Cortical motor activity and reorganization following upper-limb amputation and subsequent targeted reinnervation

Abstract

Previous studies have postulated that the amount of brain reorganization following peripheral injuries may be correlated with negative symptoms or consequences. However, it is unknown whether restoring effective limb function may then be associated with further changes in the expression of this reorganization. Recently, targeted reinnervation (TR), a surgical technique that restores a direct neural connection from amputated sensorimotor nerves to new peripheral targets such as muscle, has been successfully applied to upper-limb amputees. It has been shown to be effective in restoring both peripheral motor and sensory functions via the reinnervated nerves as soon as a few months after the surgery. However, it was unclear whether TR could also restore normal cortical motor representations for control of the missing limb. To answer this question, we used high-density electroencephalography (EEG) to localize cortical activity related to cued motor tasks generated by the intact and missing limb. Using a case study of 3 upper-limb amputees, 2 of whom went through pre and post-TR experiments, we present unique quantitative evidence for the re-mapping of motor representations for the missing limb closer to their original locations following TR. This provides evidence that an effective restoration of peripheral function from TR can be linked to the return of more normal cortical expression for the missing limb. Therefore, cortical mapping may be used as a potential guide for monitoring rehabilitation following peripheral injuries.

Keywords: Amputation; Brain reorganization; Cortical activity; Electroencephalography; Motor; Targeted reinnervation.

Fig. 1

Schematics of the targeted reinnervation (TR) procedures for all three subjects. (a) Subject A1 — left transhumeral amputee. (b) Subject A2 — right shoulder disarticulation amputee. (c) Subject A3 — bilateral shoulder disarticulation amputee.

Fig. 2

Experimental setup for the performance of motor tasks. (a) Experimental setup with EMG and EEG electrodes attached to amputee subject A1. (b) Visual display for indicating motor task timing information.

Fig. 3

Motor representations for subject A1. (a) Active cortical sources within M1 for each task and side before and after TR surgery. Activity was measured in units of current density (μA/mm²), and then normalized to the maximum strength. (b) Centers of gravity (COG) for the elbow extension (triangle) and hand closing (square) motor representations with respect to the motor homunculus. The COGs for intact arm movements (orange) were mirrored across the midline for display purposes. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

Fig. 4

Motor cortical representations for subject A2. Centers of gravity (COG) for each motor representation before and after targeted reinnervation, with respect to the motor homunculus.

Fig. 5

Motor cortical representations for subject A3. Centers of gravity (COG) are shown for each motor representation before and after targeted reinnervation, with respect to the motor homunculus.