Experimental manipulations of body ownership have indicated that multisensory integration is central to forming bodily self-representation. Voluntary self-touch is a unique multisensory situation involving corresponding motor, tactile and proprioceptive signals. Yet, even though self-touch is frequent in everyday life, its contribution to the formation of body ownership is not well understood. Here we investigated the role of voluntary self-touch in body ownership using a novel adaptation of the rubber hand illusion (RHI), in which a robotic system and virtual reality allowed participants self-touch of real and virtual hands. In the first experiment, active and passive self-touch were applied in the absence of visual feedback. In the second experiment, we tested the role of visual feedback in this bodily illusion. Finally, in the third experiment, we compared active and passive self-touch to the classical RHI in which the touch is administered by the experimenter. We hypothesized that active self-touch would increase ownership over the virtual hand through the addition of motor signals strengthening the bodily illusion. The results indicated that active self-touch elicited stronger illusory ownership compared to passive self-touch and sensory only stimulation, and indicate an important role of active self-touch in the formation of bodily self.

This paper introduces an approach for manipulating bodily self-consciousness, in particular, senses of body ownership and illusory self-touch by using a robotic/haptic technology. This study especially focuses on the rubber hand illusion (RHI). In the classic RHI paradigm, an invisible participant's hand and a visible fake rubber hand are manually stimulated by an experimenter. However, our approach, which is termed active self-touch, allows the participants to administrate the tactile stimulation with variable force feedback by using a robotic master-slave system. Three RHI experiments with active self-touch were performed to demonstrate the utility of the proposed approach for assessment of bodily self-consciousness. The embodiment of a fake hand was assessed with the classic RHI paradigm, and the RHI paradigm with somatic sensation was applied to examine the effect of variable force feedback on the sense of illusory self-touch. These experiments verified that active self-touch allows participants to experience the classic RHI. In addition, the results implied that the sense of illusory self-touch can be induced, even if force feedback is absent in the dynamic self-touch condition, and that illusory sensation is easily modulated by variable force feedback during quasi-static self-touch rather than during dynamic self-touch. These findings suggest that our proposed approach has the potential to reveal relationships between action/haptic perception and the sense of body ownership.

Thought insertion (TI) is characterized by the experience that certain thoughts, occurring in one's mind, are not one's own, but the thoughts of somebody else and suggestive of a psychotic disorder. We report a robotics-based method able to investigate the behavioral and subjective mechanisms of TI in healthy participants. We used a robotic device to alter body perception by providing online sensorimotor stimulation, while participants performed cognitive tasks implying source monitoring of mental states attributed to either oneself or another person. Across several experiments, conflicting sensorimotor stimulation reduced the distinction between self- and other-generated thoughts and was, moreover, associated with the experimentally generated feeling of being in the presence of an alien agent and subjective aspects of TI. Introducing a new robotics-based approach that enables the experimental study of the brain mechanisms of TI, these results link TI to predictable self-other shifts in source monitoring and specific sensorimotor processes.

Considering that humans recognize mirror images as copies of the real world despite misinterpreting optical reflections, spatial disagreement may be accepted in rubber hand illusion (RHI) settings when a mirror is used to show a fake hand. The present study performed two experiments to reveal how self-body recognition of a fake hand via a mirror affects RHI. First, we tested whether illusory ownership of a fake hand seen in a mirror could be induced in our experimental environment (screening experiment). Subjective evaluations using an RHI questionnaire demonstrated that embodiment of the rubber hand was evoked in the presence or absence of a mirror. We then examined whether using a mirror image for RHI allows disagreement in orientation (45 ${}^{\circ }$ ) between the rubber and actual hands (main experiment). The participants experienced RHI even when the actual and rubber hands were incongruent in terms of orientation. These findings suggest that using a mirror masks subtle spatial incongruency or degrades the contribution of visual cues for spatial recognition and facilitates multisensory integration for bodily illusions.