Extracochlear Electric Stimulation - Toward Non-Invasive Hearing Restoration

<jats:p>Background: Hearing aids and cochlear implants (CIs) are the primary interventions for sensorineural hearing loss, restoring auditory function through amplification and intracochlear electrical stimulation, respectively. For those with residual low-frequency hearing, the combined electric-acoustic stimulation (EAS) has demonstrated superior speech perception, particularly in noisy environments, compared to either modality. However, CI surgery carries inherent risks, including postoperative hearing loss, which undermines EAS benefits and limits future rehabilitation options. To overcome these limitations, we propose a non-invasive alternative: extracochlear electric and acoustic stimulation (EEAS), delivering electrical stimulation via transcutaneous electrodes without surgery. Here, we present a first systematic investigation of non-invasive extracochlear electrical stimulation using ear canal electrode montages, evaluating its feasibility, perceptual effects, and key parameters across diverse hearing statuses. Methods: We conducted a controlled, within-subject study with 15 participants: 5 with normal hearing (NH), 5 with high-frequency hearing loss (HI), and 5 with severe-to-profound deafness (PL). We used charge-balanced sinusoidal stimuli (125-4000 Hz) applied via an ear canal electrode and four return electrode montages, including contralateral ear canal, contralateral mastoid, ipsilateral mastoid, and forehead electrodes. Participants rated auditory sensations, including loudness, sound quality, and lateralization, as well as side effects on separate 0-10 scales, with current intensity increased up to 2 mA/cm^2. Thresholds and perceptual responses were analyzed across frequencies, electrode configurations, and hearing groups. Results: Reliable auditory percepts were elicited across all groups. NH participants reported pure-tone sensations, whereas HI and PL participants perceived broadband, noise-like sounds. Loudness decreased with increasing frequency, particularly for HI and PL, with minimal responses in the high-frequency range. The current threshold increased with stimulation frequency, whereas the threshold expressed as charge per phase remained constant, suggesting that charge per phase primarily determines neural activation, whereas current amplitude is more closely associated with the intensity of auditory and side effect perception. Contralateral montages produced significantly higher loudness ratings than ipsilateral or forehead configurations. The forehead montage was poorly tolerated, leading to early termination due to discomforting side effects. Sound lateralization was predominantly central or bilateral with contralateral setups, while ipsilateral and forehead configurations yielded ipsilateral perceptions. Conclusions: Non-invasive extracochlear electrical stimulation via ear canal electrodes is feasible and perceptually effective across a spectrum of hearing statuses. Perceptive outcomes are strongly influenced by electrode montage and residual hearing, with evidence of electrophonic excitation in NH individuals and electroneural activation in HI and PL participants. Contralateral mastoid electrode configurations offer the optimal balance of perceptual strength, tolerability, and spatial localization. These findings establish a critical foundation for the development of EEAS devices, demonstrating that non-invasive electrical stimulation can generate meaningful auditory percepts, paving the way for safe, accessible, and integrated hearing rehabilitation solutions. This work informs future EEAS developments and advances the path toward clinically viable, non-invasive cochlear stimulation.</jats:p>