The rubber tool illusion reveals how body image modifies body schema

Abstract

Rubber hand illusion and tool-use paradigms have been extensively used to investigate body representation. Although both approaches rely on multisensory integration and external object incorporation, they are typically studied in isolation. Here, we introduce a novel paradigm that combines these methods to investigate whether perceptual modifications to body representation can induce motor changes, and vice versa. First, participants completed a tool-use task, actively using a short or long grabber tool to move cubes. When asked to point toward the forearm midpoint, only long tool users exhibited a distal shift, denoting an expansion in motor representation. Next, participants experienced the "rubber tool illusion" by passively holding the same tool while observing a rubber hand grasp an identical-looking tool. Notably, participants holding a short tool exhibited an expanded forearm representation when they observed a synchronously stroked long tool during illusion. Control experiments revealed that this effect depended on prior active tool use, embodiment of the observed rubber hand/tool, and a length mismatch between the held and observed tools. These findings reveal for the first time that motor representation of forearm length, a component of body schema, can be modulated by changes in body image. Public Significance Statement Over the past 25 years, research on how the brain represents the body has identified (at least) two distinct systems: body image, which governs conscious perception, and body schema, which guides unconscious movement. While body schema is known to influence body image, evidence for the reverse has been limited. This study provides new evidence that changes in body image can, in fact, alter body schema. By combining two classical methods for studying the embodiment of external objects-tool use and the rubber hand illusion-we found that merely perceiving a longer tool as part of the body had a similar effect on unconscious motor responses as actively using it. These findings challenge traditional views by showing that conscious body perception can shape unconscious movement.TÜBİTAK ; Bogazici Universit