Cochlear implantation in adults with acquired single-sided deafness improves cortical processing and comprehension of speech presented to the non-implanted ears: A longitudinal EEG study

Former studies have established that individuals with a cochlear implant (CI) for treating single-sided deafness (SSD) experience improved speech processing after implantation. However, it is not clear how each ear contributes separately to improve speech perception over time at the behavioral and neural level. In this longitudinal EEG study with four different time points, we measured neural activity in response to variously degraded spoken words presented monaurally to the CI and non-CI ears in 10 single-sided CI users and 10 age- and sex-matched individuals with normal hearing. Subjective comprehension ratings for each word were also recorded. Data from single-sided CI participants were collected : pre-CI implantation, and at 3, 6, and 12 months after implantation. We conducted a time-resolved representational similarity analysis (RSA) on the EEG data to depict whether and how neural patterns became more similar to those of normal hearing individuals. The analysis was performed separately for the CI and non-CI ears. At 6 months after implantation, the speech comprehension ratings for the degraded words improved in both ears. Notably, the improvement was more pronounced for the non-CI ears than the CI ears. Furthermore, the enhancement in the non-CI ears was paralleled by increased similarity to neural representational patterns of the normal hearing control group. The maximum of this effect was observed between 600 and 1200 ms after stimulus onset, coinciding with the peak decoding accuracy for spoken-word comprehension. The present study demonstrates that cortical processing gradually normalizes when speech was presented to the non-CI ears after CI implantation within months. The CI enables the deaf ear to provide afferent input, which, according to our results, complements the input of the non-CI, gradually improving its function. These novel findings underscore the feasibility of tracking neural recovery after auditory input restoration using advanced multivariate analysis methods, such as RSA.