TABLE 1.
Summary of stimulation parameters and main results of in vivo human tACS studies outlined in the review.
| Study | Electrode montage and size | Stimulation intensity | Stimulation duration | Stimulation frequency | Method(s) | Sham included? | Main results |
| Ahn et al. (2019) | Cz: 5 cm × 7 cm F3, Fp1, T3 and P3: 5 cm2 |
1 mA | 20 min Twice a day for 5 days | 10 Hz | EEG, tDCS, auditory steady state auditory hallucinations questionnaire |
Yes, between-subjects | Restored alpha power in a group of participants who received 10-Hz alpha tACS, in comparison with groups who received tDCS or no stimulation. Furthermore, the study also reported an increase in the auditory steady-state response and a decrease in auditory hallucinations in the group that received tACS versus the groups that received tDCS or no stimulation. |
| Antal et al. (2008) | Left motor cortex: 4 cm2 Contra-lateral orbit: 5 cm × 10 cm |
0.4 mA | 5 min | 1 Hz, 10 Hz, 15 Hz, 45 Hz | TMS, Transcranial sinusoidal direct current (tSDSC), EEG Serial reaction time task |
Yes, within-subjects | Improved implicit motor learning after 10 Hz alternating current only. No lasting behavioral effects after 1 h. No changes in any frequency bands in the EEG post versus pre-stimulation. No significant changes in MEP amplitudes. |
| Gundlach et al. (2016) | CP3/CP4: 4 cm2 | 1 mA | 5 min | Individual somatosensory alpha (mu-α) | EEG, somatosensory detection task | Yes, within-subjects | Somatosensory perception thresholds were the same in the stimulation condition and sham. During mu-tACS, somatosensory detection thresholds were modulated as a function of the tACS phase. |
| Harada et al. (2020) | C3 and above right supra orbital 5 cm × 7 cm | 1 mA | 10 min | 10 Hz 20 Hz |
EEG, Visuomotor learning task | Yes, between-subjects | Performance on a visuomotor learning task improved after 10-Hz alpha stimulation in comparison with a 20-Hz stimulation and sham conditions. However, they did not find aftereffects for either stimulation condition. |
| Study 1: Cz and Oz: 5 cm × 7 cm Study 2: 10 electrodes around P& and P8 targeting the extra-striate: Ø 1.2 cm |
1 mA | 2 min | 10 Hz 40 Hz |
EEG | Yes, within-subjects | Alpha stimulation increased phase amplitude coupling and gamma power became preferentially locked to the trough of the alpha oscillation. Gamma stimulation increased the amplitude envelope correlations, and reduced alpha power. | |
| Helfrich et al. (2014a) | 10 electrodes around P7 and P8 targeting the extra-striate visual cortices: Ø 1.2 cm | 1 mA | 40 min (2 min × 20 min blocks) | 40 Hz | EEG, Ambiguous motion task |
Yes, within-subjects | Increased coherence in the gamma frequency band for in-phase stimulation compared with anti-phase stimulation during and up to 20 min after the task. Increase in phase coherence was associated with better performance on the ambiguous motion task. Increased gamma coherence was confined to the parieto-occipital areas (i.e., coherence was location specific). In both stimulation conditions, there was a decrease in alpha power. |
| Helfrich et al. (2014b) | Cz and Oz 5 cm × 7 cm | 1 mA | 20 min | 10 Hz | EEG | Yes, within-subjects | Increased power in the alpha frequency band post- and during stimulation versus pre-stimulation. Increase phase locking during stimulation. |
| Kasten et al. (2016) | Cz: 5 cm × 7 cm Oz: 4 cm2 |
Individually adapted 0.44–1.8 mA |
20 min | IAF | EEG, vigilance task | Yes, between-subjects | Alpha power increased post-versus pre-stimulation compared to sham. Alpha power was significantly higher in the stimulation condition than the sham for 70 min. After 70 min, alpha power in the sham condition increased, and diminished the difference between the sham and stimulation groups. |
| Kasten et al. (2019) | Cz: 5 cm × 7 cm Oz: 4 cm2 |
1 mA | 20 min | IAF | MEG, vigilance task | Yes, between-subjects | Variability of tACS aftereffects was significantly predicted by stimulation parameters of individual electric field modeling suggesting that individual stimulation protocols should be utilized. |
| Ketz et al. (2018) | F3/F4 Return at mastoids 5 cm2 |
1.5 mA | 5 cycles at detected slow-wave frequency | Closed-loop delta (0.5–1.2 Hz) | EEG, Target detection paradigm |
Yes, within-subjects | Closed-loop tACS during sleep enhanced sleep target detection accuracy post versus pre-sleep. |
| Kleinert et al. (2017) | F4, P4 and Cz: 5cm2 | 1 mA | 26 min | 5 Hz | EEG Match-to-sample task motor task |
Yes, within-subjects | No differences in behavioral task performance following in- or out of-phase stimulation. No differences post-versus pre-stimulation power in the stimulation frequency band; However, alpha power decreased post- versus pre- sham condition but not the stimulation condition. |
| Laczó et al. (2012) | Cz: 4 cm × 7 cm Oz: 4 cm2 |
1.5 mA | 45min ± 10min | 40 Hz 60 Hz 80 Hz |
Forced-choice detection task | Yes, within-subjects | Significantly decrease of contrast-discrimination thresholds during 60 Hz tACS, but no effect of 40 and 80 Hz stimulation |
| Lafleur et al. (2021) | C3 and C4 Ø 1.2 cm |
1 mA | 20min | 10 Hz 20 Hz |
EEG | Yes, within-subjects | No increase in alpha or beta power post- versus pre-stimulation after 10 and 20Hz stimulation respectively. |
| Lustenberger et al. (2016) | F3, F4: 3 cm2 Cz: 5 cm2 |
1 mA | 1.5 s stimulation trains. Total duration was variable per person | 12 Hz | EEG, EOG, EMG | Y, within-subjects | Closed-loop tACS selectively enhanced spindle activity. Enhanced spindle activity pre- versus post- sleep was correlated with improved motor memory in the stimulation and sham conditions. |
| Müller et al. (2015) | Cz and Oz 5 cm × 7 cm |
Individually determined (1.51 ± 0.38 mA) | 20 min over 5 consecutive days | IAF | EEG, Visual search task | Yes, between-subjects | Performance in the conjunction condition of a visual search task improved pre- versus post- stimulation in the group that received IAF stimulation versus sham. There was no significant difference in the performance in the easy or hard feature search task conditions. |
| Neuling et al. (2013) | Cz and Oz 5 cm × 7cm |
1.5 mA | 20 min | IAF | EEG, Vigilance task |
Yes, between-subjects | Sustained increase in power in the alpha band in the eyes-open condition for 30 min, but not in the eyes-closed, or sham conditions. |
| Noury et al. (2016) | O10and CP4: Ø 1.2 cm | 1 mA | 20 min total (2 min × 10 min blocks) | 11 Hz 62 Hz |
EEG, MEG |
Yes, between-subjects | tACS stimulation artifacts not only include the stimulation current but non-linear effects of heart beat and respiration. Existing stimulation artifact removal method still leave traces of the stimulation artifact. |
| Raco et al. (2016) | primary motor cortex: ring electrode internal Ø: 2.5 external Ø: 5 cm Pz: 5 cm × 6 cm |
1 mA | ∼20 min (3 min stimulation trains with 1 min break) | 20 Hz | EEG, TMS, MEP | Yes, within-subjects | Phase dependent modulation of the MEP when TMS was applied in four different parts of the beta phase, suggesting that the neural state during stimulation is important for accounting for variations in MEPs. |
| Riecke et al. (2015) | T7 and T8: 5 cm2 Return electrodes: symmetrically to the left and right side of the midline: 5 cm × 7 cm |
Individually determined 0.8 ± 0.1 mA | 39.6 min total (4 min × 9.9 min blocks) | 4Hz | Near-threshold auditory detection task | Yes within-subjects | Near threshold auditory train detection was modulated by the phase of delta stimulation. |
| Schwab et al. (2019) | 4-in-1 montage (area at P7-PO7 and P8-PO8) Ø 1.2 cm |
2 mA | 13 min | 10 Hz | EEG, ECG |
No | Post- versus pre-stimulation, connectivity between two hemispheres at the sensor level was greatest when stimulation was in-phase between the two hemispheres, followed by jittered phase and then anti-phase. These effects decayed in the first 120 milliseconds after stimulation offset. |
| Stecher and Herrmann (2018) | Cz: 5 cm × 7cm Oz: 4.5 cm2 |
1 mA | 38 min (1-, 3-, 5-, and 10-min blocks and reverse order) | IAF | EEG, visual vigilance task | Yes, between-subjects | No increase in alpha power post stimulation compared to sham. Follow up analysis suggests that a mismatch between stimulation frequency (IAF determined at the start of the experiment) and IAF at the end of the experiment may partially explain the lack of power enhancement. |
| Stecher et al. (2021) | Cz and Oz 5 cm × 7 cm |
1 mA | 20 min in total (150 8 s trains) | IAF (fixed) and closed-loop IAF | EEG, Visual detection task |
Yes, between-subjects | Fixed IAF stimulation produced an increase in alpha power pre- versus post- stimulation compared to closed-loop IAF stimulation and sham. There was no phasic modulation of visual stimulus detection in any condition. |
| Strüber et al. (2015) | Cz and Oz 5 cm × 7 cm |
Individually adjusted 0.76 ± 0.30 mA in IAF session 0.88 ± 0.37 mA in sham | 600 1 s stimulation trains | IAF | EEG, Visual detection task |
A control frequency (IAF × 3.1), within-subjects | No increase in alpha power post- versus pre-stimulation after short (1 s) trains of IAF stimulation in comparison to sham. No significant differences in performance in the visual detection task between conditions. |
| Vossen et al. (2015) | PO7, PO9, PO8, and PO10 5 cm × 7 cm |
Individually adjusted 1.35–2 mA | 22–30 min – depending on individual stimulation frequency | IAF *determined once for all 4 sessions | EEG | Yes, within-subjects | Increased alpha power post-versus pre-long stimulation (8 s) trains in comparison with short stimulation trains (3 s) and sham. Increase in alpha power occurred irrespective of phase continuity between long stimulation trains. |
| Wilsch et al. (2018) | T3 and T4: 4.18 cm Cz:5 cm × 7 cm |
Individually adjusted | 20 min per session | Speech envelope stimulation | EEG, Speech intelligibility task |
Yes, within-subjects | Intelligibility of speech in noise was better when speech envelope tACS was applied in comparison with noise. 5.12 Hz sinusoidal fit described the modulation of sentence comprehension better than linear and quadratic fits. There was also a significant 5Hz peak in the average power spectrum post versus pre-stimulation. |
| Wischnewski et al. (2019) | C3 T7/F3/Cz/P3 Ø 1 cm |
2 mA | 15 min | 20 Hz | EEG, TMS |
No, but control condition was the placebo group | No increase in beta power or MEP amplitudes post – versus pre-stimulation for participants who received an NMDAR antagonist (to block the cellular mechanism thought to underlie LTP) in comparison to a group that received a placebo. |
| Zaehle et al. (2010) | PO9 and PO10 5 cm × 7 cm |
Individually adjusted. 1.12 ± 0.49 mA | 10 min | IAF | EEG | Yes, between-subjects | Alpha power in the centro-parietal electrodes of the EEG increased post- versus pre-stimulation in the stimulation versus the sham group. |