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Abstract

In previous studies, we found effects of the Ebbinghaus (or Titchener) illusion on grasping. This contradicts the notion that the motor system uses visual transformations which are (a) different from the perceptual transformations and (b) unaffected by visual illusions [Milner and Goodale, 1995 The Visual Brain in Action (Oxford: Oxford University Press)]. Here, we tested whether the grasp effects are generated independently from the perceptual illusions. This could be the case if the motor system treated the illusion-inducing context elements as obstacles and tried to avoid them. To test this hypothesis, we varied the distance between context elements and target. Aluminum discs (31, 34, or 37 mm in diameter) were surrounded by small or large context circles (10 or 58 mm in diameter) at one of two distances (24 or 31 mm midpoint target disc to nearest point on context circles). In the perceptual task, fifty-two participants adjusted the size of a comparison stimulus to match the size of the target disc. In the grasping task, participants grasped the target disc. The trajectories were recorded and the maximum grasp apertures determined. The motor illusion responded to the variation of distance between context elements and target disc in exactly the same way as the perceptual illusion. This suggests that the same neuronal signals are responsible for the perceptual and for the motor illusion.