*3.1. Subject S1*

Using stimulus type A−, first the right ear was tested for side effects. On electrodes E3 and E7, the patient felt some vibration in the outer ear at very loud auditory perception (loudness 10, charge level 36 nC (E3) and 27 nC (E7)). On electrode E11, no non-auditory sensation was reported up to a charge level of 33 nC (loudness = 10). These three electrodes correspond approximately to electrodes e14, e11, and e8 of the previous CI (see Figure 2a), where severe FNS had previously been perceived at soft loudness levels. Thereafter, measurements were performed on the left side on electrodes E7 and E16, again without any non-auditory sensation (loudness 10, charge level 27 nC (E7) and 33 nC (E16). These electrodes matched approximately electrodes e11 and e4 of the previous CI (see Figure 2b).

Finally, measurements on electrode E5 (left) unveiled certain non-auditory side effects. Specifically, a peculiar sensation was reported at a charge level of 30 nC (perceived loudness level at 10). When the charge was increased to 33 nC, the subject described feeling a vibration in the outer ear (see Table 2 for a summary of these findings). Further, at a charge level of 36 nC, this sensation was accompanied by a facial tingle. At this point, a stapedial reflex was objectively confirmed using tympanometry, and we discovered that

the reflex was elicited at charge levels from 30 nC upwards, matching the subject's reported perceptions. We selected this E5 electrode for our further investigation.

**Table 2.** Results of the search for electrodes with side effects. FNS thresholds taken from subject S1's prior implantation (left and right) are compared with observations using the re-implanted OM device and a similar stimulation type (A−). The OM device exhibited higher FNS thresholds for this subject during the acute testing.


While the type A− stimulus presented a rather rapid increase in loudness perception, type B+ exhibited a shallower growth and did not induce any non-auditory side effects (Figure 3a and Table 3). With the use of type B+ stimulation, the loudness level did not exceed 9, and even at charge levels of 75 nC—the maximum possible in our setup—the stapedial reflex was not elicited.

**Table 3.** Current and duration parameters used while testing subject S1 left ear electrode E5 using either stimulation style A− or B+ (Table 1) and the reported loudness (0 = unheard, 10 = very loud) perceptions for each of these. Comments and observations from the subject concerning any non-auditory sensations are highlighted with footnotes.


\* The lowest possible value. <sup>1</sup> The subject reported 'feeling' something. <sup>2</sup> The subject reported an 'outer ear vibration'. <sup>3</sup> The subject reported stronger 'outer ear vibration' and a facial tingle.

#### *3.2. Subject S2*

The search for electrodes with side effects began with electrode E18, which was approximately equivalent to electrode e2 of the previous implant. This was where a comfortably loud stimulation level had previously evoked FNS (refer to Figure 2c). Using the A− stimulus, the patient experienced vertigo at 15 nC with a loudness level of 7. When the charge was increased to 18 nC, the patient reacted and described a mild but discomforting sensation akin to 'pain in the head' and a loudness level of 10 (Table 4). We selected this E18 electrode for our further investigation.

**Table 4.** Results of the search for electrodes with side effects. FNS thresholds taken from subject S2's prior implantation (right) are compared with observations using the re-implanted OM device and a similar stimulation type (A−). For this subject, the OM device exhibited non-auditory thresholds at slightly lower charge levels during acute testing.


With the anodic-leading, passive-discharge B+ stimulus, the patient reported no nonauditory side effects, even up to a charge level of 32 nC (loudness 10). When testing with the reversed polarity using stimulus A+, the patient described an unpleasant buzzing sensation at 15 nC (perceived loudness at level 4). By 18 nC, the subject experienced a sensation akin to 'a force pulling on the head' and rated the loudness at level 8. The buzzing persisted at 19 nC with a loudness perception of level 9. By the time stimulation reached 21 nC (loudness level 10), nystagmus was observed—indicative of an activation of the vestibular system due to the electrical stimulation.

In contrast, with cathodic-leading passive-discharge stimulation (B−), there was only a modest rise in perceived loudness, saturating at a level 2 perception between 32 and 42 nC with no additional growth. At 40 nC, the subject described a deterioration in auditory quality. By 42 nC, the sensations reported were uncomfortably familiar to those previously experienced with her prior cochlear implant, characterized by a blend of vague pain and dizziness. All loudness growth functions for subject S2 are shown in Figure 3b, and corresponding data is listed in Table 5.

**Table 5.** Current and duration parameters used while testing subject S2 right ear electrode E18 using stimulation styles A−, A+, B−, or B+ (Table 1) and the associated reported loudness (0 = unheard and 10 = very loud) for each of these. Comments and observations from the subject concerning any non-auditory sensations are highlighted with footnotes.



**Table 5.** *Cont.*

<sup>1</sup> The subject reported that the sound was 'not unpleasantly loud' but that it came with vertigo. <sup>2</sup> The subject winced and reported an unpleasant but mild 'pain in the head'. 3,5 The subject reported an unpleasant buzzing sound. <sup>4</sup> The subject experienced a feeling similar to 'a force pulling on her head'. <sup>6</sup> Observed nystagmus (involuntary eyes moving), a vestibular effect. <sup>7</sup> The subject reported deterioration of sound quality. <sup>8</sup> Unpleasant non-auditory sensations. The subject reported that this is 'super unpleasant' but still auditorily soft; the sensation reminded her of her previous CI.
