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      The Effect of Increasing Interphase Gap on N1 Latency of the Electrically Evoked Compound Action Potential and the Stimulation Level Offset in Human Cochlear Implant Users

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      Ear & Hearing

      Ovid Technologies (Wolters Kluwer Health)

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          Most cited references 19

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          Detecting outliers: Do not use standard deviation around the mean, use absolute deviation around the median

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            Examining the electro-neural interface of cochlear implant users using psychophysics, CT scans, and speech understanding.

            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.
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              Effect of interphase gap and pulse duration on electrically evoked potentials is correlated with auditory nerve survival.

              We investigated the effect of pulse duration (PD) and interphase-gap (IPG) on the electrically-evoked auditory brain stem response (EABR) and viiith nerve compound action potential (ECAP) of deafened guinea pigs in order to test the hypothesis that the extent of change in these neural responses is affected by the histological status of the auditory nerve. Fifteen guinea pigs were deafened by co-administration of kanamycin and furosemide. Animals were acutely implanted with an 8-band electrode array at 1, 4 or 12 weeks following deafening. EABR and ECAP input/output functions were recorded in response to charge balanced biphasic current pulses. We determined the change in current required to equalize; (i) the EABR amplitude when the duration of the current pulse was doubled (104-208 micros/phase); and (ii) the EABR and ECAP amplitudes when the IPG was increased from 8 to 58 micros using a 104 micros/phase current pulse. Following the completion of each experiment the cochleae were examined quantitatively for spiral ganglion neuron survival. As expected, the current level required to evoke an EABR with equal amplitude was lower when the animal was stimulated with current pulses of 208 compared with 104 micros/phase. Moreover, the current level required to evoke EABR/ECAPs with equal amplitude was lower when current pulses had an IPG of 58 versus 8 micros. Importantly, there was a reduction in the magnitude of this effect with greater neural loss; the reduced efficacy of changing both PD and IPG on these electrically-evoked potentials was statistically correlated with neural survival. These results may provide a tool for investigating the contribution of auditory nerve survival to clinical performance among cochlear implant subjects.
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                Author and article information

                Journal
                Ear & Hearing
                Ovid Technologies (Wolters Kluwer Health)
                1538-4667
                2021
                January 2021
                July 16 2020
                : 42
                : 1
                : 244-247
                Article
                10.1097/AUD.0000000000000919
                © 2020

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