MTr remodels the motor system by functionally connecting hand movement to the ipsilateral SMC. On a system level, it leads to interference of the neural circuit related to motor programming and observation of the trained hand with the illusionary movement of the untrained hand.
The contralesional primary motor cortex (M1) has been suggested to be involved in the motor recovery after mirror therapy, but whether the ipsilesional M1 is influenced by the contralesional M1 via transcallosal interhemispheric inhibition (IHI) is still unclear. The present study investigated the change of IHI as well as the intracortical inhibition and intracortical facilitation of both M1 induced by training in a mirror with the use of transcranial magnetic stimulation (TMS). In this 2 × 2 factorial design (time × group), healthy subjects exercised standardized motor skills with their right hand on four consecutive days. Either a mirror (mirror group) or a board (control group) was positioned between their hands. Before and after training TMS was applied along with training tests of both hands. Tests were the same motor skills exercised daily by both groups. Tests of the untrained left hand improved significantly more in the mirror group than in the control group after training (P = 0.02) and showed a close correlation with an increase of intracortical inhibition of M1(left). IHI did not show any difference between investigation time points and groups. The present study confirms the previous suggestion of the involvement of the "contralesional" left-side (ipsilateral to the hand behind the mirror) M1 after mirror therapy, which is not mediated by IHI. Even with the same motor skill training (both groups performed same motor skills) but with different visual information, different networks are involved in training-induced plasticity.
Most neuroimaging studies on planning report bilateral activations of the dorsolateral prefrontal cortex (dlPFC). Recently, these concurrent activations of left and right dlPFC have been shown to double dissociate with different cognitive demands imposed by the planning task: Higher demands on the extraction of task-relevant information led to stronger activation in left dlPFC, whereas higher demands on the integration of interdependent information into a coherent action sequence entailed stronger activation of right dlPFC. Here, we used continuous theta-burst stimulation (cTBS) to investigate the supposed causal structure-function mapping underlying this double dissociation. Two groups of healthy subjects (left-lateralized stimulation, n = 26; right-lateralized stimulation, n = 26) were tested within-subject on a variant of the Tower of London task following either real cTBS over dlPFC or sham stimulation over posterior parietal cortex. Results revealed that, irrespective of specific task demands, cTBS over left and right dlPFC was associated with a global decrease and increase, respectively, in initial planning times compared to sham stimulation. Moreover, no interaction between task demands and stimulation type (real vs. sham) and/or stimulation side (left vs. right hemisphere) were found. Together, against expectations from previous neuroimaging data, lateralized cTBS did not lead to planning-parameter specific changes in performance, but instead revealed a global asymmetric pattern of faster versus slower task processing after left versus right cTBS. This global asymmetry in the absence of any task-parameter specific impact of cTBS suggests that different levels of information processing may span colocalized, but independent axes of functional lateralization in the dlPFC.
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