The long sixth finger illusion: The representation of the supernumerary finger is not a copy and can be felt with varying lengths

We can have a distorted perception of our body, instantly induced with multisensory illusions, anaesthesia or Virtual Reality, and recent studies show we can also feel extra body parts. Newport and colleagues (Newport et al., 2016) created an illusion that induces the feeling of having a sixth finger on one's hand, for a brief moment. By changing the paradigm with a double back and forth stroking, we were able to extend the duration of this illusion (Cadete & Longo, 2020), which can reflect an endured representation of a supernumerary finger. This innovation allowed us to test one specific distortion in the supernumerary finger: length. Patients with supernumerary phantom limb syndrome feel like they have an extra limb, as if one of their limbs was duplicated (Staub et al., 2006), resembling the same size and shape of the existing one. It is unclear from existing studies whether a supernumerary limb is represented as a copy of the existing limb, or if it is represented independently, with its own features. We therefore aimed to investigate whether the properties of the supernumerary sixth finger could be altered, independently of the actual little finger. Hence, we tested whether we can embody a sixth finger with double the size of the average little finger, and half its size. Participants reported feeling a long and a short sixth finger, and gave visual judgements on the felt length of the supernumerary finger, that matched the condition length. Overall, the results show that the supernumerary sixth finger is not a mere copy of the little finger but is represented independently, with distinct features from the existing finger. Moreover, the representation of the supernumerary finger is flexible, allowing the embodiment of a long or a short sixth finger.