Neural Health and Pitch Perception with Cochlear Implants

Neural Health and Pitch Perception with Cochlear Implants Neural Health and Pitch Perception with Cochlear Implants Specific Aims: Although cochlear implant (CI) is successful as a neural prosthesis that restores hearing in patients with severe-to-profound sensorineural hearing loss, there is a large variability in CI outcomes in terms of speech recognition performance across patients (e.g., Blamey et al., 2012; Holden et al., 2013). A good understanding of the underlying factors for this inter-subject variability is needed before customized processor fitting strategies can be applied to improve speech recognition performance of individual CI users (e.g., Bierer, 2010; Garadat et al., 2013; Zhou, 2017; Goehring et al., 2019). One of the most important factors that contribute to the variable CI outcomes is the health of spiral ganglion neurons (SGNs), which are the first gateway to receive speech information encoded by electrical stimulation. Different neural health measures have been shown to be correlated with the SGN density in animal studies and to be independent of the non-neural factors such as electrode-modiolus distance and electrode impedance in human CI users (e.g., Jahn and Arenberg, 2019; Mesnildrey et al., 2020; Zhou and Pfingst, 2014; Schvartz-Leyzac et al., 2020; Brochier et al., 2020). However, there is no consensus on which neural health measure is more appropriate for the applications of predicting CI outcomes and guiding the customized fitting of CI processors. In fact, studies have shown that different neural health measures may reflect different aspects of SGN status (e.g., Brochier et al., 2021). Also, the correlations between neural health measures and speech recognition performance have been mixed (e.g., Kim et al., 2010; Zhou and Pfingst, 2014; Schvartz-Leyzac and Pfingst, 2018; Imsiecke et al., 2021), possibly because high-level language processing and cognitive functions are also involved in speech recognition. To understand how the neural health measures are related to auditory perception with CIs and to validate the neural health measures for clinical applications, psychophysical measures of the sensitivity to basic auditory cues encoded by CIs (i.e., the temporal rate and place of electrical stimulation) should be compared with the different neural health measures. Pitch perception based on the temporal rate and place of electrical stimulation may have less dependence on top-down processing than speech recognition, and can be tested locally on the same set of electrodes as the neural health measures. As shown in our pilot study, neural health measures may have a strong correlation with pitch perception performance across electrodes and CI users. However, the relationship between neural health and pitch perception with CIs has not been examined in the past. This project is proposed to use a systematic approach to filling this knowledge gap. The long-term goal of our research is to understand the neural mechanisms of pitch perception with CIs and use such knowledge to develop innovative and effective processing strategies and rehabilitation protocols for better CI outcomes. The overall objective of this proposal is to identify the aspects of neural health that are critical for CI users sensitivity to temporal and place cues for pitch and speech perception. Towards this objective, we will pursue the following specific aim. Specific Aim: Investigate the relationship between neural health measures and pitch discrimination thresholds across electrodes and CI users. Three neural health measures will be tested. Polarity effect (PE) will be measured as the difference between psychophysical detection thresholds of the cathodic- and anodic-centered triphasic pulse trains, multipulse integration (MPI) will be measured as the slope of the function relating psychophysical detection threshold to pulse rate, and interphase gap (IPG) offset of the electrically evoked compound action potential (ECAP) amplitude growth function (AGF) will also be measured. Modeling studies showed that PE may be related to the peripheral process degeneration of SGN, MPI may be related to the standard deviation of SGN firing thresholds, while IPG offset of ECAP AGF may be related to the central axon demyelination of SGN (Brochier et al., 2021). Two pitch discrimination thresholds will be measured, including amplitude modulation (AM) frequency and virtual channel (VC) discrimination thresholds based on the temporal rate and place of stimulation, respectively (Luo et al., 2012). In addition, speech recognition in noise will be measured for each tested ear using the clinical CI processor. Our general hypotheses are that all the neural health measures and pitch discrimination thresholds would vary greatly across electrodes within each CI user, and that for each measure, the pattern of results across electrodes would differ for different CI users, reflecting the subject-specific patterns of neural survival. Neural health measures will have stronger correlations with pitch discrimination thresholds than with speech recognition performance. The expected outcome of the proposed research will have significant impacts both theoretically and clinically, because the findings will increase our understanding of the neural mechanisms for pitch perception with CIs and increase the accuracy of diagnosis for customized processor fitting of individual CI users.