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RESEARCH PRODUCT

P 56 tDCS shows no effects on motor cortex excitability at rest

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Introduction Transcranial direct current stimulation (tDCS) is a widely used technique in research and clinics. However, the underlying physiological mechanisms are not yet clear. Modeling studies suggest that the electric field during tDCS is dominated by a tangential component which predominantly modifies active synaptic connections ( Radman et al., 2007 ). We have previously shown that tDCS with an increased tangential compared to the conventional montage introduced by Nitsche and Paulus (2000) , suppresses the effects of paired associative stimulation of the supplementary motor area and the primary motor cortex (PASSMA-M1) independent of tDCS polarity (Faber et al., under submission). Here, we tested the influence of tDCS with increased tangential and absolute electric field strength on M1 excitability at rest. Methods 13 healthy right-handed male subjects took part in this pseudo-randomized, double-blinded, crossover study. They received two blocks (spaced by 5 min) of 5 min of 1 mA of anodal, cathodal or sham tDCS. Modeling studies showed an increase of tangential and general electric field strength between of the electrodes ( Miranda et al., 2013 , Opitz et al., 2015 ). Therefore, we placed the electrodes 3 cm behind the primary motor cortex to maximize the tangential field strength in the hand knob area. Sessions were separated by at least three days. We evaluated M1 excitability by changes in motor-evoked potentials (MEP) in the right first dorsal interosseus muscle of the right hand at baseline versus for up to two hours after the intervention. Results The repeated measurement Analysis of Variance showed neither an effect of time, nor Condition or Time * Condition Interaction on MEP amplitude. This displays the lack of effects of tDCS in a posterior montage in a system at rest. Conclusion tDCS placed 3 cm behind the primary motor cortex with an increased absolute as well as tangential electric field strength has no effects on M1 excitability in the motor system at rest. This finding confirms experiments on slice preparation and modeling studies that showed that the tangential component of the electric field during tDCS predominantly affects active synapses.