Stecher, H. I. | Search bioRxiv

bioRxiv2025

In-ear-tACS: Auditory Perception and Side Effects depend on Electrode Montage, Frequency, and DC-Offset

BackgroundNon-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). ObjectiveTo 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. ApproachHearing-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 ResultsWe 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. SignificanceOur 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.

neuroscience↗

bioRxiv2025

Carbonized rubber electrodes can cause a DC-offset in transcranial alternating current stimulation

IntroductionCarbonized rubber electrodes are widely used in non-invasive brain stimulation studies. Due to their polarizable nature, however, they can cause a voltage offset, which might be problematic for concurrent EEG studies. ObjectiveIn this study, we aim to describe the voltage offset and ensure that the offset does not alter the intended waveform of applied stimulation. MethodsUsing data from 2 human studies and phantom measurements, which employed carbonized rubber electrodes, we quantify the magnitude and frequency of DC-offsets and contrast this against pilot-measurements using Ag/AGCl-electrodes. In a further phantom study, we record the offset-voltage that arises from the electrode/electrolyte interface and compare this to the voltage put out by the stimulation device. ResultsA non-zero voltage offset is present in all human and phantom studies employing carbonized rubber electrodes, while the offset using Ag/AgCl electrodes is close to zero. Direct measurements of the stimulator output in the presence of a measurable voltage offset at the stimulation electrodes shows that the offset originates from the electrodes and not from the current provided by the stimulation device. ConclusionUsing carbonized rubber-electrodes for stimulation can result in the emergence of a measurable voltage offset, due to their polarizable nature. We argue that this offset can be problematic in concurrent EEG recordings, as they pose the risk of amplifier saturation and distortions of the recorded stimulation waveform.

neuroscience↗