Table 2.

Partial parameters and results of auditory process studies.

Reference	Sample Size	Stimulation Types	Stimulation Position and Electrode Size	Reference Electrode Position and Size	Stimulation Parameters	The Number of Stimulation Sessions	EEG Recording Time	Auditory Paradigm	Results
Ladeira, et al., 2011 [61]	11	tDCS: anodal/cathodal/sham	Simultaneous T3 and T4, 35 cm2	Contralateral deltoid muscle 35 cm2	2.0 mA for 10 min	Three stimulation sessions	unspecified	Random gap detection test	Anodal tDCS enhanced temporal resolution and cathodal diminished temporal resolution
Heimrath, et al., 2014 [62]	15	tDCS: anodal/sham	T7, T8, 25 cm2	Contralateral C4, C3, 50 cm2	1.5 mA for 13 min	Two stimulation sessions	unspecified	Between channel gap detection task	Anodal tDCS over the left auditory cortex diminished temporal resolution
Baltus, et al., 2018 [65]	26	IGF ± 4 Hz tACS	Left hemispheric: FC5 and TP7/P7 Right hemispheric: FC6 and TP8/P8 Each hemispheric two round electrodes (2.5 cm diameter)	unspecified	1.0 mA for 7 min	Each participant had one stimulation session within two days	Day 1: 40 min in the IGF estimation. Day 2: 1.5 min in the aftereffect estimation.	Between channel gap detection task	Participants in Group A (tACS frequency above IGF) performed significantly better during tACS compared to participants in Group B (tACS frequency below IGF) Both groups had significant relative changes in the amplitude of ASSR
Baltus, et al., 2020 [66]	16	IGF + 3 Hz tACS or IGF − 4 Hz tACS	Left hemispheric: FC5 and TP7/P7 Right hemispheric: FC4 and TP8/P8 Each hemispheric two round electrodes (2.5 cm diameter)	unspecified	1.0 mA for 8.5 ± 1.7 min	Each participant had two stimulation sessions within two days	Day 1: 20 min in the IGF estimation.	Between channel gap detection task	The IGF was significantly related to the gap detection performance under the condition that the baseline and tACS frequency were higher than IGF 3 Hz Under the condition that the tACS frequency was higher than IGF 3 Hz, tACS could modulated gap detection performance
Rufener, et al., 2017a [67]	20	HF-tRNS (100–640 Hz)/sham	Over T7 and T8 35 cm2	unspecified	1.5 mA for 40 min	Two stimulation sessions. Stimulation twice in each session.	Record EEG during gap task and pitch task. Unspecified time.	Between channel gap detection task and pitch discrimination task.	Auditory tRNS only increased the detection rate in the temporal domain, no such effect on the discrimination of spectral features The facilitation effect of tRNS was limited to the processing of stimuli near the threshold
Rufener, et al., 2017b [68]	18	tRNS/sham unspecified frequency	LAC, RAC, BAC unspecified size	unspecified	1.5 mA for 30 min	Four stimulation sessions.	tRNS simultaneous EEG About 30 min	Between channel gap detection task and pitch discrimination task.	The application of tRNS on LAC and BAC was related to task performance LAC had functional relevance when dealing with temporal features, and this effect was modulated by the the inter-tone interval between the tone triplets
Lewald 2016 [70]	74	tDCS: anodal and cathodal/sham	(1) superior temporal gyrus (STG, n = 24) (2) inferior parietal lobule (IPL, n = 28) (3) somatosensory-motor cortex (SMC, n = 22). Two round electrodes (diameter 21 mm, 3.5 cm2)	unspecified	0.4 mA for 12 min	Each participant had one stimulation session at a different stimulation site	unspecified	Simulated “cocktail-party” situation	The tDCS over superior temporal gyrus could enhance the accuracy of target localization in left hemispace TDCS over IPL and off-target active stimulation over SMC found no significant effects
Lewald 2019 [71]	24	tDCS: anodal/sham	Midpoint between C5 and T7 locations and C6 and T8 locations 35 cm2	over the contralateral shoulders 7 × 14 cm	1.0 mA for 32 min	Two stimulation sessions	unspecified	Simulated “cocktail-party” situation	TDCS over the region of posterior STG, significantly improved the performance in localizing a target speaker Compared with sham tDCS, the average increase in correct responses by 3.7% after active tDCS
Hanenberg, et al., 2019 [72]	39	tDCS: anodal/cathodal/sham	Midpoint between C6 and T8 35 cm2	over the contralateral shoulder(left) 7 × 14 cm	1.0 mA for 16 min	Three stimulation sessions.	Whole session about 91 min;	Simulated “cocktail-party” situation	The amplitude of the response of the N2 component on the contralateral (left) increased significantly within 15 min after tDCS At the same point in time, the posterior parietal cortex (PPC) electrical activity reduced on the same side (right)
Malte, et al., 2018 [78]	20	10 Hz tACS and sham or 47 Hz tACS and sham	FC5 and TP7 Round electrodes of 3 cm diameter	unspecified	1.0 mA for 25 min	Each participant received two sessions, each session included one sham and one real tACS	unspecified	Dichotic listening task	Unihemispheric α-tACS relatively reduced the recall of targets contralateral to stimulation, increased recall of ipsilateral targets Contrary to α-tACS, γ-tACS relatively increased the recall of targets contralateral to stimulation, decreased recall of ipsilateral targets
Deng, et al., 2019 [79]	38	10 Hz HD-tACS/6 Hz HD-tACS/sham	P2 ring electrode unspecified size	CP2, P4, Pz, PO4 ring electrode unspecified size	1.5 mA for 20 min	Each participant received one sham and one HD-tACS	unspecified	Selective auditory attention task	RIPS’ α-HD-tACS disrupts auditory spatial attention for leftward No effect on performance in the myriad control conditions (i.e., gender, continuous/switching experiment, theta stimulation)
Rufener, et al., 2018 [82]	20	HF-tRNS (100–640 Hz)/sham	F3 25 cm2	right shoulder 35 cm2	1.5 mA for 30 min executed twice per session	Two stimulation sessions.	tRNS simultaneous EEG About 60 min	Oddball paradigm (target tone recognition)	TRNS regulates the excitability of the left DLPFC TRNS reduced the subject’s response time to identify the target sound, and fastened up the latency of the P3 component