Neuromodulation of parietal and motor activity affects motor planning and execution

Transcranial direct current stimulation (tDCS) is a non-invasive tool, which effectively modulates behavior, and related brain activity. When applied to the primary motor cortex (M1), tDCS affects motor function, enhancing or decreasing performance of both healthy participants and brain-damaged patients. Beyond M1, the posterior parietal cortex (PPC) is also crucially involved in controlling and guiding movement. Therefore, we explored whether the modulation of cortical excitability within PPC can also affect hand motor function in healthy right-handed participants. In Experiment 1, anodal tDCS (2 mA, 10 min) was applied to PPC and to M1 of both hemispheres. Skilled motor function of the non-dominant left hand, measured using the Jebsen-Taylor Hand Function Test (JTT), improved after anodal tDCS of the right, contralateral M1, as well as after the anodal stimulation of the left, ipsilateral PPC. Conversely, in Experiment 2, cathodal tDCS of the left PPC, or of the right M1, reduced motor performance of the left hand. Finally, Experiment 3 shows that the anodal tDCS of the left PPC selectively facilitated action planning, while the anodal tDCS of the right M1 modulated action execution only. This evidence shows that motor improvement induced by left parietal and right motor stimulations relies on substantial different mechanisms, opening up novel perspectives in the neurorehabilitation of stroke patients with motor and apraxic disorders.