Abstract
While many individuals can benefit substantially from cochlear implantation, the ability to perceive and understand auditory speech with a cochlear implant (CI) remains highly variable amongst adult recipients. Importantly, auditory performance with a CI cannot be reliably predicted based solely on routinely obtained information regarding clinical characteristics of the CI candidate. This review argues that central factors, notably cortical function and plasticity, should also be considered as important contributors to the observed individual variability in CI outcome. Superior temporal cortex (STC), including auditory association areas, plays a crucial role in the processing of auditory and visual speech information. The current review considers evidence of cortical plasticity within bilateral STC, and how these effects may explain variability in CI outcome. Furthermore, evidence of audio-visual interactions in temporal and occipital cortices is examined, and relation to CI outcome is discussed. To date, longitudinal examination of changes in cortical function and plasticity over the period of rehabilitation with a CI has been restricted by methodological challenges. The application of functional near-infrared spectroscopy (fNIRS) in studying cortical function in CI users is becoming increasingly recognised as a potential solution to these problems. Here we suggest that fNIRS offers a powerful neuroimaging tool to elucidate the relationship between audio-visual interactions, cortical plasticity during deafness and following cochlear implantation, and individual variability in auditory performance with a CI.
Highlights
Given the multi-modal rather than sensory-specific nature of these regions for language processing (Petitto et al, 2000; Straube et al, 2012), superior temporal cortex (STC) activations to visual speech may reflect activation elicited by amodal linguistic analysis such as phonological processing (Hall et al, 2005), a cognitive function that is implicated in successful speechreading (Andersson et al, 2001; Lyxell et al, 2003)
Various clinical characteristics are known to influence cochlear implant (CI) outcome, yet from these factors alone we are currently not able to reliably predict how well an individual will perform with a CI (Blamey et al, 2013; Lazard et al, 2012a; Summerfield and Marshall, 1995)
A better understanding of the factors and mechanisms underlying variability in CI outcome is of clinical importance as sensitive prognostic tools are needed to help more accurately predict clinical outcomes of individual cochlear implant recipients in order to set and counsel their expectations most effectively
Summary
Over the past few decades, continued developments in cochlear implantation have enabled many individuals with severe-toprofound sensorineural hearing loss to benefit substantially from a cochlear implant (CI). Evidence from multiple studies consistently suggests that there is pronounced variability in speech perception abilities across adult CI recipients, even in quiet listening conditions (Blamey et al, 2013; Gantz et al, 1993; Holden et al, 2013; Lazard et al, 2010a; Summerfield and Marshall, 1995; Tyler et al, 1997) Both the rate and trajectory of auditory performance over time is seen to vary across individuals (Holden et al, 2013; Tyler et al, 1997), and word identification across a cohort of CI users can span the entire possible range of test scores (0e100% correct, Lazard et al, 2010a). We evaluate the current evidence of deafness-related changes within the bilateral superior temporal cortex (STC)
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