Abstract
Congenital mirror movements (CMMs) are involuntary, symmetric movements of one hand during the production of voluntary movements with the other. CMMs have been attributed to a range of physiological mechanisms, including excessive ipsilateral projections from each motor cortex to distal extremities. We examined this hypothesis with an individual showing pronounced CMMs. Mirror movements were characterized for a set of hand muscles during a simple contraction task. Transcranial magnetic stimulation (TMS) was then used to map the relative input to each muscle from both motor cortices. Contrary to our expectations, CMMs were most prominent for muscles with the strongest contralateral representation rather than in muscles that were activated by stimulation of either hemisphere. These findings support a bilateral control hypothesis whereby CMMs result from the recruitment of both motor cortices during intended unimanual movements. Consistent with this hypothesis, bilateral motor cortex activity was evident during intended unimanual movements in an fMRI study. To assess the level at which bilateral recruitment occurs, motor cortex excitability during imagined unimanual movements was assessed with TMS. Facilitory excitation was only observed in the contralateral motor cortex. Thus, the bilateral recruitment of the hemispheres for unilateral actions in individuals with CMMs appears to occur during movement execution rather than motor planning.
Highlights
Some individuals manifest mirror movements throughout their lifetime (Schott and Wyke 1981)
Studies using transcranial magnetic stimulation (TMS) have found that, in persons afflicted with congenital mirror movements (CMMs), stimulation over either hemisphere can evoke muscle activity bilaterally (Konagaya, Mano, and Konagaya 1990; Farmer et al 1990; Cohen et al 1991; Britton et al 1991; Cincotta et al 1994; Balbi et al 2000; Cincotta et al 2003; Farmer et al 2004)
If CMMs are caused by an abnormal quantity of uncrossed corticospinal projections, as suggested by the unilateral control hypothesis, across a set of muscles, one would expect to observe a correlation between the magnitude of CMMs expressed behaviorally and ipsilateral motor evoked potentials (MEPs) elicited by TMS
Summary
Some individuals manifest mirror movements throughout their lifetime (Schott and Wyke 1981). The condition may be associated with developmental abnormalities such as in KlippelFiel syndrome or X-linked Kallmann’s syndrome, or may be present even in the absence of other neurological or peripheral abnormalities (for review see Vulliemoz, Raineteau, and Jabaudon 2005) For the latter group, mirror movements are frequently observed in multiple family members, indicating a genetic basis. The observation of bilateral evoked responses has led to the general consensus that CMMs reflect unilateral control signals that are projected bilaterally According to this unilateral control hypothesis, commands from the motor cortex used to produce volitional movements with the contralateral hand are, at least weakly, projected by ipsilaterally descending projections to homologous muscles, resulting in mirror movements. At least for the individual studied here, this bilateral recruitment is likely due to the distributed representation of hand muscles across the two hemispheres
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