The use of functional near-infrared spectroscopy for measuring cortical reorganisation in cochlear implant users: a possible predictor of variable speech outcomes?

Continued developments in cochlear implantation have enabled a majority of patients to benefit substantially from their cochlear implant (CI) and to achieve a good level of speech understanding. However, some people receive less benefit from their implant than others, and large variability still exists in how well individuals can understand speech through their CI (Lazard et al., 2012). While some influential factors have been identified, including age at onset of hearing loss, the duration of deafness, and duration of CI experience, currently there is no accurate predictor of how well an individual will perform with a CI (Lazard et al., 2012). However, a better understanding of the mechanisms underlying the variability in CI outcome is of clinical importance. This information may inform clinicians in counselling patients prior to implantation about their likely experiences with a CI and to help shape the rehabilitation that they receive post-implantation. It could also help to identify those individuals who are most likely to benefit from a CI, helping to ensure that limited healthcare resources are directed effectively. Emerging evidence suggests that ‘cross-modal’ reorganization of auditory brain regions could be an important factor in understanding and predicting how much benefit an individual will receive from their CI. Following deafness, cortical areas that would usually process auditory information can reorganize and become more sensitive to the intact senses, such as vision (see Fig. 1). The extent of this visual takeover of auditory brain regions may affect the ability of a CI recipient to process auditory information from their implant effectively. For example, Sandmann et al. (2012) demonstrated an inverse relationship between the response of right auditory cortex to a visual chequerboard stimulus and auditory speech perception scores. That is, a high level of visual takeover of auditory brain regions may be predictive of a poor CI outcome. As well as these changing responses to non-linguistic, ‘low-level’ visual stimuli, it is important to understand how auditory deprivation and subsequent implantation impact on the processing of ‘high-level’ stimuli like speech. It is widely accepted that everyday speech perception is multimodal in nature: auditory and visual speech cues are integrated to form a unified percept. Cross-modal interactions in speech processing are observed in healthy individuals both behaviourally and at the cortical level. For instance, research has revealed responses to visual speech information (in silence) in the auditory cortex of normal hearing individuals (Calvert et al., 1997). In a similar population, responses to auditory speech information have been found in the visual cortex (Giraud and Truy, 2002). While cross-modal interactions in speech perception are therefore the norm, it is thought that this inherent synergy between auditory and visual speech might be altered in deaf individuals and in CI recipients, in a way that may benefit perception. It has been proposed that individuals with a CI rely on a heightened synergy between audition and vision. For example, Giraud et al. (2001) found that CI users Correspondence to: Carly A Lawler, NIHR Nottingham Hearing Biomedical Research Unit, Ropewalk House, 113 The Ropewalk, Nottingham NG1 5DU. Email: [email protected]