Electrically evoked compound action potentials in adult and pediatric cochlear implant users: II. Assessing cochlear nerve health status with the interphase gap effect and the phase locking value.

This study characterized and compared neural synchrony in the cochlear nerve (CN) and the number of CN fibers activated by electrical stimulation. Neural synchrony was quantified using the phase locking value. The number of activated CN fibers was assessed using the effect of increasing the interphase gap (IPG) of biphasic pulses on the electrically evoked compound action potential (eCAP), referred to as the IPG effect. The study included 50 postlingually deafened adults and 51 pediatric cochlear implant (CI) users with Cochlear™ Nucleus® devices. PLV and IPG effect measures were obtained at four and three electrode locations in adults and pediatric participants, respectively. In adults, CN neural synchrony was significantly poorer at basal compared with more apical electrode locations. No such electrode-location effect was observed in pediatric participants. No electrode-location differences in the IPG effect were observed in either group. Compared with pediatric participants, adults showed significantly poorer CN neural synchrony at the most basal electrode location, but not at the most apical location. No group differences were observed in the IPG effect. No association was found between PLV and the IPG effect at any electrode location in either group. These findings are consistent with differences in CN health status between adult and pediatric CI users and further support the notion that PLV and the IPG effect reflect distinct underlying biological mechanisms rather than non-neural factors. Further studies are needed to clarify the relationship between age and these eCAP-derived measures in adult CI users.