28/11/18
Speaker: Elias Casula, Ricercatore presso Laboratorio di Neuropsicofisiologia Sperimentale, Fondazione Santa Lucia IRCCS
The sense that we own a body (ownership) and we are responsible of its actions (agency) is fundamental for embodiment and bodily self-consciousness. Immersive virtual reality (IVR) proved to be a powerful tool to induce the illusory experience of embodying a virtual body. The real-time neural mechanisms underlying the embodiment process of artificial limbs are still unclear. Here we combined for the first time the novel TMS-EEG (transcranial magnetic stimulation and electroencephalography) approach with IVR in order to investigate the real-time cortical dynamics underlying the embodiment of a virtual limb.
We tested 19 healthy volunteers in three TMS-EEG sessions during which they observed through a head-mounted display a virtual right upper limb, overlapped to the real one, from a first-person perspective (1PP). The participants were instructed to passively observe the virtual limb and to refer whether he/she feels it as a part of his/her body. In two sessions, the participants observed a full right upper limb and received 160 TMS single-pulses over the left (full-lM1 condition) or over the right primary motor cortex (full-rM1 condition) (fig. 1A). In another session, the participant observed the right upper limb with detached hand due to a missing wrist during TMS of the left M1 (detached-lM1 condition). All sessions were preceded and followed by a TMS-EEG block of stimulation during which 120 TMS single-pulses were delivered over the corresponding M1 to monitor possible long-lasting effects of IVR. Behavioral effects of IVR were investigated in terms of self-reported sense of ownership and agency, whereas neurophysiological effects were investigated in terms of TMS-evoked cortical excitability and oscillations.
Behavioral results showed that the observation of a right full limb induced the highest embodiment feeling (p<0.01). Neurophysiological results showed a strong reduction of TMS-evoked cortical activity (p<0.001) and alpha oscillations (p<0.01), when we stimulated lM1, during the observation of the right full virtual limb. These effects were found mainly over a region comprising left M1 and pre-motor areas (fig. 1B). A trial-by-trial analysis conducted during the observation of the full virtual limb, showed that suppression of cortical activity started concurrently with the self-reported feeling of embodiment (fig. 1C). No effects were found when the participant observed the detached limb nor when we stimulated the right M1 (p>0.05).
Our results provide the first evidence of the real-time brain dynamics underpinning the embodiment of a virtual limb. More specifically, embodying a virtual limb brings about a dramatic decrease of the motor cortex activity contralateral to the virtual limb, which could possibly reflect a disembodiment of the real hand. Interestingly, the timing of the cortical activity decrease was concurrent with the start of the embodiment self-reported feeling.