Mirror-symmetrical bimanual movement is more stable than parallel<br />bimanual movement. This is well established at the kinematic level. I used functional<br />MRI (fMRI) to evaluate the neural substrates of the stability of mirror-symmetrical<br />bimanual movement. Right-handed participants (<i>n</i>= 17) rotated disks with their<br />index fingers bimanually, both in mirror-symmetrical and asymmetrical parallel<br />modes. I applied the Akaike causality model to both kinematic and fMRI time-series<br />data. I hypothesized that kinematic stability is represented by the extent of neural<br />"cross-talk": as the fraction of signals that are common to controlling both hands<br />increases, the stability also increases. The standard deviation of the phase difference<br />for the mirror mode was significantly smaller than that for the parallel mode, <br />confirming that the former was more stable. I used the noise-contribution ratio<br />(NCR), which was computed using a multivariate autoregressive model with latent<br />variables, as a direct measure of the cross-talk between both the two hands and the<br />bilateral primary motor cortices (M ls).The mode-by-direction interaction of the<br />NCR was significant in both the kinematic and fMRI data. Furthermore, in both sets<br />of data, the NCR from the right hand to the left was more prominent than vice versa<br />during the mirror-symmetrical mode, whereas no difference was observed during<br />parallel movement or rest. The asymmetric interhemispheric interaction from the left<br />M l to the right M l during symmetric bimanual movement might represent<br />cortical-level cross-talk, which contributes to the stability of symmetric bimanual<br />movements.
Asymmetric control mechanisms of bimanual coordination: an application of directed connectivity analysis to kinematic and functional MRI data
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