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Review Article
Background
Cochlear implantation is an established technique used to enhance auditory perception in individuals with profound hearing loss. The cochlear implant stimulates the auditory nerve directly. However, the auditory signal delivered by the implant is both temporally and spectrally degraded.1Despite this degraded signal, patients with cochlear implants have the potential to hear and discriminate speech. Many patients with cochlear implants develop good open-set speech perception, resulting in marked improvements in their quality of life.2,3Nevertheless, a significant proportion of patients with an implant have poor speech perception.
Before we can determine the mechanisms responsible for the poor outcomes following cochlear implants, it is necessary to understand how individuals learn to process speech using the implant. However, the exact central processes responsible for speech perception in implantees remain unknown. This review aimed to provide a summary of the present knowledge regarding how patients process speech using a cochlear implant.
Recipients of cochlear implants may be broadly divided into two groups. The pre-lingual group refers to patients who develop hearing loss prior to the acquisition of speech and language (i.e. children), and the post-lingual group refers to patients who become deaf after the acquisition of speech. The ages and aetiology for hearing loss differ significantly between pre- and post-lingual hearing loss groups. This review focused on the adult post-lingual group.
To understand the cortical processes involved in speech processing, it is necessary to observe the cortical changes in the brain when an auditory stimulus is presented. As cochlear implants possess an internal magnet, the only brain imaging technique that can be used on implantees is positron emission tomography (PET), which involves radiation.
Current literature suggests that in normal-hearing individuals, language processing occurs in the temporal lobes. Specifically, basic acoustic analysis occurs in the primary auditory cortex, which is located in the superior temporal gyrus of the temporal lobe. Higher processing of language takes place in additional areas of the temporal gyri, in particular, the auditory association area and prefrontal cortices bilaterally.4-9Higher processing involves the processing of phonetic and sublexical speech sounds, and the accessing of the lexicon and semantics to retrieve the meaning of the presented word.
The studies by...