Purpose: Previously, we reported effects of the Titchener / Ebbinghaus illusion on grasping (Franz et al., 2000). These contradict a strong version of the action versus perception hypothesis (Milner & Goodale, 1995) which states that the motor system is unaffected by visual illusions. Here, we test whether our grasp effects were artifacts (i.e. generated by non-perceptual mechanisms). This could be the case if the motor system treated the illusion inducing context elements as obstacles and tried to avoid them. To test for this possibility, we varied the distance between context elements and target. Methods: An aluminum disc (31, 34, or 37 mm in diameter, 5 mm in height) was positioned as target on a board. Around the target either small or large context discs (10 or 58 mm in diameter) were drawn at near or far distances (24 or 31 mm midpoint to nearest point on context circles). Close to the board a monitor was mounted on which a comparison disc was displayed. In the perceptual task 52 subjects adjusted the size of the comparison stimulus to match the size of the target. In the grasping task subjects grasped the target. Subjects wore shutter glasses and could not see their hand during grasping. The grasp trajectory was recorded and the maximum preshape aperture was calculated. Results: The motor illusion responded to the variation of distance between context elements and target in exactly the same way as the perceptual illusion. None of three different obstacle avoidance hypotheses can explain these results. Conclusions: Our results suggest that the same signals are responsible for the perceptual and for the motor illusion. This either indicates that the action versus hypothesis needs modification, or that the Titchener illusion is generated before the separation of the perceptual and the motor streams.
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