Dual-hemisphere tDCS on parietal operculum does not affect the programming of intra-limb anticipatory postural adjustments

Evidences show that the postural and focal components within the voluntary motor command are functionally unique (doi: 10.1007/s00221-014-3866-4). However, we found that the SMA processes anticipatory postural adjustments (APAs) separately from the command to focal muscles (10.1016/j.bbr.2015.05.044). So we are still searching for a hierarchically higher area able to process both components. Among these, the parietal operculum (PO) seems to be a good candidate, as it integrates both sensory and motor streams (10.1177/1073858414531657). We reported that changing the excitability of PO contralateral to the moving segment did not affect intra-limb APAs (10.3389/fphys.2019.01159). In that paper, transcranial direct current stimulation (tDCS) was applied with an active electrode on PO vs. a much larger reference electrode on the opposite forehead. However, literature reports that two active electrodes of opposite polarities, one on each PO (dual-hemisphere tDCS), elicit stronger effects than the “active vs. reference” arrangement. Thus, APAs stabilizing the arm when the right index-finger is briskly flexed were recorded before, during and after dual-hemisphere tDCS on PO. Ten right-handed subjects were tested for each polarity: anode on the left vs. cathode on the right, and vice versa. Again, tDCS was ineffective on APAs amplitude and timing. Also index-finger kinematic was unchanged. These results confirm the conclusion that PO does not take part in intra-limb APA central control.