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Abstract

In recent years the view has been advocated that the motor system is immune to visual size illusions. This is regarded as a consequence of a fundamental division of labor in the primate brain: vision for action versus vision for perception. We tested this claim for the Mueller-Lyer illusion and the Parallel-Lines illusion. Both illusions clearly affected grasping. The effects on grasping were similar though not perfectly equal to the effects on perception. We present evidence that these small differences are due to problems in matching the perceptual and the motor tasks. We argue that grasping is an inherently unipolar measure because only one object can be grasped with the same hand at a time, while perceptual measures usually are bipolar because they require a comparison of two objects. We show that this difference is a central problem in matching the perceptual and the motor task.