In-Ear Transcranial Alternating Current Stimulation: Auditory Perception and Side Effects Depend on Electrode Montage, Frequency, and Direct Current-Offset.

BACKGROUND: Noninvasive transcranial alternating current stimulation (tACS) of the cochlea might be a promising new therapeutic option for patients with chronic tinnitus. However, electric stimulation of small, sensitive target regions, such as the inner ear, can cause adverse side effects (SEs). OBJECTIVE: The objective of this study was to identify stimulation parameters with low SE profiles while reliably stimulating the cochlea, thereby improving patient comfort and safety. This pilot study also aims to identify stimulation settings suited for developing a medical device intended to treat patients with chronic tinnitus. APPROACH: Participants with healthy hearing were stimulated with electrodes in the ear canal. Stimulation of the cochlea elicits a soft hearing impression (HI) in participants, indicating successful stimulation of the auditory pathway. We systematically compare HIs and SEs across distinct electrode configurations and stimulation parameters, including stimulation frequency and the presence of a direct current (DC)-offset. We record SE severity ratings through visual analog scales after stimulation. PRIMARY RESULTS: We find that tACS between 250 Hz and 2000 Hz reliably elicits HIs in participants. SE severity depends on the stimulation parameters. No magnitude of SEs caused participant withdrawal or severe adverse events, with only phosphenes, skin tingling, and a sense of vibration being reported as impactful. The addition of a slight DC-offset increases the magnitude of phosphenes but does not alter HI occurrence. SIGNIFICANCE: Our results show the feasibility of tACS to stimulate the auditory pathway noninvasively with minimal adverse SEs. Stimulation parameters with a low SE profile can be applied in further studies with patients with tinnitus. CLINICAL TRIAL REGISTRATION: This study was preregistered at the German Register of Clinical Studies (DRKS, https://drks.de/search/de/trial/DRKS00033120).