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 of 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 straight forward 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 used TMS (Tremblay et al., 2016). However, the use of current flow models have 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 have 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 current flow was parallel to the gyrus of the motor cortex. When current flow was orthogonal to the gyrus of M1, current had a more consistent direction along the gyrus. In contrast, when current flow was perpendicular to the gyrus of M1 there was less consistently in the direction of current. There was also a decrease in MEP amplitude with 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.
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