*4.5. Summary and Further Considerations*

For our two subjects utilizing the OM Neuro ZTI implant, we observed that traditional symmetric biphasic cathodic-leading pulses in monopolar stimulation mode could elicit FNS. However, when using pseudo-monophasic anodic-leading pulses in DAP grounding mode—the clinical standard setting of the ZTI implant—it was impossible to trigger FNS even when raising charge levels at the subjects' maximum tolerable loudness.

Conversely, and of significant note, we found that pseudo-monophasic cathodicleading pulses in all-polar grounding mode could induce FNS at lower charge levels, while provoking auditory sensations required much higher charge, and even then, the auditory sensations were only soft. This striking contrast between these two conditions strongly suggests that anodic stimulation is primarily effective at exciting auditory nerve structures, while cathodic stimulation appears to predominantly activate other neural structures, such as the facial nerve.

Our findings, though just from two subjects, lend further support to an expanding body of research suggesting that the auditory and facial nerves exhibit differential sensitivity to electrical stimulation based on polarity [20–24]. However, unlike previous studies, which have primarily relied on action potential recordings in CI subjects elicited with active biphasic pulses, our study provides novel evidence from direct subjective feedback obtained from two human subjects using pseudo-monophasic pulses. We demonstrate that anodic currents are markedly more effective in selectively stimulating the neural structures associated with the auditory nerve while minimizing activation of the facial and other nonauditory neural structures. The hypothesis that the auditory nerve may be more sensitive to anodic stimulation while the facial nerve is more responsive to cathodic stimulation could also partially explain the reduced FNS symptoms observed with triphasic stimulation in the MED-EL device, which also uses a longer and presumably dominant anodic phase. Using an anodic-leading (or anodic dominant) pulse could, in theory, allow for more targeted stimulation of the auditory nerve, potentially reducing unwanted activation of other nerves like the facial nerve [25].

## **5. Conclusions**

We conclude that CI stimulus parameters and grounding rather than surgical or electrode array changes were key factors in reducing FNS for our two subjects, and we suggest that this may hold true more generally. Our data indicates that the active anodic phase of the stimulus predominantly activates the auditory nerve fibers. In contrast, the cathodic phase seems more inclined to stimulate other neural structures, such as the facial nerve, leading to undesired side effects. Further untangling the relative contributions of polarity, pulse shape, pulse current vs. duration, and grounding to FNS will be a rich area for future investigations.

**Author Contributions:** Conceptualization, L.G., A.B., B.C.B. and N.L.G.; methodology, A.B., A.M. and B.C.B.; software, B.C.B.; validation, B.C.B. and L.G.; investigation, A.B., A.M., B.C.B., L.G. and N.L.G.; resources, T.L. and A.B.; data curation, B.C.B., N.L.G., L.G., A.M. and A.B.; writing—original draft preparation, A.B., A.M., B.C.B., L.G. and N.L.G.; writing—review and editing, A.B., A.M., B.C.B., L.G., N.L.G. and T.L.; visualization, B.C.B. and L.G.; supervision, A.B., T.L. and N.L.G.; project administration, A.B., B.C.B., T.L. and N.L.G. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research received no external funding.

**Institutional Review Board Statement:** The study was conducted in accordance with the Declaration of Helsinki, conducted during the normal clinical fitting routine, and approved by the ethics board of the Hannover Medical School (No. 1897-2013).

**Informed Consent Statement:** Subjects gave their written informed consent for taking part in these additional diagnostic measurements and for publishing the data.

**Data Availability Statement:** Data tables are included within the manuscript.

**Conflicts of Interest:** Authors L.G., A.M., T.L. and A.B. declare no conflict of interests. The other authors of this study declare competing interests: B.B. and N.L. are paid employees of Oticon Medical (Vallauris, France; https://www.oticonmedical.com/, accessed on 15 August 2023). They had roles in conceptualization, methodology, software, validation, investigation, data curation, writing—original draft preparation, writing—review and editing, visualization, supervision, and project administration. The specific roles of the authors employed by Oticon Medical are articulated in the 'author contributions' section. There are no patents, products in development, or marketed products associated with this research to declare.
