Investigation of the effect of speech processing strategy and timing synchronization on temporal and spectral resolution in bimodal cochlear implant users

Objective: The aim of this study was to examine speech discrimination performance in noise, as well as spectral and temporal resolution abilities, in bimodal cochlear implant users with eliminated delay mismatch. In addition, we aimed to investigate the temporal synchronization of speech processing strategies at constant and variable rates in individuals with synchronized settings. Method: The study included 41 bimodal cochlear implant users aged 15–45 years. Participants were divided into two groups based on the presence or absence of temporal mismatch (20 Cochlear, 21 Med-El). The following tests were administered sequentially: Modulation Detection Threshold (easy MDT), Random Gap Detection Test (RGDT), Spectral-Temporal Modulation Discrimination Test, and the Turkish Matrix Test. Initial session data were collected in asynchronous mode. In the Med-El group, latency mismatches were corrected using the cochlear implant software according to the brand and model of the participants' hearing aids. Both the Fine Structure Processing (FSP) combined with Continuous Interleaved Sampling (HDCIS), and HDCIS alone, were tested separately in synchronous mode. Results: Significant differences were observed in the RGDT composite score, easy MDT, and both Adaptive and Non-Adaptive Matrix tests among Med-el users using the FSP strategy in synchronous mode (p <0.05). Comparisons between manufacturers in asynchronous mode did not reveal any significant differences. Conclusion: Eliminating temporal mismatch improves the integration between the cochlear implant and the hearing aid, thereby enhancing temporal resolution and speech perception in noise. Temporal mismatch elimination should be considered an important factor in maximizing the benefits of bimodal hearing and in reducing variability across users, and it may serve as a basis for individualized implant programming.