Illusions of supernumerary fingers are not constrained by posture congruency

Perceptual illusions of having extra body parts offer an experimental method to investigate the limits of body perception. It is well established that the illusory perception of an artificial hand as one's own is dependent on spatial congruency. That is, the seen hand needs to be in a posture congruent with the actual hand. In this study, we aimed to investigate how constrained the representation of a supernumerary body part is by systematically varying the perceived rotation of an illusory sixth finger. Surprisingly, participants felt a sixth finger on their hand consistently for all induced orientations of finger extension and abduction (0°, 90°, 135°, 180°). The illusion showed no apparent decrease with increased induced rotation of the extra finger. We also measured the perceived orientation of the sixth finger, and our results show that participants felt an extended and an abducted sixth finger increasingly more rotated as the induced rotation also increased, while feeling their actual little finger in a normal position. Our results indicate that one can feel a supernumerary finger in an incongruent spatial location from one's actual fingers and hand, to an extent of 180° of extension (finger up) and 180° of abduction (finger to the side). We therefore propose that the representation of the supernumerary finger has a strong independence from the actual finger and hand-frame reference.