, called the motion induced position shift, has been studied since the 1990s. But here we introduce an illusion that shows the opposite effect. Randomly positioned dots moved radially within an imaginary annular window centered on a mid-gray background. The dots’ motion periodically changed the direction, leading to an alternating percept of expanding and contracting motion. Strikingly, the apparent size of the inner annular boundary shrank during the dots’ expanding phase and dilated during the contracting phase. Size matching measurements revealed about a 10% difference in diameter between the perceived sizes for expanding and contracting motion. We also tested whether the illusion depends upon a global percept of 3D motion, rather than simply the presence of energy at the local kinetic edge. We divided the annulus in half and rendered the dots within each half annulus to move in opposite directions. The local account predicts that this should cause the two halves of the circle defining the annulus’ inner edge to appear different sizes. However, subjects reported no difference in size between the two halves, and instead reported illusory lateral motion of the circle. Adding a texture inside the circular region weakened but did not eliminate the illusion, suggesting that the uniform gray region was not crucial. To test whether the illusion mainly arose from a monocular processing stage, we measured it when the dots were presented either monocularly or dichoptically, and found stronger illusion for the dichoptic condition. Combining these findings, our observed illusion is likely induced by a global percept of motion in depth controlled by higher level mechanisms. Expanding dots are consistent with self-motion that approaches the circle in depth. Given its constant retinal size, this may make the circle appear smaller.

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