Examining the Electro-Neural Interface of Cochlear Implant Users Using Psychophysics, CT Scans, and Speech Understanding

Abstract

This study examines the relationship between focused-stimulation thresholds, electrode positions, and speech understanding in deaf subjects treated with a cochlear implant (CI). Focused stimulation is more selective than monopolar stimulation, which excites broad regions of the cochlea, so may be more sensitive as a probe of neural survival patterns. Focused thresholds are on average higher and more variable across electrodes than monopolar thresholds. We presume that relatively high focused thresholds are the result of larger distances between the electrodes and the neurons. Two factors are likely to contribute to this distance: (1) the physical position of electrodes relative to the modiolus, where the excitable auditory neurons are normally located, and (2) the pattern of neural survival along the length of the cochlea, since local holes in the neural population will increase the distance between an electrode and the nearest neurons. Electrode-to-modiolus distance was measured from high-resolution CT scans of the cochleae of CI users whose focused-stimulation thresholds were also measured. A hierarchical set of linear models of electrode-to-modiolus distance versus threshold showed a significant increase in threshold with electrode-to-modiolus distance (average slope = 11 dB/mm). The residual of these models was hypothesized to reflect neural survival in each subject. Consonant-Nucleus-Consonant (CNC) word scores were significantly correlated with the within-subject variance of threshold (r(2) = 0.82), but not with within-subject variance of electrode distance (r(2) = 0.03). Speech understanding also significantly correlated with how well distance explained each subject's threshold data (r(2) = 0.63). That is, subjects with focused thresholds that were well described by electrode position had better speech scores. Our results suggest that speech understanding is highly impacted by individual patterns of neural survival and that these patterns manifest themselves in how well (or poorly) electrode position predicts focused thresholds.

Relationship between multipulse integration and speech recognition with cochlear implants

“…In damaged ears, thresholds for activation can be elevated due to loss of the most sensitive fibers or increased distance between the implant electrodes and the most sensitive nerve fibers (Bierer and Faulkner, 2010;Long et al, 2014). Our data showed that in some ears, stimulation sites that had shallow slopes did also tend to have higher absolute detection thresholds.…”

Deactivating stimulation sites based on low-rate thresholds improves spectral ripple and speech reception thresholds in cochlear implant users

“…Spatial summation of stimulation would not only cause spectral information to smear but also cause the subgroup of auditory fibers to be stimulated at a rate that is much higher than the singleelectrode rate, increasing the probability of neural adaptation (Boulet et al, 2016). Because neural survival, electrode position, and other pathological variables are not homogeneous across the tonotopic axis in an implanted ear (Nadol, 1997;Long et al, 2014), some stimulation sites are expected to interact more than others. The hypothesis tested in the current study was whether removing the stimulation sites estimated to produce broad neural excitation would improve spectral resolution, which in turn would improve speech recognition.…”

“…The factors that contribute to channel interaction include but are not limited to electrode impedance, electrode position relative to the modiolus, and survival pattern of the auditory nerve fibers (e.g., Long et al, 2014). High impedance, large electrode-neuron distance, or a significant loss of nerve fibers near the stimulation site, would result in an increased spread of neural excitation from the targeted tonotopic place.…”

Contribution of formant frequency information to vowel perception in steady-state noise by cochlear implant users

“…By optimizing the JND parameters for model fits to each subject's data in the quiet condition, we are controlling for individual factors that affect CI users' discrimination for place of stimulation in the cochlea such as electrode placement, cochlear size, neural survival, etc. (Bierer, 2010;Long et al, 2014). From a modeling perspective it is more parsimonious to minimize the number of degrees of freedom, and assuming an underlying constant JND for place of stimulation allows us to do so.…”