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. 2020 Dec 2;10(12):932. doi: 10.3390/brainsci10120932

Table 1.

Overview of study designs, tasks, stimulation parameters, electrode positions, as well as risk of bias for all included studies.

Authors Participants and Design Task and Dependent Variables Electrode Montage and Device Stimulation Parameters Outcome/Risk Bias
Attention
Alpha
[30] N = 36 (mean age: 21.56 years, range 18–29 years)
Crossover within-subject design (tACS vs. sham)
Endogenous and exogenous spatial attention task
Reaction time bias score
Left parietal cortex (P3) + centered above target electrode
Device: high-definition tES (NeuroConn, Ilmenau, Germany)
Frequency: 10 Hz/sham
Intensity: 1 mA peak-to-peak
Duration: 35–40 min
Electrode size: target electrode (round-shaped, diameter: 2.1 cm), return electrode (round-shaped, outer diameter: 11 cm, inner diameter: 9 cm)
Timing: online
↗ (for endogenous task only)
Low risk
[31] Exp. 1
N = 20 (mean age: 26.6 ± 4.1 years)
Exp. 2
N = 18 (mean age: 22.11 ± 2.4 years)
All Exp.
Crossover within-subject design (tACS vs. sham)
Selective auditory attention task
Number of /ga/-syllables in spatial (left or right direction) or non-spatial trials (female/male speakers)
Parietal cortex (P2) + (CP2/P4/Pz/PO4)
Soterix M × N 9 high definition-tES Stimulator (Soterix Medical, New York, NY)
Frequency: Exp. 1
10 Hz/sham; Exp. 2 6 Hz/sham
Intensity: 1.5 mA peak-to-peak
Duration: 20 min incl. 30 s ramp-in/-out
Electrode Size: not provided
Timing: online
↘ (with alpha-tACS)
− (with theta-tACS)
Low risk
[32] Exp. 1
N = 24 (mean age: 21)
Exp. 2
N = 12 (mean age: 21.2 ± 0.2 years)
All Exp.
Crossover within-subject design (tACS vs. sham)
Temporal order judgement task
Forced-choice verbal report of stimuli order
Exp. 1
Left somatosensory cortex (C3) or left posterior parietal cortex (P3) + ipsilateral lower cheek
Exp. 2
Right posterior parietal cortex (P4) + ipsilateral lower cheek
Device: Eldith (NeuroConn GmbH, Ilmenau, Germany)
Frequency: 10 Hz/sham
Intensity: 1 mA peak-to-peak
Duration: 270 s
Electrode size: both 5 × 5 cm
Timing: online

Low risk
Alpha and Beta
[33] Exp. 1
N = 18 (mean age: 20.66 ± 2.53 years)
Crossover within-subject design (10 Hz versus 20 Hz versus sham)
Exp. 2
N = 15 (mean age: 20.26 ± 2.35 years)
Crossover within-subject design (sham vs. 10 Hz in-phase vs. 10 Hz anti-phase vs. 20 Hz in-phase vs. 20 Hz anti-phase)
Attentional blink task
Detection of second target stimuli
Exp. 1
Right posterior parietal cortex (P4) + right deltoid
Exp. 2
Right posterior parietal cortex (P4) and left frontal cortex (F3) + right deltoid
Device:
Exp. 1 BrainStim (EMS Medical, Bologna, Italy);
Exp. 2 StarStim 8 (Neuroelectrics, Boston, Massachusetts)
Frequency:
Exp. 1 10 Hz/20 Hz/sham;
Exp. 2 10 Hz in-phase/10 Hz anti-phase/20 Hz in-phase/20 Hz anti-phase/sham
Intensity: 0.35 mA
Duration:
Exp. 1 35 min; Exp. 2 50 min
Electrode size:
Exp. 1 both 7 × 5 cm; Exp. 2 both round-shaped electrodes (25 cm2)
Timing: online
↗ (with anti-phase beta-tACS)
− (with alpha-tACS or in-phase beta-tACS)
Low risk
Alpha and Gamma
[34] Exp. 1
N = 48 (mean age: 22.7 ± 2.9 years)
Exp. 2
N = 37 (mean age: 22.7 ± 3.5 years)
Exp. 3
N = 41 (mean age: 23.2 ± 2.7 years)
Exp. 4
N = 43 (mean age: 23.1 ± 3.1 years)
Crossover within-subject design (tACS vs. sham vs. control (50 Hz) tACS)
Visual or auditory sustained attention tasks
Correct responses, reaction times
Occipital cortex (Oz) + vertex (Cz)
Device: Starstim (Neuroelectrics, Barcelona)
Frequency: 10 Hz/50 Hz/sham
Intensity: 2 mA peak-to-peak
Duration: 11 min
Electrode size: both 25 cm2 (round-shaped)
Timing: online
↗ (with alpha-tACS)
− (with gamma-tACS)
Low risk
[35] N = 23 (age range: 18–27 years)
Crossover within-subject design (alpha tACS vs. gamma tACS vs. sham)
Selective spatial attention task (endogenous or exogenous)
Reaction times
Inferior parietal lobe (P6) + vertex (Cz)
Device: NeuroConn DC Stimulator Plus (NeuroCare, Ilmenau, Germany)
Frequency: 10 Hz/40 Hz/sham
Intensity: 2 mA peak-to-peak
Duration: Endogenous blocks: ~33 min
Exogenous blocks: ~21 min
Electrode size: target electrode 5 × 5 cm, return electrode 5 × 7 cm
Timing: online
↗ (with alpha-tACS)
− (with gamma-tACS)
Low risk
[36] N = 20 (age range: 19–31 years)
Crossover within-subject design (tACS vs. control vs. sham)
Dichotic Listening task
Correct responses
Fronto-central and temporo-parietal cortex (FC5) + TP7
Device: NeuroConn DC Stimulator Plus (NeuroCare, Ilmenau, Germany)
Frequency: 10 Hz/47.1 Hz/sham
Intensity: 1 mA peak-to-peak
Duration: 25 min
Electrode Size: both 7 cm2 (round-shaped, 3 cm in diameter)
Timing: online
alpha-tACS:
↗ (for left-attend condition)
↘ (for right-attend condition)
gamma-tACS:
↘ (for left-attend condition)
↗ (for right-attend condition)
Low risk
[37] N = 26 (mean age: 28.5 ± 7.9 years)
Crossover within-subject design (tACS vs. sham)
Dichotic Listening task
Correct responses
Multifocal approach: (FT9 + FT10/F3 + F4/CP5 + CP6/P5 + P6)
Device: NeuroConn DC Stimulator Plus (NeuroCare, Ilmenau, Germany)
Frequency: 40 Hz/sham
Intensity: 1 mA peak-to-peak
Duration: 20 min
Electrode Size: all electrodes round-shaped, 12 mm in diameter
Timing: online
↗ (for left ear-perception only)
Low risk
Gamma
[38] N = 20 (mean age: 25.8 ± 6.2)
Crossover within-subject design (40 Hz vs. 60 Hz vs. 80 Hz vs. sham)
Visual spatial attention task
Correct responses
Occipital area (Oz) + vertex (Cz)
Device: DC-Stimulator Plus (NeuroConn, Ilmenau, Germany)
Frequency: 40 Hz/60 Hz/80 Hz/sham
Intensity: 1.5 mA peak-to-peak
Duration:
Exp. 1: 45 ± 10 min,
Exp. 2: 15 ± 5 min
Electrode size: target electrode 4 × 4 cm, return electrode 7 × 4 cm
Timing: online
↗ (with 60 Hz gamma-tACS)
− (with other tACS)
Low risk
Intelligence
Theta
[39] N = 20 (mean age: 24.9 ± 3.3 years)
Crossover within-subject design (tACS vs. sham)
Raven’s progressive matrices
Correct responses
left temporal cortex (T7) + C3
Device: DC-Stimulator Plus (NeuroConn, Ilmenau, Germany)
Frequency: 5 Hz/sham
Intensity: 1.5 mA peak-to-peak
Duration: 15 min
Electrode size: both 5 × 7 cm
Timing: offline
↗ (for difficult items only)
Low risk
Alpha
[40] N = 20 (mean age: 20.18 ± 0.40)
Crossover within-subject design (tACS vs. sham)
Raven’s progressive matrices
Correct responses
Frontal cortex (F3) + F4
Device: NeuroConn DC Stimulator Plus (NeuroCare, Ilmenau, Germany)
Frequency: 1 Hz above IAF/sham
Intensity: Modus = 1.75 mA peak-to-peak (Range = 1 mA to 2 mA)
Duration: 15 min
Electrode Size: both 7 × 5 cm
Timing: offline

Low risk
Alpha and Gamma
[41] N = 22 (mean age: 23.00 ± 2.85 years)
Crossover within-subject design (alpha tACS vs. gamma tACS vs. sham)
Verbal intelligence task
Correct responses
Frontal cortex (F3) + F4
Device: Eldith (NeuroConn GmbH, Ilmenau, Germany)
Frequency: 10 Hz/40 Hz/sham
Intensity: 1 mA peak-to-peak
Duration: 30 min incl. 100 cycles ramp-in/-out
Electrode size: both 5 × 7 cm
Timing: online

Low risk
Gamma and Theta
[42] Exp. 1
N = 24 (mean age: 23.8 ± 3.14 years)
Exp. 2
N = 34 (mean age: 24.3 ± 2.76 years)
Crossover within-subject design (tACS vs. sham vs. control)
Modified version of Raven’s progressive matrices
Correct responses, response times
Frontal cortex (left MFG) + vertex (Cz)
Device: Eldith DC-Stimulator, NeuroConn, Germany
Frequency:
Exp. 1 40 Hz/ 5 Hz/sham;
Exp. 2 40 Hz/sham
Intensity: 0.75 mA peak-to-peak
Duration: ~30 min
Electrode Size: both 5 × 5 cm
Timing: online
↗ (with gamma-tACS, benefit for low performer only)
− (with other tACS)
Low risk
Multiple Frequencies
[43] N = 20 (mean age: 20.2 ± 12.3 years)
Crossover within-subject design (tACS vs. sham vs. control)
Raven’s progressive matrices
Correct responses, response times
Frontal cortex (left MFG) + vertex (Cz)
Device: Eldith DC-Stimulator, NeuroConn, Germany
Frequency: 5 Hz/10 Hz/20 Hz/40 Hz/sham
Intensity: 0.75 mA peak-to-peak
Duration: through task (~48 min), per condition
Electrode Size: both 5 × 7 cm
Timing: online
↗ (with gamma-tACS)
− (with other tACS)
Low risk
Visual Perception
Alpha
[44] N = 96, divided into 4 groups with each N = 24
6 Hz (mean age: 21 ± 2.4 years)
10 Hz (mean age: 21 ± 3.2 years)
25 Hz (mean age: 22 ± 2.1 years)
sham (mean age: 22 ± 2.3 years)
Between-subject design
Visual detection and discrimination task
Correct responses
Parieto-occipital cortex (PO7/PO8) + vertex (Cz)
Device: NeuroConn DC Stimulator (NeuroCare, Ilmenau, Germany)
Frequency: 6 Hz/10 Hz/25 Hz/sham
Intensity: 1 mA peak-to-peak
Duration: 5 min
Electrode size: target electrode 16 cm2, return electrode 35 cm2
Timing: online
↘ (with theta- or alpha-tACS)
Low risk
[45] N = 15 (mean age: 24.3 ± 0.7 years)
Crossover within-subject design
Visual rotation detection task
Correct responses
Visual cortex stimulation montage (Oz) + Cz
Retinal stimulation montage (Fpz) + Cz
Device: NeuroConn DC Stimulator Plus (NeuroCare, Ilmenau, Germany)
Frequency: IAF/IAF+4 Hz/IAF–4 Hz
Intensity: Oz–Cz: 0.96 ± 0.3; Fpz–Cz: 2.31 ± 1.1 mA peak-to-peak
Duration: 27 min (~5.4 s per frequency-montage combination)
Electrode size: both
5 × 5 cm
Timing: online

Low risk
Gamma
[46] N = 60 (mean age: 32.22 ± 7.96 years)
40 Hz (N = 12)
40 Hz (N = 16)
5 Hz (N = 16)
sham (N = 16)
Between-subject design
Face perception task, object perception task,
Correct responses, reaction times
Occipital cortex (PO8) + frontal cortex (FP1)
Device: Neuroelectrics, Barcelona, Spain
Frequency: 40 Hz/5 Hz/sham
Intensity: 1.5 mA peak-to-peak
Duration: 20 min
Electrode size: both
25 cm2
Timing: offline
↗ (with gamma-tACS)
− (with theta-tACS)
Low risk
[47] N = 45 (mean age: 24.9 ± 4.1)
Exp. 1 N = 17
Exp. 2 N = 13
Exp. 3 N = 15
All Exp.
Crossover within-subject design
Stroboscopic alternative motion task
Motion dominance index (% duration of perceived motion direction)
Exp. 1 and Exp. 2: parieto-occipital cortex (P7-PO7) + (P8-PO8)
Exp. 3: Central (C3/C4) + occipital areas (O1/O2)
Device: NeuroConn, Ilmenau, Germany
Frequency:
Exp. 1 anti-phase 40 Hz tACS/sham;
Exp. 2 antiphase 6 Hz/sham;
Exp. 3 in-phase 6 Hz/40 Hz/sham
Intensity: 0.5–1.5 mA peak-to-peak
Duration: 15 min
Electrode size:
Exp. 1 and Exp. 2: both 5 × 7 cm; Exp. 3: both 3.9 × 3.9 cm
Timing: online
↘ (with anti-phase gamma-tACS)
− (with other tACS)
Low risk
Auditory Perception
Delta/Theta
[48] N = 17 (mean age: 33 ± 8 years)
Crossover within-subject design
Word detection task
Correct responses
Left temporal cortex (T7) + C3
Device: DC-Stimulator MR (NeuroConn, Ilmenau, Germany)
Frequency: 3.125 Hz/sham
Intensity: 1.7 mA peak-to-peak
Duration: ~30 min
Electrode size: target electrode 3 × 3 cm, return electrode 5 × 7 cm
Timing: online

High risk (small sample sizes)
[49] Exp. 1
N = 27 (mean age: 31 ± 7 years)
Exp. 2
N = 19 (mean age: 21 ± 2 years)
Crossover within-subject design
Speech perception task
Correct responses
Exp. 1
Unilateral montage: left temporal cortex (T7) + C3
Exp. 2
Bilateral montage: temporal cortex (T7/T8) + 2 large ring electrodes (around target electrode)
Device: NeuroConn DC Stimulator MR (NeuroCare, Ilmenau, Germany)
Frequency: 3.125 Hz/sham
Intensity:
Exp. 1
1.2 mA peak-to-peak;
Exp. 2 1.7 mA peak-to-peak
Duration: ~30 min
Electrode Size: both 5 × 7 cm
Timing: online
Bilateral stimulation:
↗ (when stimulation and speech were aligned)
↘ (when stimulation and speech were not aligned)
− (unilateral stimulation)
Low risk for Exp. 1
High risk for Exp. 2 (small sample size)
Theta
[50] N = 20 (age range: 18–38 years)
Crossover within-subject design
Stream perception task
Correct responses
Temporal cortex (T7/T8) + L/R side of midline (Cz)
Device: NeuroConn DC Stimulator (NeuroCare, Ilmenau, Germany)
Frequency: 4 Hz/sham
Intensity: 0.8 ± 0.1 mA peak-to-peak
Duration: 40 min
Electrode Size: both target electrodes 5 × 5, both return electrodes 5 × 7 cm
Timing: online
↗ (when streams were synchronous with entrained oscillatory activity)
Low risk
[51] N = 22 (age range: 18–28 years)
Crossover within-subject design
Speech perception task
Correct responses
Temporal cortex (T7/T8) + Cz
Device: DC-Stimulator MR (NeuroConn, Ilmenau, Germany)
Frequency: 4 Hz/sham
Intensity:
Exp. 1: 0.9 ± 0.1 mA peak-to-peak,
Exp. 2: 1 ± 0.1 mA peak-to-peak
Duration: 36 min
Electrode size: both target electrodes 5 × 5 cm, return electrode 10 × 7 cm
Timing: online

Low risk
Alpha and Beta
[52] N = 26 (mean age: 21.4 ± 4.7 years)
Crossover within-subject design
Phonological decision task
Correct responses, reaction times
Fronto-temporal cortex (Crossing point of T3-Fz × F7-Cz) + (T4-Fz × F8-Cz)
Device: NeuroConn DC Stimulator (NeuroCare, Ilmenau, Germany)
Frequency: 10 Hz/16.18 Hz/sham
Intensity: 1 mA peak-to-peak
Duration: 20 min
Electrode Size: both 9 cm2 (round-shaped)
Timing: offline
With alpha-tACS:
↗ (reaction time)
↘ (error rates)
− (with other tACS)
Low risk
Gamma and Theta
[53] Exp. 1
N = 21 (mean age: 24.3 ± 2.0 years)
Exp. 2 (control)
N = 17 (mean age: 27.5 ± 3.3 years)
All Exp.
Crossover within-subject design
Phonetic categorization task
Correct responses, perceptual learning
Exp. 1 only temporal areas (T7) + T8
Device: NeuroConn DC Stimulator (NeuroCare, Ilmenau, Germany)
Frequency:
Exp. 1
40 Hz/6 Hz;
Exp. 2 No stimulation
Intensity: 1 mA peak-to-peak
Duration: 18 min in total, in 6 min blocks
Electrode Size: both 5 × 7 cm
Timing: online
↗ (with gamma-tACS in older adults)
↘ (with gamma-tACS in younger adults)
− (with theta-tACS)
Low risk
[54] N = 25 (mean age: 24.1 ± 2.24 years)
N = 20 (mean age: 69.8 ± 4.4 years)
Between-subject/Crossover design
Phonetic categorization task
Correct responses, perceptual learning
Temporal cortex (T7) + T8
Device: NeuroConn DC Stimulator (NeuroCare, Ilmenau, Germany)
Frequency: 40 Hz/6 Hz
Intensity: 1 mA peak-to-peak
Duration: 10 min
Electrode Size: both 5 × 7 cm
Timing: online
↘ (with gamma-tACS)
↗ (with theta-tACS)
Low risk
Executive functions
Theta
[55] N = 23 (mean age: 22.91 years, range 18–30)
Crossover within-subject design
Go/NoGo task
Correct responses, reaction times
Fronto-temporal area (T4-Fz × F8-Cz) + supraorbital
Device: NeuroConn DC Stimulator (NeuroCare, Ilmenau, Germany)
Frequency: 6 Hz/sham
Intensity: 1 mA peak-to-peak
Duration: 20 min
Electrode Size: both 5 × 5 cm
Timing: online/offline

Low risk
[56] N = 90 (N = 30 for each experiment)
Exp. 1
mean age: 26 years
Exp. 2
mean age: 27 years
Exp. 3
mean age: 26 years
Crossover within-subject design
Time estimation task
Correct responses
Exp. 1/3 medial frontal and lateral prefrontal cortex (right: MFC/IPFC) + 4 surrounding return electrodes
Exp. 2 similar to Exp. 1, but left hemisphere
Device: M x N 9 high-definition Stimulator (Model 9002A, Soterix Medical, New York, NY)
Frequency: 6 Hz in-phase/anti-phase/35 Hz/sham
Intensity: 1 mA peak-to-peak
Duration:
Exp. 1/2: 20 min; Exp. 3: 20 min
Electrode size: high definition-tACS
Timing: online
↗ (tACS on right hemisphere)
− (tACS on left hemisphere or gamma-tACS)
Low risk
Theta and Alpha
[57] N = 31 (age range: 19-31 years)
Crossover within-subject design
Simon task
Correct responses, response times
left frontal cortex (between FCz and Cz) + both cheeks
Device: DC-Stimulator MR (NeuroConn, Ilmenau, Germany)
Frequency: ITF, IAF
Intensity: 2 mA peak-to-peak
Duration: ~20 min
Electrode size: target electrode 9 cm2, both return electrodes 35 cm2
Timing: online
↗ (with theta-tACS)
− (with alpha-tACS)
Low risk
Alpha
[58] N = 20 (mean age: 26 ± 3 years)
tACS (N = 10)
sham (N = 10)
Between-subject design
Mental rotation task
Correct responses, reaction times
Occipital cortex (Oz) + vertex (Cz)
Device: NeuroConn (NeuroCare, Ilmenau, Germany)
Frequency: 10.5 ± 0.9 Hz/sham
Intensity: 0.7 ± 0.3 mA peak-to-peak
Duration: 20 min
Electrode size: target electrode 4 × 4 cm, return electrode 5 × 7 cm
Timing: online
↗ (accuracy)
Low risk
Alpha and Beta
[59] N = 19 (mean age: 24 ± 4 years)
Crossover within-subject design
Temporal bisection task
Choice (long or short) Reaction times
SMA (FC1) + FC2
Device: NeuroConn MagStim (NeuroCare, Ilmenau, Germany)
Frequency: 10 Hz/20 Hz/sham
Intensity: 1.5 mA peak-to-peak
Duration: 20 min
Electrode size: both 5 × 5 cm
Timing: online

Low risk
Multiple Frequencies
[60] N = 36 (mean age: 24.4 ± 3.5 years)
Crossover within-subject design
Flanker task
Speed-accuracy-trade off and reaction times
Medial-frontal Cortex (FCz) +
Pz
Device: Neuroelectrics, Barcelona, Spain
Frequency: 2 Hz/6 Hz/11 Hz/21 Hz/60 Hz/sham
Intensity: 1.5 mA peak-to-peak
Duration: 20 min
Electrode size: both
25 cm2
Timing: online
↗ (with theta-tACS)
− (with other tACS)
Low risk
Decision-making
Multiple Frequencies
[61] N = 34 in total
left frontal montage:
N = 17 (mean age: 20.52 ± 2.52 years)
right frontal montage:
N = 17 (mean age: 21.17 ± 2.78 years)
Between-subject design
Decision making task
Gains and losses and response times
Left frontal cortex (F3) + ipsilateral shoulder
Right frontal cortex (F4) + ipsilateral shoulder
Device: BrainStim, EMS Medical, Bologna, Italy
Frequency: 5Hz/10 Hz/20 Hz/40 HZ/sham
Intensity: 1 mA peak-to-peak
Duration: 40 min
Electrode Size: both 5 × 7 cm
Timing: online
↘ (with beta-tACS to left hemisphere = riskier decisions)
↗ (with tACS to the right hemisphere)
Low risk
Procedural Memory (motor learning)
Theta
[62] N = 26 (mean age: 21.38 ± 1.52 years)
Crossover within-subject design
Implicit motor learning task
Correct responses, reaction times
Frontal area (FPz) + Pz
Device: NeuroConn DC Stimulator Plus (NeuroCare, Ilmenau, Germany)
Frequency: 6 Hz/sham
Intensity: 1 mA peak-to-peak
Duration: 20 min
Electrode Size: both 5 × 5 cm
Timing: online

Low risk
Alpha
[63] N = 20 (mean age: 23.8 ± 3.90 years)
N = 15 (mean age: 61.66 ± 3.71 years)
Between-subject design (older vs. younger participants)/Crossover within-subject design
Implicit motor learning task
Correct responses, reaction times
Central area (C3) + supraorbital region
Device: Eldith DC Stimulator, NeuroConn ((NeuroCare, Ilmenau, Germany)
Frequency: IAF/IAF + 2 Hz/sham
Intensity: 1.5 mA peak-to-peak
Duration: 10 min
Electrode Size: target electrode 5 × 7 cm, return electrode 10 × 10 cm
Timing: offline
− (accuracy)
Reaction times:
↗ (in older adults with IAF and IAF + 2 Hz)
↗ (in younger adults with IAF + 2 Hz only)
Low risk
Alpha and Beta
[64] N = 38 (mean age: 23 ± 3 years)
Crossover within-subject design
Movement selection task
Duration between stimulus onset and verbal responses
Central area (C3 or C4) + Pz
Device: NeuroConn DC Stimulator Plus (NeuroCare, Ilmenau, Germany)
Frequency: 10 Hz/20 Hz/sham
Intensity: 1.1 ± 0.3 mA peak-to-peak (range: 0.4–1.9 mA)
Duration: ~30 min
Electrode Size: both 5 × 5 cm
Timing: online
↗ (with alpha-tACS ipsilateral to requested hand)
− (with beta-tACS)
Low risk
Beta and Gamma
[65] N = 20 (mean age: 24.9 years)
Crossover within-subject design
Cued reaction time task
Reaction times
Central area (left M1) + supraorbital region
Device: NeuroConn DC Stimulator (NeuroCare, Ilmenau, Germany); BiStim (MagStim)
Frequency: IBF (or fixed 20.1 Hz)/75 Hz/sham
Intensity: 0.69 ± 0.11 mA peak-to-peak; 1.3 ± 0.36 mA peak-to-peak
Duration: 20 min
Electrode Size: both 5 × 7 cm
Timing: online
↗ (with gamma-tACS)
− (with beta-tACS)
Low risk
Gamma
[66] N = 17 (mean age: 24.5 ± 3.5 years)
Crossover within-subject design
Implicit motor learning task
Reaction times
Central area (left M1) + right supraorbital region
Device: NeuroConn DC Stimulator (E.M.S., Bologna, Italy)
Frequency: 1 Hz/40 Hz/sham
Intensity: 2 mA peak-to-peak
Duration: 5.08 ± 1.13 min
Electrode Size: both 5 × 5 cm
Timing: online
↘ (with gamma tACS)
− (with other tACS)
Low risk
Multiple Frequencies
[67] N = 55 (mean age: 32.7 ± 6.8 years)
Crossover within-subject design
Implicit motor learning task
Correct responses, Reaction times
Central area (C3) + supraorbital
Device: NeuroConn DC Stimulator Plus (NeuroCare, Ilmenau, Germany)
Frequency: 10 Hz/20 Hz/70 Hz/sham
Intensity: 1 mA peak-to-peak
Duration: 10 min
Electrode Size: both 5 × 7 cm
Timing: offline
↗ (with gamma-tACS)
− (with other tACS)
Low risk
Working memory
Theta
[68] N = 25 (mean age: 23.5 ± 2.9 years)
Crossover within-subject design
N-back task
Correct responses, reaction times
Frontal/parietal cortex
Sync: AF3/P3 in-phase + AF4/P4 in-phase
Desync: AF3/AF4 in-phase + P3/P4 in-phase
Device: Starstim (Neuroelectrics, Barcelona)
Frequency: 6 Hz/sham
Intensity: 2 mA peak-to-peak
Duration: 17–19 min
Electrode Size: 3.14 cm2 (round-shaped)
Timing: online
↘ (with anti-phase tACS)
− (with in-phase tACS)
Low risk
[69] Exp. 1
N = 18 (mean age: 29.8 ± 8.3 years)
Exp. 2
N = 14 (mean age: 21.9 ± 5.9 years)
Crossover within-subject design
Delayed match-to-sample task
Working memory capacity
Parietal cortex (P4) + (Cz/Oz/T8)
Device: Starstim (Neuroelectrics, Barcelona)
Frequency: 4 Hz/7 Hz/sham
Intensity:
Exp. 1 1 mA peak-to-peak; Exp. 2 2 mA peak-to-peak
Duration: at least 6 min per condition
Electrode Size: target electrode 19.6 cm², each return electrode 4.9 cm²
Timing: online
↗ (with low theta-tACS, for items presented contralateral to stimulation site only)
− (with high theta-tACS)
Low risk for Exp. 1
High risk for Exp. 2 (small sample size)
[70] N = 36 (mean age: 20 years ± 4.25 months)
Between-subject design
Corsi block-tapping task, Digit-span task, N-back task
Working memory capacity, correct responses (n-back only)
Left parietal cortex (P3) + supraorbital region
Frontal cortex (F3) + supraorbital region
Right parietal cortex (P4) + supraorbital region
Device: NeuroConn DC Stimulator (NeuroCare, Ilmenau, Germany)
Frequency: ITF/sham
Intensity: 1–2.25 mA peak-to-peak
Duration: 15 min
Electrode Size: both 5 × 7 cm
Timing: offline

Low risk (but imbalanced gender distribution: 27 females)
[71] N = 48 (mean age: 23 years)
Crossover within-subject design
Visuo-spatial working memory task
Working memory capacity
Parietal cortex (P3/P4) + left cheek
Exp. 1
in-phase between P3 and P4
Exp. 2
anti-phase between P3 and P4
Device: NeuroConn DC Stimulator MC (NeuroCare, Ilmenau, Germany)
Frequency: 6 Hz/sham
Intensity: 1.6 mA peak-to-peak
Duration: 20–24 min
Electrode Size: target electrode 4 × 4 cm, return electrode 5 × 7 cm
Timing: online
↗ (with in-phase for low performer)
− (with anti-phase or in-phase for high performer)
Low risk
[72] N = 46 (age range: 22–30 years)
Exp. 1: N = 10
Exp. 2: N = 18
Exp. 3 (control): N = 18
Crossover within-subject design
Letter discrimination task
Reaction times
Exp. 1: no stimulation
Exp. 2/3:
Frontal/parietal cortex (F3/P3) + Vertex (Cz)
Device: NeuroConn (NeuroCare, Ilmenau, Germany)
Frequency:
Exp. 1: no stimulation;
Exp. 2: 6 Hz in-phase or anti-phase/sham;
Exp. 3: 35 Hz in-phase or anti-phase/sham
Intensity: Exp. 2/3 1 mA peak-to-peak
Duration: Exp. 2/3 14 ± 1.5 min
Electrode size: Exp. 2/3
all 5 × 5 cm
Timing: Exp. 2/3 online
↗ (with in-phase theta-tACS)
− (with anti-phase theta- or gamma-tACS)
Low risk
[73] Exp. 1
N = 10 (mean age: 28.6 ± 5.0 years)
Exp. 2
N = 21 (mean age: 27.38 ± 4.56 years)
Crossover within-subject design
Working memory tasks
Exp. 1
2-back/1-back vs. choice reaction task
Exp. 2
2-back vs. choice reaction task + MRT
Correct responses, reaction times
Exp. 1 and Exp. 2 frontal/parietal areas (F4/P4) + T8
All Exp.
either in-phase or anti-phase between F4 and P4
Device: NeuroConn DC Stimulator MR (NeuroCare, Ilmenau, Germany)
Frequency: 6 Hz/sham
Intensity: 1 mA peak-to-peak
Duration:
Exp. 1: 26.5 min;
Exp. 2: 20 min in short runs of 20 s or 30 s
Electrode Size: all 5 cm diameter (‘donut’-shaped)
Timing: online
↗ (for choice reaction task with in-phase tACS)
− (accuracy)
High risk in Exp. 1 (small sample size)
Low risk in Exp. 2
[74] N = 2 × 16 (mean age: 28.3 years (±7.6)/22.8 years (±5.2)
Between-subject design (experimental vs. control montage)/Crossover within-subject design
Visuo-spatial working memory task
Correct responses, K-Value (Load x (Hits – False))
Target stimulation: Parietal cortex (P4) + supraorbital region
Active control:
Parietal cortex (P4) + Vertex (Cz)
Device: Magstim DC-Stimulator Plus
Frequency: 4 Hz/7 Hz/control (5.5 Hz)
Intensity: 1.24 ± 0.3 mA peak-to-peak
Duration: ~12 min
Electrode Size: both 5 × 5 cm
Timing: online
for arrays pointing to the left:
↗ (with 4 Hz-tACS)
↘ (with 7 Hz-tACS)
− (for arrays pointing to the right)
Low risk
Alpha
[75] N = 22 (mean age: 24.1 years)
Between-subject design
Spatial updating paradigm
Updating bias
Left parieto-occipital cortex (P3-O1) + vertex (Cz)
Right parieto-occipital cortex (P4-O2) + vertex (Cz)
Device: NeuroConn (NeuroCare, Ilmenau, Germany)
Frequency: 10 Hz
Intensity: 1 mA peak-to-peak
Duration: 25 min
Electrode size: target electrode 4 × 3 cm, return electrode 9 × 5 cm
Timing: online

Low risk
Gamma
[76] N = 18 (mean age: 29.3 ± 7.65 years)
Crossover within-subject design
N-back tasks
Correct responses, reaction times
Frontal cortex (F3) + subraorbital region
Device: NeuroConn DC Stimulator Plus (NeuroCare, Ilmenau, Germany)
Frequency: 40 Hz/tDCS/sham
Intensity: 1.5 mA peak-to-peak
Duration: 20 min
Electrode Size: both 5 × 7 cm
Timing: online
↗ (for 3-back condition when applied offline)
− (with online-tACS, as well as for the 1- and 2-back condition)
High risk (small sample size)
Gamma and Theta
[77] Exp. 1
N = 20 (mean age: 21 years)
Exp. 2
N = 20 (mean age: 23 years)
Control 1
N = 20 (mean age: 23 years)
Control 2
N = 20 (mean age: 23 years)
Crossover within-subject design
Visuo-spatial working memory task
d′
Centro-to-temporal areas (CP1/T5) + right cheek
Exp. 1
anti-phase between CP1 and T5
Exp. 2
in-phase between CP1 and T5
Control 1
in-phase between CP1 and T5
Control 2
anti-phase between CP1 and T5
Device: NeuroConn DC Stimulator MC (NeuroCare, Ilmenau, Germany)
Frequency:
Exp. 1/2: 40 Hz/sham; Control 1/2: 6 Hz/sham
Intensity: 1.5 mA peak-to-peak
Duration: 20 min
Electrode Size: both target electrodes 5 × 5 cm; return electrode 5 × 7 cm
Timing: online
↗ (with anti-phase gamma-tACS)
− (with in-phase gamma-tACS or theta-tACS)
Low risk
Multiple Frequencies
[78] N = 25 (mean age: 69.1 ± 4.5 years)
Exp. 1 no stimulation
Exp. 2/3
Crossover within-subject design
Retro-cue working memory paradigm
Correct responses
Parietal cortex (P3) + P4 Frequency:
Exp. 1: no stimulation;
Exp. 2: 4 Hz/10 Hz/40 Hz; Exp. 3: 10 Hz/sham
Intensity: 1.5 mA peak-to-peak
Duration: 20 min
Electrode Size: both 5 × 7 cm
Timing: online
↗ (with alpha-tACS but only in Exp. 2, no replication in Exp. 3)
− (with other tACS)
Low risk
Declarative memory
Theta
[79] N = 12 young (YA, mean age: 22.3 ± 1.5 years)
N = 12 older adults (OA, mean age: 66.3 ± 3.9 years)
Crossover within-subjects design
Language-learning paradigm
Correct responses
Centro-parietal area (CP5) + supraorbital region
Device: NeuroConn (NeuroCare, Ilmenau, Germany)
Frequency: 6 Hz/sham
Intensity: 1 mA peak-to-peak
Duration: 20 min
Electrode size: target electrode 5 × 7 cm, return electrode 10 × 10 cm
Timing: online
↗ (in older adults)
− (in younger adults)
High risk (small sample size)
[80] N = 72, (N = 24 for each experiment)
Exp. 1 (mean age: 23.5 ± 3.1 years)
Exp. 2 (mean age: 24.3 ± 2.9 years)
Exp. 3 (mean age: 23.2 ± 2.2 years)
Between-subject design
Paired-associative learning task
Correct responses
Left temporal cortex (T7) + (FPz/T8)
Device: NeuroConn (NeuroCare, Ilmenau, Germany)
Frequency: 5 Hz modulated bursts of 80 Hz gamma/sham
Intensity: 1 mA peak-to-baseline
Duration: 10 min
Electrode size: all 3 cm²
Timing: online
↘ (with gamma-bursts coupled with troughs of theta-waves)
− (with every other coupling)
Low
[81] N = 28 (mean age: 71.2 ± 6.4 years)
Crossover within-subject design
Face-occupation task (Paired-associative learning)
Correct responses
Ventrolateral PFC (intersection of T3-F3 and F7-C3 and midpoint F7-F3) + supraorbital region
Device: NeuroConn (NeuroCare, Ilmenau, Germany)
Frequency: 5 Hz/sham
Intensity: 1 mA peak-to-peak
Duration: 20 min
Electrode size: target electrode 5 × 7 cm, return electrode 10 × 10 cm
Timing: online

Low risk
[82] N = 25 (age range: 18–28 years)
Crossover within-subjects design
Short-term recognition task
Correct responses
Long-term memory recognition task
Strength of recollection and familiarity
Target stimulation: Right parietal cortex (P4) + (T8/C2/CP1/Oz)
Active control:
Left parietal cortex (P3) + (T7/C1/CP2/Oz)
Device: Multi-channel stimulator (StarStim, Neuroelectrics, Barcelona, Spain)
Frequency: 4 Hz/sham
Intensity: 3 mA peak-to-peak
Duration: 20 min
Electrode size: 5 round-shaped electrodes with 1 cm radius
Timing: online
↗ (with theta-tACS; for familiarity)
Low risk
[83] N = 60 (age range: 18–45 years)
N = 19 (mean age: 28.4 ± 6.9 years)
N = 21 (mean age: 25.3 ± 5.6 years)
N = 19 (mean age: 24.9 + 4.7 years)
Between-subject design
Visual associative memory task
Correct responses
Right fusiform cortex (P10) + (FP1/P2/P3/PO7)
Device: Soterix MxN TES device (Soterix Medical Inc., New York, USA)
Frequency: 6 Hz/sham
Intensity: 2 mA peak-to-baseline
Duration: 10 min
Electrode size: HD-tES electrodes
Timing: online
↗ (accuracy and forgetting)
Low risk
Gamma
[84] N = 70 (mean age: 22.12 + 2.16 years)
Congruent group 60-60 (N = 17)
Congruent group 90-90 (N = 18)
Incongruent group 60-90 (N = 17)
Incongruent group 90-60 (N = 18)
Between-subject design/Crossover within-subjects design
Word-list learning
Correct responses;
d’; Reaction times
Dorsolateral prefrontal cortex (F3) + left wrist
Device: NeuroConn DC Brain Stimulator Plus (NeuroCare, Ilmenau, Germany)
Frequency: 60 Hz/90 Hz/sham
Intensity: 1.5 mA peak-to-peak
Duration: 15 min during encoding, 15 min during retrieval
Electrode size: both 5 × 7 cm
Timing: online
↗ (with the same gamma-tACS during encoding and retrieval)
− (with different gamma-tACS during encoding and retrieval)
Low risk
[85] N = 36 (mean age: 21.3 ± 0.5 years)
N = 18 (mean age: 21.2 ± 0.4 years)
N = 18 (mean age: 21.3 ± 0.5 years)
Between-subject design, mixed with repeated-measures within-subject design
Episodic memory task (Word learning and recognition)
Correct responses,
d’, Reaction times
PFC (F3) + left wrist
Device: Eldith Brain Stimulator (NeuroCare, Ilmenau, Germany)
Frequency: 60 Hz/sham
Intensity: 0.75 mA peak-to-peak
Duration: 15 min during encoding, 15 min during retrieval
Electrode size: both 5 × 7 cm
Timing: online
↗ (accuracy)
− (reaction times)
Low risk
[86] Exp. 1
N = 36 (mean age: 20.03 ± 2.38)
Exp. 2
N = 36 (mean age: 20.97 ± 2.22)
Crossover within-subject design
Episodic memory task (Word and face learning and recognition)
Correct responses
Left IFG (FP1) + C5
Right IFG (FP2) + C6
Device: 4-channel DC Stimulator MC (NeuroCare, Ilmenau, Germany)
Frequency: 18.5 Hz/control frequencies: 6.8 Hz, 10.7 Hz, 30 Hz, 48 Hz/sham
Intensity:
Exp. 1: 2 mA peak-to-peak Exp. 2: 1.6 mA peak-to-peak
Duration: event-related stimulation for 2 s during each stimulus presentation (2 s × 360 = 720 s total stimulation duration)
Electrode size:
Exp. 1: 4 donut-shaped electrodes, each 14 cm2; Exp. 2: 4 round electrodes, each 10.75 cm2
Timing: online

Low risk

Note. tACS, transcranial alternating current stimulation (HD-tACS, high-definition tACS); tDCS, transcranial direct current stimulation (HD-tDCS, high-definition tDCS); SMA, supplementary motor area; Exp., experiment; Hz, Hertz; IAF, individual alpha frequency; ITF, individual theta frequency; IBF, individual beta frequency; mA, Milliampere; min, minutes. Low risk bias = Sufficient sample size and control conditions (sham condition and/or control frequencies), description of blinding procedure and efficiency, randomization procedure described, detailed stimulation protocol and description of statistical analysis. High risk bias = Small sample size, no control parameters. Electrode positions were reported as follows: target electrode(s) + return electrode(s). The direction of the arrows indicates improvement (↗), decrease (↘), or no change (−) in performance with tACS.