The electrically evoked auditory change complex: preliminary results from nucleus cochlear implant users

Abstract

Objectives: The purpose of this study was to determine if changes in the position of the stimulating electrode in the cochlea could be used to elicit the electrically evoked auditory change complex (EACC) from Nucleus cochlear implant users.

Design: Nine postlingually deafened adults participated in this study. Each study participant had been using his or her Nucleus CI24 cochlear implant for at least 3 mos before testing. The speech processor was bypassed and the output of the implanted receiver/stimulator was controlled directly. The stimulus was a 600 msec burst of a biphasic pulse train (1000 pps). In control conditions, the stimulating electrode was held constant and stimulation continued throughout the 600 msec recording interval. In experimental conditions, the EACC was elicited by introducing a change in the stimulating electrode 300 msec after the onset of the pulse train. The EACC was recorded using surface electrodes. Three recordings of 100 sweeps each were obtained for each stimulus condition. Bandpass filtering (1-100 Hz) was used to minimize contamination of the recordings by stimulus artifact. Averaged responses were then smoothed using a 40-msec wide boxcar filter and standard peak picking procedures were used to analyze these responses in the time domain.

Results: In each case, a clear onset response (P1-N1-P2) was recorded. In the experimental conditions, a second evoked potential, the EACC, was also recorded after the change in stimulating electrode. This second response had general morphological characteristics that were very similar to those of the onset response. Increasing the separation between the two stimulating electrodes in the experimental conditions resulted in a general trend toward increased EACC amplitudes.

Conclusions: This report describes results of a set of experiments in which the speech processor of the cochlear implant was bypassed and the EACC was recorded in response to a change in stimulating electrode position. EACC amplitude was shown to increase as the separation between the two stimulating electrodes increased. Although preliminary in nature, these results demonstrate the feasibility of recording the EACC in response to changes in stimulating electrode position from individual cochlear implant users.

Figure 1

Electrically evoked cortical potentials are shown for one subject (M66). The stimulus was a 600 ms burst of biphasic current pulses (1000 pps, 25 us/phase). In the control/no change condition (upper panel) stimulation was applied to electrode 10 for the full 600 ms recording interval. This recording contains only an onset response. In the lower panel, stimulation began on electrode 10 and at the 300 ms point was changed to electrode 15. This recording includes both an onset response and an EACC.

Figure 2

Thin lines represent waveforms recorded from each of the 9 adult study participants (10 ears). The thick line is the group mean average waveform. The upper panel is a control condition. The lower panel shows recordings from the same set of individuals. In this case, the stimulation level was held constant but 300 ms after stimulation began, the stimulating electrode was changed. In each case, the change was to an electrode located 4–5 electrodes apical to the initial stimulating electrode.

Figure 3

The waveforms shown in the lower panel of Figure 2 have been have been shifted along the ordinate and re-plotted. The column on the right indicates the stimulating electrodes used to elicit these responses. For each subject, an onset response as well as an EACC is recorded and the dots indicate the N1 peaks picked for both the onset responses and for the EACC. More details about the stimulation parameters are listed in Table 2.

Figure 4

The left panel shows a series of onset and EACC responses recorded from an individual study participant (M66). In each case, stimulation began on electrode 10. The upper waveform is a control/no change condition. As indicated on the figure, the stimulating electrode was changed 300 ms after stimulation was begun to a second electrode located apical to electrode 10. The right panel shows amplitude of these EACC responses as measured between N1 and P2. In this graph, response amplitude is plotted as a function of the second electrode in the stimulus pair. In the control/no change condition, the second electrode stimulated was electrode 10. The separation between the two stimulating electrodes increased as the second electrode was changed from electrode 11 to electrode 15.

Figure 5

Change in amplitude of the EACC as a function of the distance between the two stimulating electrodes used to elicit the EACC is shown. The ordinate shows the difference between the EACC N1-P2 amplitudes measured in stimulation conditions that included a change in the stimulating electrode and the maximum peak-to-peak amplitudes recorded over the same time window in the control or “no change” condition. Changes in the stimulating electrode in a basal direction (e.g. from electrode 15 to electrode 10) are indicated by negative numbers. Changes in the stimulating electrode in the apical direction (e.g. from electrode 15 to electrode 20) are indicated by positive numbers.