*2.5. Cochlear Implant System Activation*

The cochlear implant system's initial activation was carried out one month after the CI surgery.

#### *2.6. Electrophysiological Measurements*

Typical electrophysiological measurements were performed during surgery (impedances of the electrodes and neural response telemetry) and at initial system activation (impedances of the electrodes). Additionally, intraoperative and postoperative monitoring of CM electrical potentials (ECochG) was performed.

#### *2.7. ECochG Measurement*

Intraoperative and postoperative ECochG measurements were performed with the Advanced Bionics Active Insertion Monitoring AIMTM system. The system consists of the AIM System tablet, Naida CI Q90 sound processor, headpiece, cables, insert earphone and sterile inserts with an acoustic tube. Surgical preparation for the measurement was performed according to the AIMTM system Intra-Operative Guide. During electrode array insertion, a 50 ms tone burst stimulus (500 Hz) of alternating polarity was delivered at 115 dB SPL via the external ear canal with sterile inserts and an acoustic tube. The ECochG responses were recorded with the apical-most electrode contact. The acoustic feedback on changes in ECochG magnitude during the electrode array insertion was automatically provided to the surgeon. The AIM system schematic block system diagram is presented in Figure 4.

After full electrode array insertion, the ECochG responses were recorded with selected active electrode contacts to assess estimated pure-tone thresholds at audiometric frequencies in the 125–4000 Hz range. For this purpose, implemented in the device, an automatic algorithm of the ECochG signal detection and associated gradually decreasing tone burst stimulus were used (subsequently referred to in the text as the estimated audiogram measurement). The same procedure was repeated during the initial system activation.

#### *2.8. Imaging*

CT of the temporal bone (Siemens, Somatom Definition Edge, Munich, Germany) was performed the day after surgery to confirm the proper position of the electrode.

#### *2.9. Pure-Tone Audiometry*

The pure-tone audiometry measurements (Interacoustics AC40) were performed preoperatively and postoperatively to assess air conduction (AC) and bone conduction (BC) thresholds in line with ISO 8253–1:2010 standards in selected periods, no earlier than one month before surgery (AC and BC thresholds), two days after the surgery (BC thresholds only), and 1-month after the surgery (AC and BC thresholds).

**Figure 4.** The AIM system schematic block diagram.
