Embodying the other using the sixth finger illusion

Previous studies have robustly illustrated the phenomenon of embodying additional body parts. For example, in the sixth finger illusion, the perception of an extra finger is generated by conflicting visuo-tactile stimulation via a mirror box. However, it remains unclear to what extent we can incorporate extra body parts into our body representation, when the additional body part in question belongs to someone else. By distorting the boundary between the self and the other in the sixth finger illusion, this present study explores whether it is possible to embody an extra finger which in fact belongs to someone else. Rather than using virtual reality or a prosthetic, a real-life volunteer took part in the experiment so that their little finger acted as the participant's extra digit. We tested two illusory variations featuring the volunteer: 'the Other' condition, in which the volunteer's little finger was placed palm-down next to the participant's little finger; and 'the Other Upwards' condition, in which the volunteer's little finger was placed palm-up next to the participant's little finger. Participants experienced strong body ownership over the volunteer's finger in both conditions, demonstrating that we can perceive someone else's finger to be our own as an extra digit on our hands. Moreover, we can do so even when the finger is anatomically incongruent with our own existing body structure. This study offers insight into the startling flexibility of our mental body representations, and suggests that it is possible to embody someone else's body part without having to establish a clear first-person perspective or disembody our own existing bodies. This is a compelling paradigm to explore the self-other boundary in relation to embodiment.