Examining constraints on embodiment using the Anne Boleyn illusion.

Abstract

Using a mirror box, the concurrent stroking of the lateral side of the fifth finger behind the mirror along with stroking the empty space next to the mirror-reflected hand’s fifth finger results in a strong sense of having a sixth finger—the Anne Boleyn illusion. We used this illusion to understand what constraints illusory embodiment. In Experiment 1, we manipulated the anatomical constraints, posture, and stroking of the sixth finger, along with other variants. Given evidence from other body illusions, we predicted no illusory embodiment in conditions in which the sixth finger was created in a manner incompatible with a typical hand, when the mirror and viewed hands were in different posture, and when stroking differed. Surprisingly, the illusion was persistent in most variants, including those with curved fingers, elongated fingers, and even with mismatches between the posture of the viewed and hidden hand. In Experiment 2, we manipulated the orientation, shape, and length of the illusory sixth finger, presenting more extreme versions of the illusion. The illusion was significantly diminished only when the sixth finger was far from the hand, or in a very implausible posture. This evidence suggests that body representations are extremely flexible and allow for embodiment of empty space in conditions not seen in other body illusions. We suggest that bottom-up information from concurrent visuotactile input, combined with reduced constraints provided by the “blank canvas” of empty space, results in a particularly robust illusion.