TABLE 2.

Studies and experimental characteristics of the tES literature for depression.

Authors	Subjects	Type	Stimulation parameters			Neuroim aging		Conclusion
			Current	Duration	Location	Type	Feature
Nikolin et al., 2020	20	tDCS	2 mA, 40 min	3 sessions per week for 6 weeks	Anode: left DLPFC (F3), cathode: right shoulder.	EEG	Power spectral in alpha (8–13 Hz) and theta (4–8 Hz), Event-related potential.	There is a significant improvement on the behavioral performance (i.e., mood, memory, cognitive).
Nishida et al., 2019	33	tDCS	1 mA, 20 min	1 session	Anode: left DLPFC (F5) or DMPFC (Afz), cathode: left shoulder.	EEG	Power spectral in alpha (8–12 Hz) and theta band (4–8 Hz), event-related potential.	tDCS could affect the brain activity on the stimulated brain area and influence the other related resting state neural network’s cortical brain state.
Li et al., 2019	26	tDCS	2 mA, 20 min	5 sessions/week for 4 weeks	Anode: left DLPFC (F3), cathode: right DLPFC (F4).	fNIR S	Concentration change in HbO	tDCS can improve depression symptoms in behavioral domains) and influence hemodynamic metabolism.
Alexander et al., 2019	32	tACS	1/2 mA, 10/40 Hz, 40 min	5 sessions in consecutive days	Anode: left/right DLPFC (F3/F4), cathode: Cz.	EEG	Power spectral in alpha (8–12 Hz).	10-Hz tACS could significantly reduce the alpha power over the left frontal cortex. tACS has potential for the treatment of depression.
Shahsavar et al., 2018	7	tDCS	1 mA, 20 min	5 session in consecutive days	Anode: left DLPFC (F3), cathode: right DLPFC (F4).	EEG	Event-related potential and the power spectrum in the different brain wave band	It was possible to estimate the change of depressed patients treated with tDCS with reasonable precision using the alpha band wavelet coefficients.
Al-Kaysi et al., 2017	10	tDCS	2 mA, 20 min	5 sessions per week for 3 weeks	Anode: left DLPFC (F3), cathode: F8	EEG	Power spectral in theta band (4–8 Hz), alpha (8–12 Hz), beta (13–30 Hz), and, gamma (30–100 Hz).	This study demonstrated the feasibility of predicting tDCS treatment outcomes by analyzing the EEG data recorded at baseline.
Liu et al., 2016	37	tDCS	2 mA, 20 min	5 session in consecutive days	Anode: left DLPFC (F3), cathode: right supraorbital area	EEG	Power spectral in alpha (8–13 Hz) and theta band (4–8 Hz).	tDCS could improve the depression symptom, but memory function was not immediately following or persisting after the stimulation
Powell et al., 2014	18	tDCS	2 mA, 20 min	1 session	Anode: left DLPFC (F3), cathode: F8	EEG	Power spectral in alpha (8–12 Hz) and theta band (4–8 Hz), event-related potential.	Anodal tDCS with a single session from the left DLPFC for the major depressive episode resulted in modulated brain activity of EEG.
Palm et al., 2009	1	tDCS	1 mA, 20 min	16 sessions in 27 days	Anode: F left DLPFC (F3), cathode: right supraorbital area	EEG	Power spectral in delta (1–3 Hz), theta band (4–7 Hz), alpha (8–12 Hz), and beta (13—5 Hz).	tDCS did not exert clinically meaningful antidepressant effects. The results for cognitive measures and EEG suggest that beneficial effects may occur in depressed subjects.
Khayyer et al., 2018	9	tDCS	1.5 mA, 15 min	3 sessions per week for 4 weeks	Anode: left or right DLPFC (F3 or F4), cathode: Cz.	EEG	Power spectrum in Delta (1–4 Hz), Theta (4.5–7 Hz), Alpha (7.5–12 Hz), Beta (12.5–24.5 Hz), High Beta (25–30 Hz)	The combined treatment of positive psychotherapy and tDCS showed the great performance to improve the neurological and clinical condition of major depressive disorder.