Enhanced mirror activity in 'crossed' reaction time tasks in multiple sclerosis.

Abstract

Execution of unimanual voluntary motor tasks requires appropriate inhibitory control over contralateral motor output. Such inhibition should involve interhemispheric connections, which are often damaged in multiple sclerosis (MS). Twenty mildly-disabled MS patients and 13 healthy subjects performed ipsilateral and contralateral wrist-extension reactions (IR and CR, respectively) to a unilateral somatosensory cue, the latter condition requiring necessarily interhemispheric transfer of information. Prevalence, persistence, latency and amount of mirror electromyographic activity (mEMG) were calculated in each study group, as well as diffusion-tensor-imaging measures of damage in corpus callosum and brainstem for correlation with mEMG. Healthy subjects and patients showed mEMG more often in CR than in IR. In CR tasks, mEMG was larger, more persistent and occurred more often at a shorter latency with respect to voluntary reaction in patients than in healthy subjects (p<0.05 for all). Patients with mEMG had significantly higher diffusivity values of damage in corpus callosum and lower brainstem volumes than patients without mEMG (p<0.05 for all). MS patients show deficient inhibition of unintended mEMG in 'crossed' reaction time tasks. Enhanced mEMG in MS may be due to microstructural axonal damage and atrophy in inhibitory commissural connections of the corpus callosum and brainstem.