Cortical cross-modal plasticity: a predictor of variable speech outcomes in cochlear implant users? Seminar

It is well known that speech outcomes vary widely amongst cochlear implant (CI) users. Emerging evidence suggests that cortical plasticity could be an important variable in understanding and predicting how much benefit an individual will receive from their CI. Specifically, cortical areas that would usually process auditory information can become more sensitive to the intact senses, such as vision, following deafness. Indeed it has been shown that individuals with a CI rely on a heightened synergy between audition and vision. These findings emphasize the importance of understanding how the brain responds to auditory and visual stimulation before and after cochlear implantation. Unfortunately, the measurement of cortical responses in CI recipients using traditional imaging techniques has been challenging. Many established methods for non-invasive brain imaging in humans can be plagued by electrical and magnetic artifacts generated by CI stimulation.

Functional near-infrared spectroscopy (fNIRS) is a non-invasive imaging technique which, owing to its optical nature, is fully compatible with a CI. Furthermore, it is essentially silent, which is advantageous for auditory research. Together, these advantages indicate that fNIRS may provide a powerful tool to explore cortical plasticity following deafness and subsequent cochlear implantation.

At the NIHR Nottingham Hearing Biomedical Research Unit, we are using fNIRS to examine cortical plasticity associated with deafness and cochlear implantation from multiple perspectives. One strand focuses on the development of fNIRS as a tool to measure ‘low-level’ cross-modal plasticity, specifically how auditory brain regions can become more responsive to visual and touch stimulation in deaf people compared with hearing controls. Another strand uses fNIRS to examine how the brain responds to auditory and visual components of speech before and after an individual receives their CI. The aim of this longitudinal study is to understand how perceptual improvements in an individual’s ability to understand speech with their CI relate to changes in cortical responsiveness. We are also using fNIRS to examine the mechanisms through which the brains of normal hearing listeners combine information across the senses, and to understand the potential impact of auditory deprivation and cochlear implantation on these mechanisms. By developing fNIRS as a tool to study how the brain responds to multisensory stimulation before and after cochlear implantation, we aim to provide insights into the reasons for variable CI outcomes, and to develop a clinically useful prognostic tool. We also hope that this technique may provide a useful adjunct to help program speech processors, particularly for recipients who are difficult to assess using behavioral testing, such as children with complex needs.