A Double Dissociation Between Perception and Action Using Sander's Parallelogram: Demand Characteristics Come Up Short

摘要

According to the Interface Theory of Perception, perceptual representations reflect evolutionary entrainment to utility rather than reality. In contrast to perception, object-directed action involves physical interaction which can be harmful or beneficial to the agent. Thus, actions need to be accurate and the representations that underlie them need to be veridical. Arguably, utility and physical reality are perfectly aligned for action. Consistent with this view, a number of studies have shown that object-directed actions resist pictorial illusions. Nevertheless, this work has largely ignored the fact that one's belief about the properties of a stimulus can contradict moment-to-moment visual experience. It is therefore possible that actions resist illusions because they are guided by veridical beliefs whereas perceptual estimations are guided by 'fooled' visual experience. In short, demand characteristics might be responsible for dissociating action and perception. Here, we tested this possibility directly. We asked participants to manually estimate the length of different 3D-target bars embedded in Sander's Parallelogram illusion or to reach out and pick them up. Critically, we positioned the bars such that the physically-shorter ones could appear longer than their physically-longer counterparts. This arrangement served a dual role, pitting perception and action against one another while reducing demand characteristics; pictorial displays that can reverse the apparent difference in target length are rarer than the more 'traditional' displays which induce an apparent difference in target length when no difference exists. The results revealed a double dissociation: grasps reflected the real difference in target length whereas manual estimates reflected the illusory difference. At the end of the study, participants were asked whether or not they believed the targets differed in physical length. A large majority affirmed a difference existed, indicating that the difference was in the direction of the illusion. Thus, demand characteristics based on belief cannot explain the perception-action dissociation.