Over my fake body: body ownership illusions for studying the multisensory basis of own-body perception

Abstract

Which is my body and how do I distinguish it from the bodies of others, or from objects in the surrounding environment? The perception of our own body and more particularly our sense of body ownership is taken for granted. Nevertheless, experimental findings from body ownership illusions (BOIs), show that under specific multisensory conditions, we can experience artificial body parts or fake bodies as our own body parts or body, respectively. The aim of the present paper is to discuss how and why BOIs are induced. We review several experimental findings concerning the spatial, temporal, and semantic principles of crossmodal stimuli that have been applied to induce BOIs. On the basis of these principles, we discuss theoretical approaches concerning the underlying mechanism of BOIs. We propose a conceptualization based on Bayesian causal inference for addressing how our nervous system could infer whether an object belongs to our own body, using multisensory, sensorimotor, and semantic information, and we discuss how this can account for several experimental findings. Finally, we point to neural network models as an implementational framework within which the computational problem behind BOIs could be addressed in the future.

Keywords: body ownership; body semantics; causal inference; multisensory perception; rubber hand illusion.

Figure 1

Examples of body illusions. (A) The Pinocchio illusion. A blindfolded participant receives vibration on his biceps while touching the tip of his nose with his fingers. The illusory extension of the arm (Goodwin et al., 1972) generates the illusion that his nose, his fingers or both are elongating (Lackner, 1988). (B) The phantom nose illusion. The experimenter moves the finger of a blindfolded participant to tap the nose of another subject, while simultaneously tapping the nose of the participant. As the participant's movements and his finger contact with the other subject's nose are synchronous with the touch he receives on his nose, the participant experiences the illusion of tapping his very long nose (Ramachandran and Hirstein, 1998). (C) An out of body illusion. The participant sees a video of his back as if he were located behind it. The experimenter touches the back of the participant with a stick while the participant sees it online in the video. As the seen and the felt stimulation is synchronous, the participant perceives illusory drifts in his self-location toward the seen body (Lenggenhager et al., 2007). (D) The rubber hand illusion. The participant sees a rubber hand placed in front of him, while his real hand is concealed from view. The experimenter strokes both hands at the same time, and after some time the participant perceives the fake hand as if it were his own hand (Botvinick and Cohen, 1998).

Figure 2

Different induction methods of Body Ownership Illusions (BOIs). (A,D) Visuotactile triggers: the participant is watching the fake hand/body placed in a plausible posture and being touched, while receiving synchronous tactile stimulation in the real counterpart that remains out of view. (B,E) Visuomotor triggers: the participant is performing movements with his real hand/body that remains out of view, while watching the fake counterpart moving synchronously. (C,F) Visuoproprioceptive triggers: the participant is watching the fake hand/body placed in an overlapping position with the real counterpart that remains out of view.

Figure 3

Examples of objects with different semantic information. (A) Objects with non-human body shape. (B) Objects with human body shape. (C) Objects with non-human skin texture (D) Objects (blue) in anatomically implausible spatial configurations with respect to the participant's body (red). (E) Objects (blue) with anatomically implausible structure with respect to the human body.

Figure 4

A causal inference model for the classic version of the RHI. Left: one cause being responsible for all cues. In this case, the visually perceived configuration x_V, the proprioceptive perceived configuration x_P, the seen strokes ${\text{s}}_{\text{V}}^{\to \tau }$ and the felt strokes ${\text{s}}_{\text{T}}^{\to \tau }$ together with the seen morphological characteristics m_V, are mapped into a common cause (C = 1). Right: alternatively, two distinct causes may be inferred, decoupling the problem into two independent estimation problems. The brain infers whether the seen and felt spatial configurations, the tactile events and the seen morphology origin from the same causal structure, i.e., my hand (C = 1), or independent causal structures, i.e., the real and the rubber hands (C = 2), and then derives optimal predictions from this.