Recording of EABRs

The EABRs were recorded from all the electrodes within the cochlea in random order. Stimulation was performed within the subjects’ comfortably loud perception range at three various stimulus levels (expressed in units proportional to microamperes: CU). The stimulation was thus performed at the intensity rated comfortably loud by the subject, as well as 5 CU and 10 CU above that level. Comfortably loud intensities were balanced over the electrodes. High levels of stimulation were used in order to obtain EABR waves that would be good enough to measure latencies precisely. It was assumed that this choice would not keep from investigating place specific responses for several reasons. Firstly, the latency of wave Ve was found to vary little with stimulus intensity, unlike the acoustic ABR, and not to vary at all when the intensity reached upper comfortable loudness (Gallégo et al, 1997; Truy et al., 1998). Secondly, electrode pitch discrimination, which has been shown to be correlated with electrode interaction as measured with the electrical compound action potentials (ECAPs) (Hughes & Abbas, 2006), is poorer when the intensity decreases (McKay et al., 1999; Pfingst et al., 1999; Morris et Pfingst, 2000) and Cohen et al. (2003) showed that there is no significant effect of such high levels of stimulation on the widths of the ECAP functions. Therefore it seems that once the intensity is high enough the activation is initiated at the same place along the auditory pathway and such intensities would not keep from studying specific neural places. Once the latency of the waves Ve was the same for each intensity, the three recordings were averaged. Responses were recorded with a Centor C evoked response test system (Racia-Alvar) externally triggered by the stimulus output of the SCLIN 2000 software (version 1.08) and the Clinician Programming Interface (CPI) unit. The interface unit was connected to a processor itself connected to an antenna, which transmitted the stimulation parameters to the subject’s implanted device. Stimuli were biphasic current pulses of 75 µs duration alternating in polarity. The stimulation repetition rate was 21.07 Hz and monopolar mode of stimulation 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 1000 sweeps for each intensity. The time window was 10 ms (500 points) with an amplification sensitivity of 12.5 μV per division. Frequencies below 80 Hz and above 1600 Hz were cut off from the recording amplifier. Subjects were seated in a reclining chair and watching a captioned videotape. They were instructed to relax and encouraged to sleep. Trials that included muscle movements which interfered with the response were rejected. Latencies of the peak of waves IIIe and Ve were measured with respect to the onset of the first phase of the biphasic stimulus pulse.