Transcranial electrical stimulation (tES) is a means of non-invasively altering membrane excitability of a neuron by applying a weak electric current to the scalp. Conventionally, tDCS involves one anode and one cathode; current will always flow from the anode towards the cathode. One approach is transcranial direct current stimulation (tDCS), where stimulation can make action potentials less likely (with cathodal, negative polarity of tDCS) or make action potentials more likely (with anodal, positive polarity of tDCS). An alternative popular approach is to apply transcranial alternating current stimulation (tACS). With a tACS, the direction or the polarity of the current changes at a certain frequency. With tACS, changes in membrane excitability - or potential - are thought to be affected at the frequency the tACS waveform.
tDCS and tACs
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). When anodal current (left M1 —> right orbita) was applied, the amplitude of motor evoked potentials (MEPs) as evoked by transcranial magnetic stimulation (TMS) increased. In contrast, when cathodal (right supraorbita —> left M1) stimulation was applied, the amplitude of MEPs decreased. However, considerable variability has been revealed when attempting to use tDCS to change cortical excitability, particularly when the effects are probed used TMS (e.g. Tremblay et al., 2016). Recently, current flow models and comparisons with experimental data have highlighted the importance of applying current in a direction that is orthogonal to a gyrus, which reduces variability when stimulating the motor cortex at 1mA (Rawji et al., 2019).
Initially, tDCS was applied with one anode and one cathode, usually with 5 x 7cm electrodes which often has leads to peak induced electrical field at a site in between the electrodes as opposed to directly underneath the anode (Datta et al., 2009). However, the development of ‘high definition-tDCS (HD-tDCS) involving a ‘4 x 1 ring’ with four cathode surrounding one anode in a circular arrangement of disc shaped electrodes (Datta et al., 2009). A HD-tDCS montage around the motor cortex led to a peak in electric field immediately beneath the anode (in M1) but peaks in electric field elsewhere were absent (Datta et al., 2009), suggesting that HD-tDCS can be used to selectively depolarise regions beneath the anode with a reasonable degree of spatial resolution.
tACS, on the other hand, enables changes in cortical excitability to occur at a particular frequency. This usually involves sinusoidal changes in the amplitude of tES to occur at certain frequency, based on intracranial work showing that changes in local field potential were synchronised with an externally applied field (Frohlich & McCormick, 2010). In one of the first instances, tACS was applied at the individual alpha frequency (as measured in occipital cortex) and revealed an increase in alpha power during electroencephalography (EEG) measurement after tACS compared to before (Zaehle et al., 2010), which some reports of an after effect up to 30 minutes after stimulation (Neuling et al., 2013). Moreover, when applying tACS at a range of frequencies (4 - 16 Hz), an increase in alpha power was observed at 10 H, with a decrease in power that eventually fades into non-significance as frequencies outside a band of 8 - 12 Hz were used (Merlet et al., 2013). Taken together, these experiments suggest that tACS could be a promising means of providing causal evidence for oscillatory changes in voltage underlying basic neurophysiological processes.
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