Behavioral measures

Audiometric tone thresholds were recorded in a sound field before cochlear implantation. Continuous tones were presented via calibrated sound field equipment in a sound-treated booth. For M level measurements, stimuli were biphasic current pulse trains delivered on electrodes 3, 7, 11 and 15 using the HiRes-S mode. The pulse width was 10.8 µs in duration and the stimulation rate was 2486 Hz. 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. Subjects used a 10-step loudness scale (Clarion Multi-strategy Cochlear Implant System Manual Version 2.0, 1996) on which the M level is surrounded by two levels called low comfortable level and high comfortable level in order to reduce influence of subjective judgement on M level. The M levels were then balanced across the electrodes over the array.