Abstract
Mirror Activity (MA) describes involuntarily occurring muscular activity in contralateral homologous limbs during unilateral movements. This phenomenon has not only been reported in patients with neurological disorders (i.e. Mirror Movements) but has also been observed in healthy adults referred to as physiological Mirror Activity (pMA). However, despite recent hypotheses, the underlying neural mechanisms and structural correlates of pMA still remain insufficiently described. We investigated the structural correlates of pMA during isometric contractions of hand muscles with increasing force demands on a whole-brain level by means of voxel-based morphometry (VBM) and tract-based spatial statistics (TBSS). We found significant negative correlations between individual tendencies to display pMA and grey matter volume (GMV) in the right anterior cingulate cortex (ACC) as well as fractional anisotropy (FA) of white matter (WM) tracts of left precuneus (PrC) during left (non-dominant) hand contractions. No significant structural associations for contractions of the right hand were found. Here we extend previously reported functional associations between ACC/PrC and the inhibtion of intrinsically favoured mirror-symmetrical movement tendencies to an underlying structural level. We provide novel evidence that the individual structural state of higher order motor/executive areas upstream of primary/secondary motor areas might contribute to the phenomen of pMA.
Highlights
Pathological forms of Mirror Activity (MA) have previously been linked to uncrossed ipsilateral projections of the corticospinal tract[6,8] which persist due to specific mutations in genes that are crucial for corticospinal axonal development www.nature.com/scientificreports/
Motor Overflow is hypothesized to be the result of ongoing modulations of interhemispheric communication during unilateral contractions with progressively higher force demands, characterized by a gradual shift from predominantely interhemispheric inhibition (IHI) to interhemispheric facilitation (IHF), which in turn leads to bilateral activation of motor-relevant brain regions[12,28]
Side comparison of maximal isometric force during maximum voluntary contraction (MVC) testing detected a significant difference between left hand and right hand
Summary
Pathological forms of MA have previously been linked to uncrossed ipsilateral projections of the corticospinal tract[6,8] which persist due to specific mutations in genes that are crucial for corticospinal axonal development www.nature.com/scientificreports/. Motor Overflow is hypothesized to be the result of ongoing modulations of interhemispheric communication during unilateral contractions with progressively higher force demands, characterized by a gradual shift from predominantely interhemispheric inhibition (IHI) to interhemispheric facilitation (IHF), which in turn leads to bilateral activation of motor-relevant brain regions[12,28] In support of this hypothesis, a structural white matter (WM) correlate, the fractional anisotropy (FA) of transcallosal fibers connecting bilateral primary motor cortices (M1) has recently been positively associated with the amount of IHF29 and pMA12 during dominant hand contractions. Further evidence regarding the underlying neural mechanisms of pMA gained from functional magnetic resonance imaging (fMRI) points to a role of neural networks upstream of primary and secondary motor areas including the anterior cingulate cortex (ACC) and precuneus (PrC) in the precise coordination of unilateral movements[15] and proactive movement inhibition[31] It is unknown if these functional associations of higher-order executive areas are relatable to an underlying structural level
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