A Model-based Interpretation of Phantom Pain -Conservative Body Schema and Flexible Somatotopy-

Some patients with amputations are known to still experience presence of the amputated limb (phantom limb). Phantom limb is found to be accompanied by reorganizations of somatotopic representations at cortical and subcortical levels due to deafferentation, which also discloses presence of body schema, i.e., integrated image of body. In addition, phantom limb is often recognized with painful sensation, i.e., phantom pain, which is suggested to be caused by abnormally high activations in thalamocortical neurons in the deafferentated area. In this paper, simulations using a self-organizing neural network model were performed to clarify contribution of axonal sprouting and body schema to reorganization of cortical representation, and to explore a possible mechanism underlying abnormally high thalamocortical activation. The results showed that axonal sprouting could cause reorganization of cortical mapping with an aid of facilitatory inputs referring to the conserved body schema even after deafferentation. In addition, removing thalamic recurrent inhibitions was shown to induce high and sustained activations in thalamocortical neurons in the deafferentated area. Consequently, this study suggested that discordance between the recognized cortical representation and physiological process referring to the body schema could give rise to phantom pain. Phantom pain is a clue disclosing how body schema is constructed and maintained, which might be an important subject also for robotics dealing with an artificial body