Abstract
Previous evidence highlighted the multisensory‐motor origin of embodiment – that is, the experience of having a body and of being in control of it – and the possibility of experimentally manipulating it. For instance, an illusory feeling of embodiment towards a fake hand can be triggered by providing synchronous visuo‐tactile stimulation to the hand of participants and to a fake hand or by asking participants to move their hand and observe a fake hand moving accordingly (rubber hand illusion). Here, we tested whether it is possible to manipulate embodiment not through stimulation of the participant's hand, but by directly tapping into the brain's hand representation via non‐invasive brain stimulation. To this aim, we combined transcranial magnetic stimulation (TMS), to activate the hand corticospinal representation, with virtual reality (VR), to provide matching (as contrasted to non‐matching) visual feedback, mimicking involuntary hand movements evoked by TMS. We show that the illusory embodiment occurred when TMS pulses were temporally matched with VR feedback, but not when TMS was administered outside primary motor cortex, (over the vertex) or when stimulating motor cortex at a lower intensity (that did not activate peripheral muscles). Behavioural (questionnaires) and neurophysiological (motor‐evoked‐potentials, TMS‐evoked‐movements) measures further indicated that embodiment was not explained by stimulation per se, but depended on the temporal coherence between TMS‐induced activation of hand corticospinal representation and the virtual bodily feedback. This reveals that non‐invasive brain stimulation may replace the application of external tactile hand cues and motor components related to volition, planning and anticipation.
Highlights
The brain constantly receives, sends and updates information from and to the body, building association rules between different multisensory bodily signals, motor commands and related external events (e.g. Medina & Coslett, 2010; Serino & Haggard, 2010; De Vignemont, 2011; Held et al, 2011)
These data show that supra-threshold stimulation over primary motor cortex (M1) induces the illusory embodiment for the virtual hand selectively when combined with a temporally congruent visual feedback in virtual reality
A positive drift towards the virtual hand was present across all conditions, both in synchronous and asynchronous conditions and this occurred in the supra-threshold M1 and supra-threshold vertex conditions
Summary
The brain constantly receives, sends and updates information from and to the body, building association rules between different multisensory bodily signals (i.e. tactile, proprioceptive, kinesthetic, visual, auditory, vestibular), motor commands and related external events (e.g. Medina & Coslett, 2010; Serino & Haggard, 2010; De Vignemont, 2011; Held et al, 2011). Integrated signals between motor intention, execution and multisensory feedback have been proposed to lead to a sense of control for one’s own movements (sense of agency, ‘I am the one who generated that hand movement’) and to Received 3 August 2017, revised 9 February 2018, accepted 9 February 2018. Embodiment for a fake hand can be induced through multisensory stimulation using the rubber hand illusion (RHI) (Botvinick & Cohen, 1998; Tsakiris, 2010). Many other RHI-like protocols have been proposed, such as by providing visuo-motor stimulation based on participants’ movements and congruent visual feedback of a fake or a virtual hand moving Most RHI work has been based either on the direct application of somatosensory stimuli to participant’s skin or limb (i.e. tactile, Botvinick & Cohen, 1998 or proprioceptive cue, Walsh et al, 2011) or on subjects’ movements (Tsakiris et al, 2006; Riemer et al, 2013, 2014; Kalckert & Ehrsson, 2014)
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