AbstractWe previously showed that improved skill performance in the untrained hand after sensory-motor learning with reversed vision was associated with increased excitability in the human primary motor cortex (M1) and parietofrontal circuits in the untrained hemisphere. Neuroimaging studies have implicated the posterior parietal cortex (PPC) as a critical node in this reversed visuomotor mapping. Here we tested whether transcranial direct current stimulation (tDCS) to right PPC alters sensory-motor learning with reversed vision. We examined whether the degree of sensorimotor learning not only influences the transfer of skill performance in the untrained hand, but also modulates M1 excitability and PPC-M1 interactions in the untrained hemisphere. We measured transfer of skill performance in the untrained left-hand on a serial reaction-time task (SRTT) and cortical excitability with transcranial magnetic stimulation (TMS) in the untrained right hemisphere before and after four training interventions: (1) anodal tDCS to right PPC combined with directly viewing the active learning right-hand; (2) anodal tDCS to right PPC combined with viewing the 'mirrored' image of the moving right-hand superimposed over the inactive hand with left-right optical reversing spectacles (prism); (3) cathodal tDCS to right PPC combined with prism; and (4) sham tDCS combined with prism. We found that both anodal and cathodal tDCS to PPC: (i) impaired visuomotor adaptations; (ii) prevented transfer of skill learning to the untrained left-hand; (iii) decreased PPC-M1 excitability; and (iv) interfered with M1 'plasticity'. These results suggest that right PPC operates at an optimal excitability level and modulations of excitability interfere with the neural circuits for visuomotor adaptation.
Acknowledgments: NSERC & CIHR