Catodal or Anodal tDCS on Parietal Operculum do not affect Intra-limb Anticipatory Postural Adjustments

Recent data (Sepulcre 2014, Neuroscientist 20:499-508) suggests that the parietal operculum (PO) acts as an integration centre within a multimodal network, originating from different primary sensory cortices and projecting to frontal, parietal and temporal cortical hubs that govern cognitive and motor functions. Thus, PO might also play a crucial role in the control of voluntary movement and posture. In order to test this hypothesis, the Anticipatory Postural Adjustments (APAs) stabilizing the arm when the index-finger is briskly flexed (Cavallari et al 2016, Front Hum Neurosci 10:525) were recorded in three groups of 10 healthy subjects, before, during and after cathodal or anodal transcranial direct current stimulation (tDCS, 20 min at 2 mA) applied over PO. Results were compared to those obtained in a sham group. In agreement with literature data, in the sham group the activation of the prime mover Flexor Digitorum Superficialis 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 was observed in both the cathodal and anodal groups, with no significant tDCS effects on APAs amplitude or timing. Index-finger kinematics were also unchanged. These negative results suggest that PO should be excluded from the key network governing intra-limb APAs, indirectly confirming, as indicated in the literature, that such structure is mainly committed to cognitive functions (learning and memory) and in defining the aim of the motor action.