Suppressing movements with phantom limbs and existing limbs evokes comparable electrophysiological inhibitory responses

Abstract

Phantom limb is a common sensation in amputees, who often report vivid experiences of voluntarily moving their phantom. Previous studies showed that phantom movement can be functionally disentangled from imagined movement comparable to the actual movement of an intact limb. How and to what extent phantom movement and real movement share similar physiological mechanisms? Here, we focused on a specific aspect of motor control, the motor inhibition, and we asked whether inhibitory physiological responses are implemented when a phantom movement has to be suppressed. Sixteen two-handed controls and two left upper-limb amputees (with and without phantom movement) underwent a Go/Nogo paradigm, while event-related potentials (ERPs) were recorded. The task was performed with both the right (intact) and the left (phantom) hand, either in real or imagery conditions. Opposite results between the moving-phantom case and the static-phantom case were found. In the real condition, moving-phantom case showed the classical motor-inhibition related ERP pattern, with large P300 inhibitory wave when the movements of both (right) intact and (left) phantom limbs have to be suppressed. This inhibitory response was not different from that found in controls (who performed the task with an existing hand; real condition), but, crucially, it was significantly different from the imagery condition of controls. Contrariwise, in the static-phantom case, the ERP responses to Nogo trials during the real condition were different from the real condition in controls, but were not different from their imagery condition. Importantly, in the real condition, Nogo-ERP responses were significantly different between the two phantom cases. Taken together, these findings provide compelling evidence that phantom movements share the same neurophysiological correlates of real movements, not only when an action has to be executed, but also when it should be inhibited.