Table 4.

Summarization of the identified studies with low risk of bias.

Study	Population	Stimulation parameters							Research methods								Results
		Type of stimulation	Target region	Location and type of coil / Location and size of electrodes	Duration	Intensity of stimulation	Frequency of stimulation	Method of control	Diagnostic criteria	Randomization	Blinding	Allocation concealment	Interval scaling	Practice effect	Missing data and drop-outs	Other statistical practices
Bystad et al. (2016)	AD	atDCS	Left temporal cortex	According to the 10-20 EEG system: anode: 5 × 7 cm, at T3 cathode: 5 × 7 cm, at Fp2	6 sessions, 30 min/session	2 mA		30 s active stimulation	Revised NINCDS-ADRDA	Computer randomized list containing 5-digit codes provided by the manufacturer of the tDCS device	Patients and assessor blinded to the type of stimulation	Assignment disclosed until the end of the intervention	Scaling according to standardized norm tables, transformation to z-scores	Two versions of CVLT-II used	Explicitly reported no drop-outs	Sample size based on other studies	No changes in global cognition, verbal learning, attention or executive function
Khedr et al. (2014)	AD	atDCS and ctDCS	LDLPFC	Anodal: 10 x 10 cm, right supraorbital region cathodal: 4 x 6 cm, LDLPFC	10 sessions, 25 min/session	2 mA		30 s active stimulation	NINCDS-ADRDA	Computer generated randomization table	Patients and assessor blinded to group assignment the effectiveness of blinding was measured	Serials numbered opaque closed envelopes			Reportedly no drop-outs		Improvement in MMSE after both anodal and cathodal tDCS in contrast to sham, improvement in performance IQ after cathodal stimulation
Suemoto et al. (2014)	AD	atDCS	LDLPFC	Anode 5 × 7 cm, over DLPFC cathode 5 × 7 cm, right supraorbital region	6 sessions on every 2nd day, 20 min/session	2 mA		20 s active stimulation	NINCDS-ADRDA	Computer-generated list of random numbers	Patients and assessor blinded to condition, numbered	Opaque and sealed envelopes			Reasons of missing data not reported, intention to treat analyses conducted using the method of last observation carried forward	A priori sample size calculation, using the method of minimal clinically relevant difference, planned pairwise comparisons	No change in active and sham group
Wu et al. (2015)	AD	HF-rTMS	LDLPFC	Figure-of-eight coil	20 sessions, 1,200 pulses/session	80% of RMT	20 Hz	Tilted coil (180°)	NINCDS-ADRDA	Standard table of random numbers	Patients and assessor blinded to group assignment	Patients and assessor blinded to the group assignment before starting the trial, method not specified	Using cutoff scores based on the findings of other studies				Improvement of behavioral and global cognitive symptoms
Drumond Marra et al. (2015)	MCI	HF-rTMS	LDLPFC	Figure-of-eight coil 5 cm in a parasagittal plane parallel to the point of maximum rMT	10 sessions, 2,000 pulses/session	110% of RMT	10 Hz	Sham coil	Not specified, MoCA <26	Computer generated randomization	Patients and assessors blinded to group assignment, the effectiveness of blinding was measured	Different staff members responsible for the allocation				Scores adjusted according to age, gender and education level	Selective improvement in everyday memory compared to sham group
Padala et al. (2018)	MCI	HF-rTMS	LDLPFC	Figure-of-eight coil n.a.	10 sessions/condition, 3,000 pulses/session	120% of RMT	10 Hz	Sham coil	Criteria of Petersen et al. (1999)	Randomized block design	Patients and assessors blinded to condition	Independent staff member responsible for the allocation		Random subject effect calculated	Drop-outs reported and reasoned	Baseline measurements set as covariates	Improvement in apathy symptoms, global cognition, processing speed and clinical improvement compared to sham condition

AD, Alzheimer's disease; MCI, mild cognitive impairment; HF-rTMS, high frequency repetitive transcranial magnetic stimulation; atDCS, anodal transcranial direct current stimulation; ctDCS, cathodal transcranial direct current stimulation; LDLPFC, left dorsolateral prefrontal cortex; NINCDS-ADRDA, National Institute of Neurological and Communicative Diseases and Stroke/Alzheimer's Disease and Related Related Disorders Association; RMT, resting motor threshold; EEG, electroencephalography; CVLT-II, California Verbal Learning Test-II; MMSE, Mini-Mental State Examination; MOCA, Montreal Cognitive Assessment Test.