tACS involves the application of current that changes in direction through two or more electrodes. This usually involves sinusoidal changes in current direction based on work showing that changes in local field potential were synchronised with an externally applied field (Frohlich & McCormick, 2010). It is also possible to use non-sinusoidal waveforms, such as sawtooth waveforms, which have been combined with electroencephalography (EEG) to reduce stimulation artefacts (Dowsett et al., 2019). Current that changes in direction does not have to change in polarity either. It is also possible to apply oscillating tDCS (tSDCS or otDCS), where current direction changes without a change in polarity (e.g. Antal et al., 2008). An appealing application of tACS is its ability to provide causal evidence for frequency dependent changes in cortical excitability (Thut et al., 2011).
Applications of tACS
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 Hz, 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). The effects of tACS on frequency-specific changes in power are thought to be caused by spike-timing depended plasticity (Vossen et al., 2015). In a series of experiments, where individual alpha frequency tACS was delivered during EEG, an increase in alpha power was observed after alpha frequency tACS (Vossen et al., 2015). The increase in alpha power did not occur at the same phase as the tACS stimulation, suggesting that alpha oscillations were not being entrained at the tACS frequency. Instead, it appears that tACS increases power of dominant alpha frequency at rest, particularly when the frequency of tACS is just below this natural frequency (Vossen et al., 2015). This evidence shows where the potential of tACS lies as a therapeutic or experimental tool.
The strength of tACS or tDCS is its ability to probe the causal relevance of brain oscillations. However, the majority of experiments so far administer sinusoidal waveforms. However, real oscillations that take place in the brain are not always sinusoidal (Cole & Voytek, 2017). A waveform can possess the properties of tACS whilst also possessing non-sinusoidal properties. For example, sawtooth waveforms with a positive ramp up at individual alpha frequency (IAF) have been demonstrated to significant alter alpha power following tACS administration (Dowsett & Herrmann, 2016). Such waveforms are also complementary to tACS artefact techniques when tACS is combined with electroencephalography (EEG) (Dowsett & Herrmann, 2016; Dowsett et al., 2019). Such waveforms are showing promise in modulating the amplitude of posterior steady state evoked potentials (SSVEPs) (Dowsett et al., 2019).
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