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High definition tDCS effect on postural control in healthy individuals: entropy analysis of a crossover clinical trial
* 1 , 1 , * 2 , 1 , 1 , 1 , 1 , 1 , 1 , 3, 4 , 1
1  University of Sao Paulo
2  Departamento de Matemática, Facultad de Ingeniería, Universidad de Atacama, Copiapó 1530000, Chile
3  Moss Rehabilitation Research Institute, Elkins Park, PA, USA.
4  Edith Cowan University, Joondalup, Australia.
Academic Editor: Stephen Meriney (registering DOI)

Objective: The identification of dose-response effects of transcranial direct current stimulation (tDCS) on postural control after stroke has highlighted this strategy as promising for post-stroke rehabilitation. Nonetheless, spatial-temporal dependence data have not been investigated using entropy analysis. Thus, we performed a nonlinear time series analysis of ground reaction force during and after the application of the high-definition transcranial direct current stimulation (HD-tDCS), over the right temporo-parietal junction (TPJ). Materials and Methods: We conducted a randomized, double-blind, placebo-controlled, crossover clinical trial. Twenty-one healthy young adults received the HD-tDCS and sham protocols. We evaluated the exchanging information (causal direction) between both force plates, using the summarized time series of transfer entropy, and compared the dose-response across the healthy subjects by a generalized linear mixed model (GLMM). Results: We found significant variation during the dynamic information flow (p<0.001) among the dominant bodyside. Specifically, all participants were right-handed, and a greater force transfer was observed from the right- to the left-side during the experiment. We observed a causal relationship in the information flow (equilibrium force transfer) from right to left and a decrease in entropy over time. Conclusions: HD-tDCS intervention induced a dynamic influence over time on postural control. Right-TPJ stimulation using HD-tDCS can induce an asymmetry of body weight distribution, leaning to the contralateral side of the stimulation, and thus a plausible post-stroke treatment.

Keywords: high-definition transcranial direct current stimulation; postural control; entropy; nonlinear time series.
Comments on this paper
Leonardo Barrios
Ground breaking on the postural manipulation.
Great work showing inovation towards the quantification of postural manipulation using HD-tDCS. Most of the works in the field only approach Entropy methods or GLM separatly, nonetheless, this paper brings together both approach complementing each other.

Marcela Di Marzo
Another perspective to the causal entropy.
Great work, most of the papers debating entropy are complex and present dense mathematical formulation without an intuitive application, but this one brings forward reasoning with statistical hypothesis testing. This paper shows relevant results and a promising application of the HD-tDCS to stroke recovering treatment.