Assessing the Relationship Between Pitch Perception and Neural Health in Cochlear Implant Users

Abstract

Various neural health estimates have been shown to indicate the density of spiral ganglion neurons in animal and modeling studies of cochlear implants (CIs). However, when applied to human CI users, these neural health estimates based on psychophysical and electrophysiological measures are not consistently correlated with each other or with the speech recognition performance. This study investigated whether the neural health estimates have stronger correlations with the temporal and place pitch sensitivity than with the speech recognition performance. On five electrodes in 12 tested ears of eight adult CI users, polarity effect (PE), multipulse integration (MPI), and interphase gap (IPG) effect on the amplitude growth function (AGF) of electrically evoked compound action potential (ECAP) were measured to estimate neural health, while thresholds of amplitude modulation frequency ranking (AMFR) and virtual channel ranking (VCR) were measured to indicate temporal and place pitch sensitivity. AzBio sentence recognition in noise was measured using the clinical CI processor for each ear. The results showed significantly poorer AMFR and VCR thresholds on the basal electrodes than on the apical and middle electrodes. Across ears and electrodes, only the IPG offset effect on ECAP AGF had a nearly significant negative correlation with the VCR threshold after removing the outliers. No significant across-ear correlations were found between the mean neural health estimates, mean pitch-ranking thresholds, and AzBio sentence recognition score. This study suggests that the central axon demyelination reflected by the IPG offset effect may be important for the place pitch sensitivity of CI users and that the IPG offset effect may be used to predict the perceptual resolution of virtual channels for CI programming.

Keywords: Cochlear implant; Neural health; Pitch ranking; Speech recognition.

Fig. 1

Boxplots of polarity effect (PE; a), multipulse integration (MPI) slope (b), interphase gap (IPG) offset of electrically evoked compound action potential (ECAP) amplitude growth function (AGF; c), amplitude modulation frequency ranking (AMFR) threshold (d), and virtual channel ranking (VCR) threshold (e) for individual ears as a function of the electrode. Each ear is represented by a different color. The horizontal black line within each boxplot represents the median value across the ears for each electrode. The bottom and top edges of each boxplot indicate the 25th and 75th percentiles, respectively. The distance between the two edges defines the interquartile range (IQR). The whiskers extend to ± 1.5 × IQR. The data points beyond the whiskers are outliers and are indicated by pink diamonds. Horizontal brackets with p values show the significant differences between electrodes based on post-hoc pairwise comparisons

Fig. 2

Pearson product-moment correlations between neural health estimates and pitch-ranking thresholds across ears and electrodes. Neural health estimates include polarity effect (PE), slope of multipulse integration (MPI), and interphase gap (IPG) offset of electrically evoked compound action potential (ECAP) amplitude growth function (AGF), while pitch-ranking thresholds include amplitude modulation frequency ranking (AMFR) threshold and virtual channel ranking (VCR) threshold. The r and p values are shown in each panel for a pair of measurements (MPI slope vs. PE in a, IPG offset effect vs. PE in b, AMFR threshold vs. PE in c, VCR threshold vs. PE in d, IPG offset effect vs. MPI slope in e, AMFR threshold vs. MPI slope in f, VCR threshold vs. MPI slope in g, AMFR threshold vs. IPG offset effect in h, VCR threshold vs. IPG offset effect in i, and VCR threshold vs. AMFR threshold in j). Each ear is represented by a different color. Each solid line shows the linear regression between a pair of measurements

Fig. 3

Across-ear correlations between mean polarity effect (PE), mean slope of multipulse integration (MPI), mean amplitude modulation frequency ranking (AMFR) threshold, mean virtual channel ranking (VCR) threshold, and AzBio sentence recognition score. The neural health estimates and pitch-ranking thresholds were obtained and averaged across all the tested electrodes of each ear. The r and p values are shown in each panel for a pair of measurements (mean MPI slope vs. mean PE in a, mean AMFR threshold vs. mean PE in b, mean VCR threshold vs. mean PE in c, AzBio sentence recognition score vs. mean PE in d, mean AMFR threshold vs. mean MPI slope in e, mean VCR threshold vs. mean MPI slope in f, AzBio sentence recognition score vs. mean MPI slope in g, mean VCR threshold vs. mean AMFR threshold in h, AzBio sentence recognition score vs. mean AMFR threshold in i, and AzBio sentence recognition score vs. mean VCR threshold in j). Each ear is represented by a different color. Each solid line shows the linear regression between a pair of measurements