TMS-induced control of bodily self-consciousness by combining non-invasive brain stimulation and immersive virtual reality

Our brain constantly receives and integrates a flow of sensory and motor information that shapes the way our body is represented. Several experimental approaches have been proposed to alter the body representation, by manipulating exteroceptive or interoceptive bodily signals. During my thesis, I studied the contribution of central, rather than peripheral, motor brain mechanisms underlying bodily self-consciousness (BSC). For this purpose, I attempted to manipulate BSC, specifically inducing embodiment for a fake hand in healthy participants, by using a novel method that combines non-invasive transcranial magnetic stimulation (TMS) with immersive virtual reality (VR). First, we managed to induce embodiment for a virtual hand utilizing the spatio-temporal coherence between an artificial TMS-induced activation of the motor system and movement-related visual feedback in VR (Study 1). I observed that embodiment for a virtual hand was induced only when the provided stimulation is supra-threshold but not when TMS is delivered sub-threshold. I argue that such TMS-induced illusory embodiment is due to neuro-visual integration between TMS-induced primary motor cortex (M1) activation and hand twitches with visual VR feedback. In a second study (Study 2), by refining the parameters of our TMS-VR protocol we investigated the different peripheral components elicited by TMS with a special focus on hand perception. Our data revealed the main features of TMS-evoked hand perceptions (in terms of threshold and evoked subjective perceptions) and show that these are discernible from MEPs and TMS-evoked movements associated with single-pulse TMS over M1. Finally, we combined the TMS-VR setup (Study 1) and our stimulation protocol (Study 2) to investigate whether the awareness of peripheral bodily stimuli is necessary to induce changes in BSC (Study 3). Our findings reveal a prominent role of conscious bodily perceptions in triggering illusory embodiment, even in the absence of peripheral stimuli. Furthermore, I have been involved in other projects investigating the plasticity of body and space representation in healthy young and elderly participants (listed in appendix 1 & 2) and the robotically controlled induction of presence hallucinations (appendix 3). The present work provides important findings and promising tools regarding the role of non-invasive brain stimulation and virtual technology in the study of BSC. In particular, I showed that it is possible to manipulate BSC by directly stimulating the M1 cortical representation of the hand inducing embodiment for a virtual hand. Furthermore, I investigated the role of the TMS-evoked hand sensations and their role in the embodiment showing that the hand ownership is mainly driven by the detection of TMS stimuli on the hand.