67 ALTERED VISUOMOTOR PROCESSING IN NA 4 Introduction When elements of the sensorimotor system are damaged, motor function can be regained through plastic adaptations within the nervous system. 12, 13 However, this reorganization is not always clinically beneficial, and may even contribute to impaired motor function, in which case it is considered maladaptive. 12, 15 Maladaptive neuroplasticity has been linked to motor dysfunction in several central and peripheral nervous system disorders. 16, 18, 19, 43, 67, 121 A striking clinical example of the latter is obstetric brachial plexus palsy, which is associated with developmental apraxia and persistent clinical motor dysfunction despite peripheral reinnervation. 42 Persistent motor dysfunctions following recovery from peripheral nerve damage offer a well-defined test case for understanding mechanisms of central neuroplasticity. Here we study cerebral changes related to persistent motor dysfunction and subsequent pain in neuralgic amyotrophy (NA), a common (incidence of 1/1000) and disabling peripheral nerve disorder that involves acute autoimmune inflammation of the brachial plexus. 3, 6, 8 NA is typically asymmetric and most often involves one upper extremity. 7 The characteristic paresis of muscles that are innervated by damaged nerves leads to motor impairment of the affected limb, most notably the shoulder. Many patients subsequently develop alternative movement patterns that can be beneficial for compensation at first, but may lead to long-term motor dysfunction, and persistent pain in the long run. These secondary impairments are related to overuse of and strain of compensating muscles and shoulder impingement due to altered scapular biomechanics. 6, 8, 10 Several clinical signs connect these residual complaints to maladaptive cerebral changes. First, patients often do not regain motor function, despite reinnervation of the affected muscles and return of muscle strength. 6-8, 10 Second, some NA patients develop abnormal and involuntary movements that resemble dystonia, a symptom which is often associated with cerebral abnormalities. 4 Third, patients can regain normal motor function through specialized rehabilitation that focuses on relearning correct movement patterns and postures, even years after onset. 1 Finally, we have recently shown in a separate behavioural study that NA patients have behavioural deficits during motor imagery of the affected limb, as evoked by a hand laterality judgment task, suggesting that NA patients have altered sensorimotor representations related to their affected limb. 122 However, it remains unclear whether these alterations arise from changes in somatomotor processes, visuomotor processes, or both. We address this issue in an independent sample of NA patients with task-based functional MRI (fMRI) during performance of the hand laterality judgment task, which involves both somatomotor and visuomotor processes, and can activate sensorimotor representations of the upper limb. 21, 58, 69, 70, 86 In this task, participants have to judge the laterality (left or right) of hand stimuli. It is believed that subjects solve this task by mentally rotating their own body part to match the stimulus, a process involving “motor imagery”: mental simulation of movement, without overt motor expression. 25, 29 This validated task involves similar cerebral processes as motor planning, 21, 24, 86 without interference of diseaserelated abnormalities in motor execution or associated afferent feedback. 29-31 The fact that subjects incorporate their own body posture when performing the task, suggests that this task has embodied components. 58, 69, 70 This task typically engages a fronto-
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