###### Abstract  
Objective: Converging evidence supporting an effect of transcranial direct current stimulation (tDCS) on postural control and human verticality perception highlights this strategy as promising for post-stroke rehabilitation. We have previously demonstrated polarity-dependent effects of high-definition tDCS (HD+DCS) on weight-bearing asymmetry. However, there is no investigation regarding the time-course of effects on postural control induced by HD+tDCS protocols. Thus, we performed a nonlinear time series analysis focusing on the entropy of the ground reaction force as a secondary investigation of our randomized, double-blind, placebo-controlled, crossover clinical trial. Materials and Methods: Twenty healthy right-handed young adults received the following conditions (random order, separate days); anode center HD+tDCS, cathode center HD+tDCS or sham HD-tDCS at 1, 2, and 3 mA over the right temporo-parietal junction (TPJ). Using summarized time series of transfer entropy, we evaluated the exchanging information (causal direction) between both force plates and compared the dose-response across the healthy subjects with a Generalized Linear Hierarchical/Mixed Model (GLMM). Results: We found significant variation during the dynamic information flow (\(p<0.001\)) among the dominant bodyside (and across time). A greater force transfer entropy was observed from the right to the left side during the cathode-center HD+tDCS up to 2 mA, with a causal relationship in the information flow (equilibrium force transfer) from right to left that decreased over time. Conclusions: HD-tDCS intervention induced a dynamic influence over time on postural control entropy. Right hemisphere TPJ stimulation using cathode-center HD+tDCS can induce an asymmetry of body weight distribution towards the ipsilateral side of stimulation. These results support the clinical potential of HD+tDCS for post-stroke rehabilitation. high-definition transcranial direct current stimulation; postural control; entropy; nonlinear time series +
Footnote †: journal: _Sci._  
Footnote †: journal: _Sci. synergies [3]. The visual verticality perception (VV) disorder, the incapacity to judge the orientation of the body or environment in relation to Earth vertical within normal limits, is commonly observed after stroke and is associated with poor balance [4]. When in a standing position, patients with VV disorder present a weight asymmetry towards the opposite side of the VV tilt as a compensatory strategy to maintain their center of gravity within the limits of stability [5; 6; 7; 8]. Lesions of the temporo-parietal junction (TPJ), a hub area for multisensory integration, can cause VV disorder and postural imbalance [9]. Non-invasive brain stimulation (NIBS) techniques, such as conventional and high-definition transcranial direct current stimulation (HD+tDCS) are current therapeutic resources with potential modulation over the pathophysiology and behavior of brain mechanisms [10]. Recently, we have observed significant effects of conventional non-invasive transcranial stimulation (tDCS) [11] and HD-tDCS [12] applied over the TPJ in both healthy subjects and patients after stroke. Clinical VV and weight-bearing asymmetry (WBA) disorders described in patients after stroke [4; 12; 13] were reproduced in healthy individuals after using our stimulation protocol. We found the effect was dependent on the cathode center condition with induction of WBA towards the side of brain stimulation [12] with no dependency on the electrical current intensity. Other studies evaluated electroencephalography (EEG) after our HD-tDCS protocol in healthy subjects and suggested entropy (nonlinear outcome) of EEG as a robust alternative for temporal data analysis complementing linear analysis [14]. We hypothesized that the HD-tDCS would induce a sequence of events on postural control demonstrated by an influence in the WBA. Thus, we analyzed the ground reaction force in each platform through the flow of information using transfer entropy.