A double dissociation between action and perception in bimanual grasping: evidence from the Ponzo and the Wundt-Jastrow illusions

Abstract

Research on visuomotor control suggests that visually guided actions toward objects rely on functionally distinct computations with respect to perception. For example, a double dissociation between grasping and between perceptual estimates was reported in previous experiments that pit real against illusory object size differences in the context of the Ponzo illusion. While most previous research on the relation between action and perception focused on one-handed grasping, everyday visuomotor interactions also entail the simultaneous use of both hands to grasp objects that are larger in size. Here, we examined whether this double dissociation extends to bimanual movement control. In Experiment 1, participants were presented with different-sized objects embedded in the Ponzo Illusion. In Experiment 2, we tested whether the dissociation between perception and action extends to a different illusion, the Wundt-Jastrow illusion, which has not been previously used in grasping experiments. In both experiments, bimanual grasping trajectories reflected the differences in physical size between the objects; At the same time, perceptual estimates reflected the differences in illusory size between the objects. These results suggest that the double dissociation between action and perception generalizes to bimanual movement control. Unlike conscious perception, bimanual grasping movements are tuned to real-world metrics, and can potentially resist irrelevant information on relative size and depth.

Figure 1

Stimuli and experimental design in Experiment 1. Participants grasped (a) or estimated the length of objects embedded in a standard (b) and inverted (c) configurations of the Ponzo illusion. Note that in both configurations, the physically shorter object appears longer.

Figure 2

The results of experiments 1a and 1b. For grasping, apertures reflected the actual size differences between the objects, even in trials in which participants made erroneous judgments of size. In contrast, manual estimations reflected the perceived, illusory size difference between the objects. Error bars represent confidence intervals in repeated measures ANOVAs.

Figure 3

Stimuli and experimental design used in Experiment 2. Participants either grasped (a) or estimated the horizontal length of the standard (b) and the inverted (c) 3D configurations of the Wundt–Jastrow illusion. The physically shorter object appeared bigger in the majority of the trials.

Figure 4

The results of experiments 2a and 2b. MGAs during bimanual grasping reflected the actual size differences between the objects, even in the trials in which participants made erroneous perceptual judgments. In contrast, manual estimations reflected the illusory, perceived size differences between the objects. Error bars represent confidence intervals in repeated measures ANOVAs.