My hand in my ear: a phantom limb re-induced by the illusion of body ownership in a patient with a brachial plexus lesion

Corporeal awareness of body unity, continuity, and integrity is hardwired in the brain, even following massive deafferentation. Following peripheral limb injury, referred phantom sensations are reported frequently on the cheek and, rarely, on the ear. Here, we explore how brain plasticity mechanisms induced by multisensory stimulation of different facial regions (cheek and ear) modulate the feeling that a complete missing limb is still attached to the body. We applied the modified rubber hand illusion (RHI) paradigm following synchronous and asynchronous stimulation of the face-hand and ear-hand in the unusual case of a patient with a brachial plexus lesion, who had lost upper-left limb sensation and developed a phantom sensation of the arm restricted to the ear. He experienced a strong illusion of ownership of the rubber hand during synchronous stroking of the ear but not the cheek and reported more defined tactile sensations in his previously numb body part during the illusion than when simply touching the ear. Phantom experiences are not exclusively based on sensory memories of the once-present body periphery, they are organized into a topographic cortical map with the ear-hand area adjoining but separate from the face. Multimodal experiences specifically modulate possible remapping of ear-hand representations and generate a more defined connection between the brain's memory of the body and what one feels of the actual physical body. We suggest that RHI is a form of sensory intervention that makes the best use of residual signals from disconnected body parts after peripheral injury, evoking and controlling the limb sensations.