The rubber hand illusion in action.

In the well-known rubber hand illusion (RHI), watching a rubber hand being stroked while one's own unseen hand is synchronously stroked, induces a relocation of the sensed position of one's own hand towards the rubber hand [Botvinick, M., & Cohen, J. (1998). Rubber hands 'feel' touch that eyes see. Nature, 391(6669), 756]. As one has lost the veridical location of one's hand, one should not be able to correctly guide one's hand movements. An accurate representation of the location of body parts is indeed a necessary pre-requisite for any correct motor command [Graziano, M. S. A., & Botvinick, M. M. (1999). How the brain represents the body: Insights from neurophysiology and psychology. In D. Gopher, & A. Koriat (Eds.), Attention and performance XVII-Cognitive regulation of performance interaction of theory and application (pp. 136-157)]. However, it has not yet been investigated whether action is indeed affected by the proprioceptive drift towards the rubber hand, nor has the resistance of visual capture in the RHI to new proprioceptive information been assessed. In the present two kinematic experiments, we show for the first time that action resists the RHI and that the RHI resists action. In other words, we show a dissociation between illusion-insensitive ballistic motor responses and illusion-sensitive perceptual bodily judgments. Moreover, the stimulated hand was judged closer to the rubber hand for the perceptual responses, even after active movements. This challenges the view that any proprioceptive update through active movement of the stimulated hand erases the illusion. These results expand the knowledge about representations of the body in the healthy brain, and are in line with the currently most used dissociation between two types of body representations so far mainly based on neuropsychological patients [Paillard, J. (1991). Knowing where and knowing how to get there. In J. Paillard (Ed.), Brain and space (pp. 461-481); Paillard, J. (1999). Body schema and body image: A double dissociation in deafferented patients. In G. N. Gantchev, S. Mori, & J.Massion (Eds.), Motor control, today and tomorrow (pp. 197-214)].

Dissociating body representations in healthy individuals: differential effects of a kinaesthetic illusion on perception and action.

Evidence from neuropsychological patients suggests that multiple body representations exist. The most common dissociation is between body schema to guide limb movements, and body image used to make perceptual judgements. In the current study we employed a kinaesthetic illusion in two experiments to dissociate body representations in healthy individuals. Tendon vibration creates an illusory lengthening of the muscle and an illusive displacement of the limb. In Experiment 1 two conditions were used. In the 'direct' condition the biceps of the dominant right arm of blindfolded participants was vibrated, creating illusory elbow extension. In the 'indirect' condition the right knee was held with the vibrated right arm, creating illusive lowering of the leg and knee. In both conditions, subjects performed with the non-vibrated arm a reaching as well as a matching response, theorized to be based on the body schema and body image, respectively. Results showed that the illusion was significantly larger for the matching as compared to the reaching response, with the most pronounced difference observed in the direct condition. In Experiment 2 reaching and matching without vibration and a passive matching response were implemented in the direct condition. The same differential effect of the illusion was found. Results further showed that passive and active matching were statistically similar but significantly different from the reaching response. In conclusion, these findings suggest that the effect of the kinaesthetic illusion on reaching and matching differed, consistent with the idea of separate underlying body representations for both responses.