No effect of transcranial direct current stimulation over the auditory cortex on auditory-evoked potentials

Transcranial direct current stimulation (tDCS) is a non-invasive brain stimulation technique to modulate cortical excitability. Various investigations have shown tDCS to modify behavior as well as symptom severity in psychiatric disorders. Despite growing popularity, underlying neural mechanisms causing behavioral effects are still hardly understood. Additionally, results in recent investigations of tDCS effects on auditory processing, i.e. on auditory-evoked potentials (AEP), were inconsistent. In these recent investigations, sample sizes were small and reported results restricted to effects after stimulation. Thus, we aimed to investigate the effects of tDCS on auditory processing in more detail by including a larger population compared to earlier publications and by comparing effects of stimulation before, during, and after tDCS application. We included 24 healthy subjects in a crossover design to receive anodal or sham tDCS in two sessions with one week apart. During an AEP paradigm, we applied 20 min of tDCS on the primary auditory cortex. Results in earlier studies suggest an increase of P50 amplitude after anodal compared to sham tDCS as well as a shortening of N100 latency. In the current study, measures of amplitude and latency for P50, N100, and P200 were compared between three time points (pre, during, after stimulation) and for two types of stimulation (anodal, sham). Our results showed no difference in AEP for anodal compared to sham stimulation and no difference in AEP after stimulation compared to baseline. Hence, we were not able to replicate earlier results even when investigating the expected effects of tDCS in a larger cohort and with a longer duration of stimulation. Our results suggest that tDCS fails to substantially modify basic processing in the auditory cortex. Further evidence for the actual effectiveness of tDCS on auditory processing and its underlying mechanisms is warranted.