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

Voluntary movements induce postural perturbations which are counteracted by anticipatory postural adjustments (APAs). These actions build up long fixation chains toward the available support points, to grant whole body equilibrium, but also develop short chains within the same limb where a distal segment is moved (intra-limb APAs), to stabilize the proximal segments. The neural structures generating intra-limb APAs still need investigations. In this view we tested the involvement of the parietal operculum (PO), which has been suggested to play a key role in motor control (Sepulcre 2014, Neuroscientist). APAs that stabilize the arm when the index-finger is briskly flexed were recorded in 9 healthy subjects, before, during and after anodal transcranial direct current stimulation (tDCS, 20 min at 2 mA) applied over PO, and compared to APAs recorded in 9 healthy subjects who underwent sham tDCS. In agreement with literature, in the sham group the activation of prime mover Flexor Digitorum Superficialis (FDS) was preceded by an inhibitory APA in Biceps Brachii and Anterior Deltoid, and almost simultaneous to an excitatory APA in Triceps Brachii. The same pattern also occurred in the anodal group, where tDCS did not significantly affect neither APAs amplitude nor timing. Index-finger kinematics were also unchanged. These preliminary results seem to exclude PO from intra-limb APA organization, though more experiments are needed to increase the sample size and also test cathodal tDCS.