gms | German Medical Science

Introduction: The residual acoustic hearing of cochlear implant (CI) users can be monitored using electrocochleography (ECochG). In intracochlear ECochG, the electrical activity of the cochlea in response to acoustic stimuli is recorded via the electrode array. The ECochG consists of responses from hair cells (e.g. CMs) as well as from neurons (e.g. CAP or ANNs). Prior research in animal models has demonstrated that the shape of the acoustic stimulus may effect amplitude and threshold of cochlear microphonics (CM) and CAP (Adel et al., 2020). The present study aimed to (1) record intracochlear ECochG responses to narrowband (1 octave) and broadband (4 octaves) chirp stimuli (NB Chirp & BB Chirp), and Tone Bursts, and (2) determine the efficiency of all stimuli by analyzing threshold and amplitude of the ECochG response.

Methods: 15 subjects (16 ears) who had undergone CI surgery with hearing preservation were enrolled in the study. Intracochlear Post surgery ECochG measurements were conducted using single electrode and electrode sweep recording modes. For both recording modes, subjects received acoustic stimuli comprising 250, 500, and 1,000 Hz Tone Bursts, NB Chirps at 250, 500, and 1,000 Hz center frequency, and BB Chirps spanning 250 Hz–4 kHz. The electrode exhibiting the highest spectral amplitude in sweep recording mode was chosen as electrode in single test mode. Loudness of the different stimuli and test level was rated between 0 (no sensation) and 10 (extremely loud). CM responses were evaluated by spectral analysis (FFT). For electrode sweep recordings, sound level was set to most comfortable level (MCL). For CAP assessment, an alternating Tone Burst at 500 Hz at MCL was employed.

Result and discussion: Averaged pure tone hearing thresholds at 250 Hz, 500 Hz, and 1 kHz were 60.3, 81.7, and 93.8 dB HL. Tone Burst thresholds were on average 68.5, 78.1, and 95 dB HL, NB Chirp thresholds 72.0, 85.4, and 102 dB HL. BB Chirp threshold was 74.3 dB HL The increase in loudness and growth of FTT-amplitude on CM amplitudes related to sound level were found to be significantly shallower for the BB Chirp stimulus. Due to the limitations imposed by the restricted choice of stimuli in the alternating recording mode, the evaluation of the CAP response was challenging.

Conclusion: Chirp stimuli demonstrate so far no clear advantage against conventional tone bursts in terms of response amplitudes and threshold estimation.