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

Abstract

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.

Auditory-Nerve Responses to Varied Inter-Phase Gap and Phase Duration of the Electric Pulse Stimulus as Predictors for Neuronal Degeneration

“…The potential of an IPG to decrease excitation threshold has been demonstrated in various neural tissues and at various levels of neural processing (van den Honert and Mortimer 1979;Shepherd and Javel 1999;McKay and Henshall 2003;Carlyon et al 2005;Prado-Guitierrez et al 2006;Cappaert et al 2013). In the present study, this potentiating effect of an IPG is confirmed unambiguously for threshold (Fig.…”

“…With increasing PD, firing thresholds for single fibers (Shepherd and Javel 1999), eABRs (Miller et al 1995) and eCAPs (Prado-Guitierrez et al 2006; this study), and psychophysical detection level thresholds (Miller et al 1999) become lower when current level is kept constant. The main reason for this is that the amount of displaced charge (PD times current level) increases, which is why we chose to express stimulation level in term of charge rather than current.…”

“…Whereas Prado-Guitierrez et al (2006) used only SGC packing density as histological measure to relate to their electrophysiological measures, we added perikaryal area as second histological measure to cover both the degree of SGC loss and the state of the surviving SGCs. The two measures are obviously related, although, as is shown in Figure 3C, variability within groups is substantial.…”

“…Overcoming these unpredictable and often confounding factors, Prado-Guitierrez et al (2006) employed a method with which the difference in excitability between two stimulus pulse shapes can be utilized as a quantitative functional measure for SGC health. The magnitude of the change in excitability brought about by either a prolonged inter-phase gap (IPG) or phase duration (PD) correlated with the amount of surviving SGCs.…”

Effects of Hearing Preservation on Psychophysical Responses to Cochlear Implant Stimulation

“…The neural representation of electrical pulse trains is essential to the performance of cochlear implants since these pulse trains serve as the carrier for the temporal envelopes that convey much of the information needed for speech recognition. The condition of the hair cells and auditory neurons near the stimulating electrodes might affect the manner in which the auditory system integrates the responses to electrical pulses by influencing the number of surviving stimulable fibers as well as temporal properties of those fibers, affecting adaptation, refractory properties, and/or the amount of across-fiber synchrony in the stimulated fibers (Shepherd and Javel 1997;Vischer et al 1997;Wilson et al 1997;Matsuoka et al 2000;Miller et al 2001;Hu et al 2003;Cartee et al 2000Cartee et al , 2006Prado-Guitierrez et al 2006).…”