Neuromagnetic approaches to measuring auditory brain function in cochlear implant recipients: markers of speech processing and evaluation of a prototype MEG system

There is substantial variability in the speech perception performance achieved by different cochlear implant (CI) recipients, for reasons that are poorly understood but that may be attributable to changes in the way that the brain functions after auditory deprivation. Objective measures of speech processing in the brains of CI recipients would be valuable for identifying the central sources of performance variability and provide potential targets for intervention. Within an adult population, the specific objectives of the present thesis were to: (1) evaluate the effects of intelligibility on a reported novel MEG brain response that tracks hierarchical linguistic structures in connected speech; (2) evaluate the effects of prior perceptual experience on the MEG response, which reflects increased intelligibility of the degraded speech signal; (3) evaluate the capabilities of a novel, prototype MEG system designed to measure auditory brain function in cochlear implant recipients. Noise vocoding was employed to degrade the speech signal to a level that approximates the signal produced by a cochlear implant. Results showed that responses to sentence- and phrase-level structures were systematically reduced with reduced intelligibility of the speech signal; and that cortical sources coherent with intelligible phrase and sentence-level structures were left lateralized. Responses to sentence-level structure were slightly but significantly enhanced by prior experience; and this enhancement was associated with greater coherence in the right cerebral hemisphere (STG). Our evaluation of the prototype MEG system showed that CI artefacts from an emulator device were strongly attenuated. However, cochlear implant recipients still showed substantial levels of noise that preclude reliable measurements of auditory evoked brain activity. We conclude that further improvements will be required before the prototype MEG system is capable of routine measurements with CI recipients. Dissociated from artefacts at stimulus presentation frequency, this brain response to spoken language may serve as a sensitive markers of speech processing in CI recipients.