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

Background Non-invasive 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 To identify stimulation parameters with low side-effect profiles while reliably stimulating the cochlea, thereby improving patient comfort and safety and paving the way for the development of a medical device for treating patients with chronic tinnitus. Approach Hearing-healthy participants 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 early auditory pathway. We systematically compare HS and SEs across distinct electrode configurations and stimulation parameters, including stimulation frequency and the presence of a Direct Current (DC)-offset. We record SE occurrence via visual analog scales after stimulation. Main Results We find that tACS stimulation between 250 Hz and 2000 Hz reliably elicits HIs in participants. SEs are generally low. No occurrence of SEs resulted in 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 occurrence and magnitude of SEs considerably. Significance Our results demonstrate the feasibility of tACS in non-invasively stimulating the auditory pathway with minimal adverse SE. Stimulation parameters with a low SE profile can be applied in further studies with tinnitus patients.