On the possible role of stimulation duration for after-effects of transcranial alternating current stimulation

Abstract

Transcranial alternating current stimulation is a novel method that allows application of sinusoidal currents to modulate brain oscillations and cognitive processes. Studies in humans have demonstrated transcranial alternating current stimulation (tACS) after-effects following stimulation durations in the range of minutes. However, such after-effects are absent in animal studies using much shorter stimulation protocols in the range of seconds. Thus, stimulation duration might be a critical parameter for after-effects to occur. To test this hypothesis, we repeated a recent human tACS experiment with a short duration. We applied alpha tACS intermittently for 1 s duration while keeping other parameters identical. The results demonstrate that this very short intermittent protocol did not produce after-effects on amplitude or phase of the electroencephalogram. Since synaptic plasticity has been suggested as a possible mechanism for after-effects, our results indicate that a stimulation duration of 1 s is too short to induce synaptic plasticity. Future studies in animals are required that use extended stimulation durations to reveal the neuronal underpinnings. A better understanding of the mechanisms of tACS after-effects is crucial for potential clinical applications.

Keywords: EEG; NIBS; electroencephalography; entrainment; non-invasive brain stimulation; synaptic plasticity; tACS.

Figure 1

Experimental procedure and results. (A) The experiment consisted of two sessions recorded on two separated days. Sessions started with 3 min of spontaneous EEG recordings to estimate the individual alpha frequency (IAF), before the thresholds for skin sensation and phosphene perception were measured. Afterwards participants completed two stimulation blocks with 300 trials each separated by a 5 min break. (B) tACS electrodes were centered over Cz and Oz of the 10/20 system. A finite-element model simulation revealed that this montage results in current densities that are highest in the posterior cortex (see Neuling et al., for details). (C) Exemplary EEG data (electrode P3) from a typical trial. The participant was stimulated with ten cycles of tACS at 9 Hz starting at 0 ms. For each trial, the pre-tACS epoch from −1100 to −100 ms relative to tACS onset and post-tACS epoch from 1500 to 2500 ms relative to tACS onset were chosen for analysis. (D) Time-frequency plots of power (upper row) and intertrial coherence (ITC, bottom row) for the pre-tACS (left column) and the post-tACS epoch (right column) in the IAF session. (E) Power (upper panel) and ITC (lower panel) averaged across the pre-tACS (blue) and the post-tACS epoch (red) in the IAF session. (F) Mean power (upper panel) and mean ITC (lower panel) at IAF ± 1 Hz do not differ significantly between the pre-tACS (blue) and the post-tACS epoch (red), neither in the IAF session, nor in the control session. (G) Time course of mean power (left) at IAF ± 1 Hz in steps of 100 trials does not show differences between the IAF session (dark gray) and the control session (light gray). (H) Time course of mean ITC (right) at IAF ± 1 Hz in steps of 100 trials also shows no differences between the IAF session (dark gray) and the control session (light gray).