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Christine Gamble, Joo-Hyun Song; Distinct patterns of size-contrast illusion effects in reaching and grasping movements. Journal of Vision 2014;14(10):408. doi: 10.1167/14.10.408.
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Past studies using size-contrast illusions to dissociate the dorsal and ventral streams and examine their roles in visually-guided action have focused on different types of movements, and yielded contradictory findings on the influence of perception on action. Here, we examine how the Ponzo illusion influences visually-guided reaching and grasping movements under the same conditions. In our Ponzo illusion paradigm, two identical circles were presented on the left and right sides of a screen with converging lines on one side causing the "inner" circle to appear larger than the "outer" circle. To assess individual susceptibility to this illusion, participants used a keyboard to adjust the sizes of outer target circles to perceptually match inner reference circles, consistently making outer targets larger than inner references in accord with the illusion. Using the adjusted circle size for each participant, we created a condition in which the two circles were perceptually the same size, but physically different (perceptual size match) in addition to the aforementioned standard illusion condition (physical size match.) We also included two control conditions reflecting the different physical sizes in the experimental conditions, but with parallel lines that fail to produce an illusion. We recorded movement time (MT) and maximum grasp aperture, respectively, during reaching and grasping movements, and calculated a difference in MT and aperture between the two circles in each of the four conditions. We then correlated this difference in the physical size match and perceptual size match conditions with the difference between the physically different controls. We found that perceptual rather than physical size determines MT in reaching, whereas physical size primarily influences grasp aperture. However, grasp aperture also appears to be impacted by perceived size to a lesser extent. Thus, the patterns of size contrast illusions' influence on action are inherently different for reaching and grasping movements.
Meeting abstract presented at VSS 2014
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