Recording of electrophysiological data

The ELARs were recorded for activation of Electrode 3 (apical), Electrodes 7 and 11 (mid), and Electrode 15 (base) in the cochlea in random order. Stimulation was delivered through SCLIN 2000 software version 1.08 (Advanced Bionics Corporation, 2000). Stimulation levels expressed in units proportional to microamperes were delivered at subjects’ most comfortable loudness level. The intensities of stimulation were balanced across the stimulation electrodes. Responses were recorded with a Centor C evoked response test system (Racia-Alvar) externally triggered by the stimulus output of the SCLIN 2000 software and the Clinician Programming Interface (CPI) unit. The CPI unit was connected to a processor itself in turn connected to an antenna, which used radio-frequency to transmit the stimulation parameters to the subject’s implanted device. Stimuli were biphasic current pulses of 75 µs duration and had alternating polarity. The repetition rate was 1.01 Hz and a monopolar stimulation mode was used. Surface recording electrodes were placed on the forehead (+), nape of the neck (-), and contralateral earlobe (common). The impedances between recording electrodes were less than 5 kΩ and balanced. Recording of electrical activity included 500 sweeps for each stimulating electrode site. The time window was 300 ms (500 points) with an amplification sensitivity of 50 μV per division. Frequency cutoffs of 1.6 and 100 Hz were used for the recording amplifier. Subjects were seated in a reclining chair, watching a captioned videotape. They were instructed to relax but not to sleep. Trials that included muscle movements which interfered with the response were rejected. The waveforms for late responses were identified according to criteria that were based on animal and human research with acoustic and electrical stimulation. Electrical N1-P2 cortical responses would consist of a negative trough, N1, at approximatively 80 to 110 msec, followed by a positive peak, P2, at approximatively 180 to 210 msec (Kraus et al., 1993; Näätanen & Picton, 1987; Picton et al., 1974; Ponton & Don, 1995). We also kept in mind the fact that some authors had found shorter latencies for cochlear implantees when they activated the electrodes directly (Ponton & Don, 1995; Ponton et al., 1996; Firszt et al., 2002a). The latencies were measured (in msec) with respect to the onset of the first phase of the biphasic stimulus pulse at the midpoint of the individual N1 and P2 peaks. If two peaks fell in the appropriate interval, the latency of the higher peak was chosen.