Relationship Between Intracochlear Electric Field and Temporal eCAP Characteristics in Cochlear Implant Patients

Goals:
Clinical observations of transimpedance matrix (TIM) measurements have suggested that apical cochlear implant (CI) electrode contacts may exhibit stronger and more broadly distributed intracochlear potentials, as well as double-peaked electrically evoked compound action potential (eCAP) morphologies. To quantify the temporal structure of eCAP responses, an iterative deconvolution framework [1] has been developed to estimate the temporal dispersion of the early and delayed neural response components. The aim of this study was to investigate whether spatial characteristics of the intracochlear electric field (EF) are associated with temporal properties of the neural components.

Material and Methods:
Routine intraoperative TIM and AutoNRT-derived eCAP measurements from 215 ears of 191 CI recipients (age 46.5 ± 28.6 years, mean ± SD; 20 perimodiolar and 195 lateral-wall arrays) were included in the study. EF parameters were estimated from TIM measurements using an exponential spatial decay model to represent local EF magnitude and spatial spread around the stimulating contact. The deconvolution framework was used to estimate compound discharge latency distributions (CDLDs), from which temporal parameters of the neural response components were derived. Associations between EF parameters and CDLD parameters were evaluated using linear mixed-effects models.

Results:
Double-peaked eCAP morphologies were observed in 1.6% of 3895 analyzed recordings and they occurred predominantly in the middle–apical region of the electrode array. Their low prevalence likely reflects the small eCAP amplitudes typical of clinical AutoNRT recordings. The deconvolution framework demonstrated stable waveform reconstruction, with a median root mean squared error corresponding to 7% of the median eCAP N1–P1 peak-to-peak amplitude. EF magnitude and spatial spread showed statistically significant positive associations with the amplitude of the delayed component of the CDLD (p < 0.05). In contrast, EF parameters showed weak or non-significant relationships with CDLD latency and temporal dispersion parameters.

Conclusion:
Routine intraoperative telemetry can be parameterized to provide objective information on electrical and neural aspects of CI stimulation. Intracochlear EF characteristics appear to be associated with the amplitude of the delayed CDLD component, which may contribute to the neural activity underlying double-peaked
eCAP responses.