In one of the first instances, tACS was applied at the individual alpha frequency (as measured in the 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 the 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 significantly 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., 2020).

Amplitude Modulated tACS (AM-tACS)

An amplitude modulated tACS (AM-tACS) waveform is illustrated in figure 1 below. Amplitude modulation involves two frequencies: a carrier frequency and an envelope frequency. The (slower) carrier frequency is the stimulation frequency of tACS whereas the (faster) envelope frequency enables the amplitude of tACS to change as a function of time. Hence, by modulating the amplitude of the envelope frequency (such as gamma), the carrier frequency can be produced (such as theta) from the formation of the sequential peaks.

(Figure 1: Akkad, H., Dupont-Hadwen, J., Kane, E., Evans, C., Barrett, L., Frese, A., ... & Stagg, C. J. (2021). Increasing human motor skill acquisition by driving theta–gamma coupling. Elife, 10, e67355. 10.7554/eLife.67355)

Amplitude-modulated tACS can be used to strengthen the coupling between 2 different frequency bands - a phenomenon hypothesised to drive cortical computation in neocortical areas (Fries, 2009; Akkad et al., 2021). For example, Akkad et al., (2021) have externally applied a theta-gamma phase-amplitude coupled tACS waveform to strengthen a hypothesised endogenous learning-related mechanism in participants. When the gamma oscillations were phase-locked to the peaks of the theta sinusoidal rhythm Akkad and colleagues found enhancements in motor learning. Conversely, when the gamma oscillations were phase-locked to the troughs of the theta sinusoidal rhythm (as an active control) there were no significant differences compared to the sham condition. This specialised asymmetrical waveform shown in Figure 1 can be delivered to a stimulator (e.g., nurostym tES) via a data acquisition device (e.g., CED Micro1401).