Abstract
Currently, it is not possible to accurately predict how well a deaf individual will be able to understand speech when hearing is (re)introduced via a cochlear implant. Differences in brain organisation following deafness are thought to contribute to variability in speech understanding with a cochlear implant and may offer unique insights that could help to more reliably predict outcomes. An emerging optical neuroimaging technique, functional near-infrared spectroscopy (fNIRS), was used to determine whether a pre-operative measure of brain activation could explain variability in cochlear implant (CI) outcomes and offer additional prognostic value above that provided by known clinical characteristics. Cross-modal activation to visual speech was measured in bilateral superior temporal cortex of pre- and post-lingually deaf adults before cochlear implantation. Behavioural measures of auditory speech understanding were obtained in the same individuals following 6 months of cochlear implant use. The results showed that stronger pre-operative cross-modal activation of auditory brain regions by visual speech was predictive of poorer auditory speech understanding after implantation. Further investigation suggested that this relationship may have been driven primarily by the inclusion of, and group differences between, pre- and post-lingually deaf individuals. Nonetheless, pre-operative cortical imaging provided additional prognostic value above that of influential clinical characteristics, including the age-at-onset and duration of auditory deprivation, suggesting that objectively assessing the physiological status of the brain using fNIRS imaging pre-operatively may support more accurate prediction of individual CI outcomes. Whilst activation of auditory brain regions by visual speech prior to implantation was related to the CI user’s clinical history of deafness, activation to visual speech did not relate to the future ability of these brain regions to respond to auditory speech stimulation with a CI. Greater pre-operative activation of left superior temporal cortex by visual speech was associated with enhanced speechreading abilities, suggesting that visual speech processing may help to maintain left temporal lobe specialisation for language processing during periods of profound deafness.
Highlights
A cochlear implant (CI) can partially restore hearing to profoundly deaf individuals
Both pre-operative brain imaging and postoperative CI outcome data were available for 15 CI users: one participant displayed excessive motion and poor contact between functional near-infrared spectroscopy (fNIRS) optodes and the scalp resulting in poor data quality
Bivariate linear regression analysis revealed that bilateral superior temporal cortex (STC) activation to visual speech before implantation was significantly predictive of future CI outcome, F(1,13) = 16.59, p = .001 (Table 2, model A)
Summary
A cochlear implant (CI) can partially restore hearing to profoundly deaf individuals. Estimates of CI outcome in adults are based on pre-operative factors that include duration of deafness (Blamey et al 2013; Holden et al 2013; Summerfield and Marshall 1995), age-at-onset of deafness (Blamey et al 2013; Teoh et al 2004), residual hearing (Gantz et al 1993; Lazard et al 2012a) and hearing-aid use (Lazard et al 2012a), amongst others. Currently, there is no accurate predictor of how an individual will fare with a CI, and identification of an accurate prognostic marker is crucial to help clinicians better predict clinical outcomes
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