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 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 a 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).
- Increasing human motor skill acquisition by driving theta–gamma coupling.. Akkad, H., Dupont-Hadwen, J., Kane, E., Evans, C., Barrett, L., Frese, A., ... & Stagg, C. J.. Elife. 2021
- Shift in lateralization during illusory self‐motion: EEG responses to visual flicker at 10 Hz and frequency‐specific modulation by tACS. James Dowsett, Christoph S. Herrmann, Marianne Dieterich, Paul C.J. Taylor. European Journal of Neuroscience. August 2019
- Brain Oscillations and the Importance of Waveform Shape. Scott R Cole, Bradley Voytek. Trends in Cognitive Sciences. February 2017
- Transcranial Alternating Current Stimulation with Sawtooth Waves: Simultaneous Stimulation and EEG Recording. James Dowsett and Christoph S. Herrmann. Frontiers in Human Neuroscience. March 2016
- Alpha Power Increase After Transcranial Alternating Current Stimulation at Alpha Frequency (α-tACS) Reflects Plastic Changes Rather Than Entrainment. Alexandra Vossen, Joachim Gross, Gregor Thut. Brain Stimulation. June 2015
- From Oscillatory Transcranial Current Stimulation to Scalp EEG Changes: A Biophysical and Physiological Modeling Study. Isabelle Merlet,Gwénaël Birot, Ricardo Salvador, Behnam Molaee-Ardekani, Abeye Mekonnen, Aureli Soria-Frish, Giulio Ruffini, Pedro C. Miranda, Fabrice Wendling. PLOS ONE. February 2013
- Entrainment of Perceptually Relevant Brain Oscillations by Non-Invasive Rhythmic Stimulation of the Human Brain. Gregor Thut, Philippe G. Schyns, and Joachim Gross. Frontiers in Psychology. July 2011
- Transcranial Alternating Current Stimulation Enhances Individual Alpha Activity in Human EEG. Tino Zaehle, Stefan Rach, Christoph S. Herrmann. PLOS ONE. November 2010
- Endogenous electric fields may guide neocortical network activity. Flavio Fröhlich and David A. McCormick. Neuron. July 2010
- Comparatively weak after-effects of transcranial alternating current stimulation (tACS) on cortical excitability in humans. Andrea Antal, Klára Boros, Csaba Poreisz, Leila Chaieb, Daniella Terney, Walter Paulus. Brain Stimulation. April 2008
- Neuronal gamma-band synchronization as a fundamental process in cortical computation. Fries, P.. Annual Review of Neuroscience. 2009
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