Effect of Increasing Pulse Phase Duration on Neural Responsiveness of the Electrically Stimulated Cochlear Nerve

Abstract

Objectives: The aim of this study is to (1) investigate the effects of increasing the pulse phase duration (PPD) on the neural response of the electrically stimulated cochlear nerve (CN) in children with CN deficiency (CND) and (2) compare the results from the CND population to those measured in children with normal-sized CNs.

Design: Study participants included 30 children with CND and 30 children with normal-sized CNs. All participants used a Cochlear Nucleus device in the test ear. For each subject, electrically evoked compound action potential (eCAP) input/output (I/O) functions evoked by single biphasic pulses with different PPDs were recorded at three electrode locations across the electrode array. PPD durations tested in this study included 50, 62, 75, and 88 μsec/phase. For each electrode tested for each study participant, the amount of electrical charge corresponding to the maximum comfortable level measured for the 88 μsec PPD was used as the upper limit of stimulation. The eCAP amplitude measured at the highest electrical charge level, the eCAP threshold (i.e., the lowest level that evoked an eCAP), and the slope of the eCAP I/O function were measured. Generalized linear mixed effect models with study group, electrode location, and PPD as the fixed effects and subject as the random effect were used to compare these dependent variables measured at different electrode locations and PPDs between children with CND and children with normal-sized CNs.

Results: Children with CND had smaller eCAP amplitudes, higher eCAP thresholds, and smaller slopes of the eCAP I/O function than children with normal-sized CNs. Children with CND who had fewer electrodes with a measurable eCAP showed smaller eCAP amplitudes and flatter eCAP I/O functions than children with CND who had more electrodes with eCAPs. Increasing the PPD did not show a statistically significant effect on any of these three eCAP parameters in the two subject groups tested in this study.

Conclusions: For the same amount of electrical charge, increasing the PPD from 50 to 88 μsec for a biphasic pulse with a 7 μsec interphase gap did not significantly affect CN responsiveness to electrical stimulation in human cochlear implant users. Further studies with different electrical pulse configurations are warranted to determine whether evaluating the eCAP sensitivity to changes in the PPD can be used as a testing paradigm to estimate neural survival of the CN for individual cochlear implant users.

Figure 1.

eCAP responses and eCAP I/O functions measured at electrode 1 in CND4 (upper panels) and electrode 3 in S1L (lower panels). Subject and electrode numbers are indicated in each panel. Normalized eCAP amplitudes measured at different PPDs are indicated using different symbols with different colors.

Figure 2.

The means and the standard deviations of the eCAP amplitude measured at the highest stimulation for different PPDs and three electrode locations in two study groups. Results measured in children with CND and children with normal-sized CNs (i.e., control) are indicated using filled circles and open triangles, respectively. Each column shows results recorded at one electrode location (i.e., basal, middle or apical electrode).

Figure 3.

The means and the standard deviations of the eCAP threshold measured at different PPDs and electrode locations in the CND group 1 (black circles), the CND group 2 (grey hexagons) and the control group (black triangles). Each column shows results recorded at one electrode location (i.e., basal, middle or apical electrode).

Figure 4.

The means and the standard deviations of the slope of the eCAP I/O function estimated using the sigmoidal regression function for results measured at different PPDs and three electrode locations in the CND group 1 (black circles), the CND group 2 (grey hexagons) and the control group (black triangles). Each column shows results recorded at one electrode location (i.e., basal, middle or apical electrode).

Figure 5.

The means and the standard deviations of the highest stimulation level used in the CND group 1 (black circles), the CND group 2 (grey hexagons) and the control group (black triangles). Each column shows results recorded at one electrode location (i.e., basal, middle or apical electrode).

Figure 6.

The means and the standard deviations of the dynamic range of the CND group 1 (black circles), the CND group 2 (grey hexagons) and the control group (black triangles). Each column shows results recorded at one electrode location (i.e., basal, middle or apical electrode).