2026 Volume 33 Issue 3 Pages 449-463
Human body representation flexibly changes through multisensory integration. The Finger-Bonding Illusion (FBI) is a tactile illusion in which synchronous stimulation of the left and right fingers induces the sensation that the fingers are fused, typically accompanied by an attraction between them. The present study examined whether similar effects occur when the fingers are arranged along the anterior–posterior axis and how distance from the body influences such distortions. Across two experiments, synchronous tactile stimulation was applied to the index and ring fingers positioned 6 cm apart in depth. In Experiment 1, participants reported perceived finger positions using a tabletop display under open-eye conditions. In Experiment 2, finger position was measured under closed-eye conditions using head tracking, together with subjective ratings and time-series analyses. Both experiments revealed a reliable reduction in inter-finger distance during synchronous stimulation, indicating that FBI-like attraction also emerges in the anterior–posterior direction. Importantly, the attraction was strongly asymmetric. In both experiments, the index finger consistently drifted toward the distal direction, whereas the ring finger exhibited little or no displacement. This asymmetry was robust across measurement methods and was also reflected in subjective reports of attraction. Time-series analyses further showed that the distal drift of the index finger emerged rapidly, even under asynchronous stimulation. These findings suggest that anterior–posterior FBI reflects not only multisensory synchrony but also distance-dependent weighting of body parts relative to the torso. The results extend previous FBI findings and highlight the role of body-centered distance in shaping multisensory body representations.
