The idea of tDCS was based on work showing that the delivery of polarising current along a neuron for 5 - 20 minutes produces a lasting change in the number of spikes a second within that neuron (Bindman et al., 1964). A similar principle was used for non-invasive stimulation in humans when a weak electric current (up to 2mA) was applied to a human scalp (Nitsche & Paulus, 2003). tDCS involves the administration of a fixed amount of direct current in the unit of milliamperes (mA) following a linear ramp up, which terminates after a ramp down. The direct current can be positive going (‘anodal’) or negative going (‘cathodal’), which usually ranges from -2mA to 2mA.
Applications of tDCS
When tDCS was first applied, anodal current (left M1 —> right orbita) was applied, the amplitude of motor evoked potentials (MEPs) produced by transcranial magnetic stimulation (TMS) increased. In contrast, when cathodal (right supraorbita —> left M1) stimulation was applied, the amplitude of MEPs decreased (Nitsche & Paulus, 2003). However, the effect of tDCS on corticospinal excitability is not as straightforward as anodal montages leading to excitatory effects and cathodal montages leading to inhibitory effects (Batsikadze et al., 2013). For instance, 1mA of cathodal stimulation is inhibitory whereas 2mA of cathodal stimulation is excitatory (Batsikadze et al., 2013). The 2mA cathodal effect is similar to the effect observed for 2mA anodal effect.
Variability has been revealed when attempting to use tDCS to change cortical excitability, particularly when the effects are probed using TMS (Tremblay et al., 2016). However, the use of current flow models has been useful in revealing how tDCS montages that claim to stimulate the ‘same’ brain region, despite having a different position of the return (or cathode) electrode (Bestmann et al., 2015). The use of current flow models has revealed that the direction of current flow relative to the orientation of the gyrus being stimulated is critical (Rawji et al., 2019). One experiment used two anodal (1mA) montages: one montage where the direction of current flow was orthogonal to the gyrus of the motor cortex and another where the current flow was parallel to the gyrus of the motor cortex. When current flow was orthogonal to the gyrus of M1, the current had a more consistent direction along the gyrus. In contrast, when the current flow was perpendicular to the gyrus of M1 there was less consistency in the direction of the current. There was also a decrease in MEP amplitude with the orthogonal current flow but not with perpendicular current flow, suggesting that a tDCS montage needs to carefully consider the underlying tissue orientation relative to the direction of induced current.
- tDCS changes in motor excitability are specific to orientation of current flow. Vishal Rawji, Matteo Ciocca, André Zacharia, David Soares, Dennis Truong, Marom Bikson, John Rothwell, Sven Bestmann. Brain Stimulation. April 2018
- Systematic assessment of duration and intensity of anodal transcranial direct current stimulation on primary motor cortex excitability. Sara Tremblay, Félix Larochelle-Brunet, Louis-Philippe Lafleur, Sofia El Mouderrib, Jean-François Lepage, Hugo Théoret. European Journal of Neuroscience. September 2016
- Understanding the behavioural consequences of noninvasive brain stimulation. Sven Bestmann, Archy O de Berker, James Bonaiuto
. Trends in Cognitive Sciences. January 2015
- Partially non-linear stimulation intensity-dependent effects of direct current stimulation on motor cortex excitability in humans. G Batsikadze, V Moliadze, W Paulus, M-F Kuo, M A Nitsche. The Journal of Physiology. April 2013
- Sustained excitability elevations induced by transcranial DC motor cortex stimulation in humans. M A Nitsche, W Paulus. Neurology. November 2001
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