Anchoring to the hand, but not spatially distinct mappings, facilitates illusory supernumerary finger embodiment

To induce the Anne Boleyn illusion, an individual's hands are placed on either side of a mirror and stroked synchronously from the thumb to the empty space neighboring the reflected fifth finger, creating the perception of a supernumerary finger. The hidden fifth finger is stroked on the medial and lateral sides, which correspond to the fifth and "sixth" finger on the visible hand. The percept induced is robust enough to withstand biologically implausible manipulations that break other visuotactile illusions, making the illusion a promising avenue for exploring multisensory integration and illusory embodiment. The present study investigates three aspects of its underlying cognitive mechanisms. First, although embodiment was theorized to require tactile stimulation of two discrete fifth finger locations, we found that stroking only one location does not abolish or reduce the illusion. Second, manipulating the starting location of strokes produced differences in body part categorization of the percept, indicating the influence of top-down constraints from pre-existing body representations. Third, we aimed to identify factors underlying the illusion's robustness to enhance our understanding of illusory embodiment mechanisms. We found support for the "anchoring" hypothesis, proposing that the sixth finger's proximity to the real hand may be a critical factor.