Cochlear implants in single-sided deaf recipients: Near normal higher-order processing

Objective: Single-sided deafness (SSD) is a condition where an individual has a severe to profound sensorineural hearing loss in one ear and normal hearing on the contralateral side. The use of cochlear implants in individuals with SSD leads to functional improvements in hearing. However, it is relatively unclear how sounds incoming via the cochlear implant (independent of the hearing ear) are processed and interpreted by higher-order processes in the brain. Methods: Scalp electroencephalography and auditory event-related potentials were recorded monaurally from nine experienced single sided cochlear implant users. Speech-in-noise and localisation tests were used to measure functional changes in hearing. Results: cochlear implant use was associated with improvement in speech-in-noise and localisation tests (compared to cochlear implant off). Significant N2 and P3b effects were observed in both cochlear implant and normal hearing ear conditions, with similar waveform morphology and scalp distribution across conditions. Delayed response times and a reduced N2 (but not P3b) effect was measured in the CI condition. Conclusion: The brain is capable of using processes similar to those in normal hearing to discriminate sounds presented to the cochlear implant. There was evidence of processing difficulty in the cochlear implant condition which could be due to the relatively degraded signals produced by the cochlear implant compared to the normal hearing ear. Significance: Understanding how the brain processes sound provided by a cochlear implant highlights how cortical responses can be used to guide implantation candidacy guidelines and influence rehabilitation recommendations.