Transcranial Electrical Stimulation (tES) is a Non-Invasive Brain Stimulation (NIBS) approach, in use for several decades, that involves applying a low electrical current on the human head and assessing its effects. tES has been shown to modulate spontaneous cortical activity and excitability, leading to alterations of behavior, cognition and sensory perception (for a review: Yavari et al., 2018). tES can be generated by applying either direct current or alternating current.

Transcranial Direct Current Stimulation (tDCS) is the most frequently used type of tES in both clinical and research domains. tDCS has been reported to modulate resting membrane potentials by depolarizing or hyperpolarizing cortical neurons, thereby altering their firing rate (Creutzfeldt et al., 1962; Bindman et al., 1964; Purpura and McMurtry, 1965; Radman et al., 2009). This technique consists of applying a weak direct electrical current through two or more electrodes (Priori et al., 1998). The polarity of the active electrode can be positive (anode) or negative (cathode), and the effects induced by tDCS notably depend on the polarity of the current applied. Anodal tDCS (a-tDCS) at 1 mA has been shown to typically have an excitatory effect as it induces a depolarization of the resting membrane potential and consequently an increase of the firing rate of neurons (Nitsche and Paulus, 2000). On the contrary, cathodal tDCS (c-tDCS) at 1 mA induces a hyperpolarization of the resting membrane potential, which thereby decreases the firing rate of neurons. These effects were demonstrated in the motor cortex by means of Motor Evoked Potentials (MEPs); a-tDCS increased MEPs amplitude and c-tDCS decreased it (Paulus, 2011). However, a reverse effect of a-tDCS and c-tDCS can be observed depending on many factors, such as the direction of current flow relative to neuronal orientation or duration of stimulation (Jefferys, 1981; Bikson et al., 2004; Kabakov et al., 2012; Paulus et al., 2013). tDCS effects have been observed during stimulation (online effect), but also after a sufficient stimulation (offline effect). Previous studies demonstrated that the after-effect of tDCS can last for several minutes and even hours following the end of stimulation. However, to induce an offline effect, the intensity and duration of stimulation have to be adjusted. This effect is mediated by mechanisms similar to Long-Term Potentiation (LTP) and Long-Term Depression (LTD) (Bindman et al., 1964; Nitsche and Paulus, 2000; Fritsch et al., 2010; Paulus et al., 2012).

Transcranial Alternating Current Stimulation (tACS) consists of a sinusoidal current applied at a specific frequency that alternates between electrodes (Reed and Kadosh, 2018). This neuromodulation technique modulates neuronal firing to the external frequency applied with tACS and thereby synchronizes cortical oscillation (Antal and Paulus, 2013; Herrmann et al., 2013). tACS therefore enables to assess the influence of cortical oscillation on perception and cognition. It has been demonstrated to have an after-effect similar to tDCS, which is linked to induced neuroplastic changes (Vossen et al., 2015).

Multiple alternating currents can also be applied simultaneously in an approach known as transcranial Random Noise Stimulation (tRNS). tRNS induces noise by means of multiple alternating currents varying in amplitude and frequency. The bandwidth can vary from 0.1 to 640 Hz, and it can also be divided into lower (0.1–100 Hz) or higher (100–640 Hz) frequency bands. The mechanism behind the effect of tRNS is Stochastic Resonance (SR) which enables the enhancement of information processing and detection of subthreshold signals by adding noise in a non-linear system, such as the human brain (McDonnell and Ward, 2011). A previous study has demonstrated that tRNS applied on the motor cortex induces an enhancement of cortical excitability and effect lasting up to 1-h post-stimulation (Terney et al., 2008). The putative mechanism of tRNS action has been linked to the potentiation of voltage-gated sodium channels (Terney et al., 2008).

tES has been primarily shown to have an influence on motor cortical excitability by increasing and decreasing MEPs (Nitsche and Paulus, 2000), however, the application of tES on the motor cortex can also modulate a number of motor functions. Indeed, a-tDCS has been shown to improve performance of different motor tasks (e.g., Boggio et al., 2006; Vines et al., 2008; Sohn et al., 2012). Recently, there has been increasing evidence of the therapeutic effects of tES, notably an enhancement of motor functions and cognitive performances in older adults and in individuals with various neurological disorders. A recent meta-analysis reported robust beneficial evidences of a-tDCS for an array of motor functions, but also for some cognitive functions, such as working memory and language production (Summers et al., 2016).

Another clinical population that seems to benefit from tES approaches is stroke patients. Specifically, multiple studies reported that electrical stimulation, and more particularly tDCS, enhanced the upper and lower limb recovery of stroke patients showing motor dysfunctions (for review see Bai et al., 2019). Interestingly, other studies reported improvement on motor tasks as well as an enhancement of the acquisition of motor tasks in stroke patients with tDCS, and the effect lasted minutes to hours post-stimulation (Hummel et al., 2005; Zimerman et al., 2012). Stroke patients can also benefit from a-tDCS for language recovery, as such stimulation has been shown to improve naming performance and naming reaction time (Baker et al., 2010; Fridriksson et al., 2011). Furthermore, tES techniques seem to have therapeutic effects for several other neurological disorders, notably Parkinson’s disease (Broeder et al., 2015; Chen and Chen, 2019) and Alzheimer’s disease (Hsu et al., 2015; Chang et al., 2018). However, it is still unclear whether these therapeutic effects would extend to the sensory field.

Here, we review the evidence that supports the use of tES as a tool for targeting and modulating plasticity in the sensory field. Indeed, if tES is recognized as a potentiator of sensitivity at the central level, it could help to alleviate sensory loss. We more specifically sought to examine the evidence revealing the impact of tES on cortical auditory excitability and its corresponding effect on auditory processing, and in particular on hearing rehabilitation.

The present review summarizes how transcranial electrical stimulation (tES) techniques can have a significant impact on the excitability of auditory cortical regions and its behavioral effects. Overall, research suggests that these techniques can be used to modulate nearly all auditory functions and abilities. However, there are tremendous discrepancies between studies, making it difficult to predict the directionality of the various effects.

Experimental reports on temporal and spectral processing are the only ones reporting constant outcomes. Indeed, all studies suggest that a-tDCS and c-tDCS have a positive effect on temporal processing, but a negative effect on spectral processing. Other results are scarce or often conflicting.

The results on the effect of tES techniques, notably bilateral tRNS, on binaural integration seem promising. However, more research is needed to determine whether tES could present a potential enhancing effect on binaural integration. Likewise, the potential improvement of auditory scene analysis with tDCS needs to be confirmed, as data are still preliminary. Finally, current knowledge does not allow the drawing of a clear conclusion on the impact of tES on speech comprehension, considering the very limited number of studies and the significant discrepancies between the results of these studies.

Since it is not possible to predict a constant improvement for one auditory process without affecting other aspects of auditory function, it is not possible to make any recommendation at this time. Indeed, from an audiological point of view, the use of this technique could even be harmful, and studies on spectral processing suggest that the potential applicability of tES techniques in auditory rehabilitation could be particularly damaging.

Stimulation parameters and electrode montage differ largely across studies, which could partially account for the discrepancies in the results. The importance of stimulation parameters on the induced effect has already been reported in the motor domain (e.g., Dissanayaka et al., 2017). Indeed, previous studies in the motor as well as in sensory domains demonstrate that an increase of current intensity, stimulation duration and electrode montage can invert the direction of the effect of tDCS (Paulus et al., 2013; Parkin et al., 2019). Indeed, Parkin et al. (2019) notably demonstrated that the classic effects of unilateral tDCS reported by Nitsche and Paulus (2000), as a-tDCS induces excitation and c-tDCS induces inhibition, are not extended to every stimulation protocols. No significant effect was obtained on MEP amplitude when bilateral 1 mA tDCS was applied or when the intensity was increased to 2 mA. The impact of changes in stimulation parameters are less known for tACS and tRNS, but some studies suggest that these techniques are frequency-dependent (Moreno-Duarte et al., 2014). The type of acquisition (i.e., online; offline) might also have an influence on the effect induced. Indeed, undergoing a task during stimulation could reverse the effect expected since there is ongoing brain activity due to the task (see Batsikadze et al., 2013). As such, until a concerted effort is made to use the same parameters and montage and to establish proper stimulation guidelines, there is no doubt that these observed discrepancies will persist.

Another possible explanation for the significant variations of tES results in the auditory domain could be the anatomical location of the auditory cortex, which is located deep in the superior temporal gyrus (STG) and extends to the lateral sulcus and Heschl’s Gyrus (HG). It was previously suggested that tES has a larger effect on superficial neurons (Paulus et al., 2013). This may therefore suggest that higher stimulation intensity and/or bilateral stimulation would be more likely to induce an effect on auditory functions, as it was demonstrated to stimulate deeper region in the motor cortex (Paulus et al., 2013).

Furthermore, the auditory cortex has different tonotopic gradients and neurons have different characteristic frequencies. As such, neuronal orientation may vary throughout the cortex (Talavage et al., 2004; Humphries et al., 2010; Costa et al., 2011; Langers and van Dijk, 2012; Tang and Hammond, 2013). Some studies demonstrated that the effect of tES, and more specifically tDCS, depends on the direction of the current relative to neuronal orientation. Indeed, a current applied parallel to a neuron can induce hyperpolarization while a current applied perpendicularly can induce depolarization or no effect at all (Jefferys, 1981; Bikson et al., 2004; Kabakov et al., 2012). The latter results may suggest that in the auditory cortex, where neuron orientation is not uniform, part of the neuronal population may be hyperpolarized, and another part depolarized depending on the direction of the current applied.

In the same vein, the variability of neuronal orientations in the auditory cortex underlines the importance of the active electrode position, since a small difference in position might change the effects due to a different alignment with neuronal orientation (Talavage et al., 2004; Humphries et al., 2010; Costa et al., 2011; Langers and van Dijk, 2012). This is indeed the hypothesis proposed by Ladeira et al. (2011) who showed that tDCS only had an effect at 4,000 Hz, but no effect at 500, 1,000, and 2,000 Hz. Indeed, lower frequencies are coded in the anterolateral portion of the HG, whereas higher frequencies are coded in the postero-medial part of the HG (Bhatgnagar, 2002; Langers et al., 2007). The stimulation electrodes used by Ladeira et al. (2011) were placed in the posterior portion of the temporal cortex, which may explain the specific improvement of random GDT at 4,000 Hz.

One could also suggest that it might be more appropriate to use tRNS on the auditory cortex to avoid effects of neuronal orientation, since tRNS stimulate neurons irrespective of their spatial orientation (Terney et al., 2008). As suggested by the present review, all the studies using tRNS reported an improvement in performance. However, the number of studies is too limited to conclude with certainty that tRNS is more efficient. In addition, there is no clear evidence as to which tES technique is optimal in the auditory domain. As such, the data do not support the use of this technique in audiological practice.

Another obvious shortcoming is the lack of data in populations with impaired hearing function or impaired auditory processing, notably older adults. Indeed, it is well known that a decrease in temporal processing occurs with age (Fitzgibbons and Gordon-Salant, 1996; Pichora-Fuller and Souza, 2003) and that the improvements induced by tDCS have been hypothesized to be greater in non-proficient systems (Reis et al., 2014). For example, some effects discussed earlier appear to be age-dependent—with an improved performance being observed in older subjects and conversely a reduced performance being seen in younger adults (Rufener et al., 2016a,b). These results suggest that the effects of tES on voice onset time categorization might depend on the efficiency of the auditory system. These elements alone could explain the improvements in temporal processing induced in older adults. Indeed, it is possible that tACS perturbs a normal, well-functioning system and leads to a processing deterioration in younger adults, because the neuronal reactivity level is already optimal in this group (Krause et al., 2013; Schaal et al., 2013). Future studies should therefore investigate the effect of tES techniques in older adults with hearing loss and/or impaired auditory processing, to determine if such techniques could have a clinical relevance in audiology.

Another important variable might also explain the divergence found between younger and older participants, namely, the homeostatic control of cortical excitability (Schaal et al., 2017). Unfortunately, the studies included in the present review were only conducted with normal-hearing individuals. One may therefore wonder if such variability in results would have been present in people with a suboptimal auditory system. For example, all studies using tDCS on spectral processing reported a decreased performance in the different tasks studied. However, it is important to note that all the studies reviewed studied young adults with no apparent spectral processing difficulties. Furthermore, a few studies even recruited participants with musical experience. Therefore, it can be hypothesized that the participants not only had no spectral processing impairment, but also had great auditory skills. As such, adding electrical stimulation could only disrupt that optimal level and decrease their performance. It would be important to assess the impact of tES on populations with impaired spectral processing, and then draw conclusion on the rehabilitation potential of this method.

In conclusion, the present review demonstrated that tES can have an influence on several auditory abilities. While the results are still inconclusive, the impact of the stimulation of other cortical structures on auditory perception would also deserve attention, particularly in relation to auditory scene analysis and speech comprehension, which depend on multisensory processes. Indeed, results of preliminary studies on the effect of tDCS on multisensory integration are promising. Marques et al. (2014) examined the influence of a-tDCS and c-tDCS on the McGurk illusion, a multisensory integration task. They reported that c-tDCS appears to reduce the McGurk illusion when active electrodes are placed on temporal cortices, but that applying a-tDCS to the same regions has no effect. However, a-tDCS applied on parietal cortices increased the McGurk illusion. It is therefore likely that further studies of the effect of tES on multisensory cortical structures could provide interesting avenues for audiological practice. Furthermore, this review underlined important methodological discrepancies between studies, therefore, future studies should determine the optimal stimulation parameters to apply.

MN, TA, KM-D, B-AB, and FC wrote the article. All authors discussed the results and commented on the manuscript at all stages.

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

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