In the present study we demonstrate that phonetic perception can be modulated by bilateral tDCS of the AC. Categorization of CV-syllables in a VOT continuum was enhanced by cathodal tDC-stimulation. In particular, concurrent cathodal tDCS steepened the slope of the identification curve indicating more consistent categorization of the syllables/ta/and/da/. This sharpening of the phonetic perception was accompanied by increased P50 amplitude in response to natural CV syllables after anodal stimulation.

In our study, cathodal tDCS improved preciseness of phonetic categorization, with no influences of anodal tDCS. In a first attempt this result might be contradictive in the light of the polarity-specific dichotomy assuming that anodal tDCS typically improves while cathodal tDCS worsens the behavioral outcome in a specific task. Notably, these dual-polarity effects have mainly been demonstrated in the motor domain but less on cognitive functions [28]. Particularly in the auditory domain, several studies demonstrated a decrement of performance induced by cathodal tDCS on auditory function [25, 29], but there is also evidence for an opposite effect showing improved performance after cathodal stimulation [30]. It can be assumed that different stimulation parameter such as stimulation power, electrode size, and electrode placement especially of the reference electrode, as well as the individual auditory stimuli contribute to the varying tDCS-effects. Furthermore, the tDCS-related alterations of the neurotransmitter level may impact homeostatic plasticity in the auditory domain. Whereas anodal tDCS reduces local concentrations of the inhibitory neurotransmitter gamma-amino butyric acid (GABA), thus, inducing improvement, cathodal tDCS reduces excitatory glutamate levels followed by impoverishment of the behavioral outcome. However, there is also evidence that cathodal tDCS can decrease GABA concentration and thus may induce improved performance as well [31, 32]. Thus, given that regional cortical excitation/inhibition balance, measured by ratios of glutamate/GABA, provide meaningful interpretations of individual cognitive as well as perceptual performance [33], cathodal tDCS may artificially change the excitation/inhibition balance towards a more optimal level in the auditory cortex.

The present results extend the view of tDCS induced modulations on temporal processing by showing improved phonetic categorization of CV-syllables with varying VOTs. This might reflect a facilitation of low-level acoustic processing of temporal features in the AC. Moreover, we assessed the electrophysiological brain activity in order to investigate tDCS induced after effects on CV-syllable perception. It has been proposed that anodal tDCS over the temporal cortex can alter AC reactivity resulting in modulation of the AEPs. As has been demonstrated previously using sinus tones [23], we found enhanced P50 amplitudes after unilateral anodal tDCS indicating that alterations in the AC reactivity impact early stages of perceptual processing. Basically, the P50 component of the AEP presumably reflects sensory representation of an acoustic stimulus in the AC [34, 35]. Accordingly, uni- as well as bilateral anodal tDCS over the AC increases P50 amplitudes to acoustic stimuli indicating a general neuromodulatory effect on early sensory acoustic processing. Remarkably, the present study shows enhancement of the AC reactivity after anodal- but not cathodal tDCS. Such anodal tDCS-related increase in cortical excitation could be assumed to be the cause of an improved auditory performance. However, our behavioral data during tDCS showed no change of performance during anodal condition. Accordingly, we cannot directly relate the observed electrophysiological modulations after tDCS to the improved auditory phonetic categorization abilities described with concurrent stimulation. However, tDCS efficiency on cortical excitability critically depends on the timing of the stimulation. Several studies showed that tDCS can result in contradictive effects during (online) and after the application of tDCS (offline). For instance, simultaneous anodal tDCS leads to an improvement in motor learning and working memory performance, whereas during the after-effect anodal tDCS results in no or opposite effects [36–38]. Such opposite online vs. offline effects have been found for the visual domain showing improved perceptual learning after but not during cathodal tDCS [39]. Analogously, online tDCS decreased motor learning, whereas motor performance was worsened during the after-effect [36, 40]. These opposing effects might be related to the underlying physiological actions of online vs. offline tDCS. While acute-effects during stimulation (online) are primary based on changed membrane potentials, post-stimulation after-effects are related to NMDA-receptor activation indicating a LTP-like mechanism for learning [41–43]. Consequently, these differential underlying physiological actions during and after stimulation may lead to opposite effects of tDCS. One might further speculate that bilateral tDCS application over the AC might influence the mutual inhibition between the two hemispheres and thereby inducing opposite effects. Moreover, contradictive results might be also related to the different auditory stimuli used during and after tDCS application. While subjects heard ambiguous syllables from the VOT continuum, during EEG-recording non-ambiguous syllables were presented.

Our results demonstrate that simultaneous cathodal tDCS can induce an enhancement of auditory performance, whereas anodal tDCS induces after-effects that enhance AC reactivity.

Nevertheless, the present findings may have clinical implications for the treatment of speech-related pathologies such as dyslexia. Dyslectic children as well as adults exhibit deficits in the processing of rapid auditory information accompanied with deficient phonological processing [13, 14, 44]. Those patients may benefit from tDCS administration as add-on to conventional therapy. Notwithstanding the fact that the neurophysiological mechanisms are still not fully understood the current results show that tDCS can be successfully used to modulate rapid temporal processing of speech sounds. Consequently, by modulating the excitability of the temporal cortex via non-invasive brain stimulation, the present study provides a novel approach that can be simply administered to address stunted temporal processing abilities in auditory disorders in the human brain.