Recording of behavioural data

Audiometric tone thresholds were recorded in the sound field before cochlear implantation. Continuous tones were presented via calibrated sound field equipment in a sound-treated booth. The M level measurements were made on the first day of cochlear implant use. Stimuli were biphasic current pulse trains delivered on electrodes 3, 7, 11 and 15 using the HiRes-S mode. The electrode configuration was monopolar, which should not result in a broader pattern of excitation than the bipolar mode of stimulation and respect the tonotopic organization of the cochlea (Kwon & van den Honert, 2006). The pulse width was 10.8 µs and the stimulation rate was 2486 pulses per second per channel (pps/ch). Testing began at a level below threshold, the stimulus slowly increasing in steps of between 3 and 5 CU (charge units) using a bracketing approach until the subject identified the M level for each electrode as precisely as possible. It is worth noting that the stimulation parameters used to record the M levels were different from those used for ELAR recording. Speech perception scores (percentage of phoneme correctly perceived) were obtained at the time ELAR recordings were made using Lafon lists (lists of three-phoneme words; Lafon, 1964) presented in an open set format in quiet at 65 dB HL in a sound field.