Bilateral Auditory Development and Function in Children with Asymmetric Hearing Loss Who Listen with Electric and Acoustic Hearing

This thesis aimed to determine whether treating children who have asymmetric hearing loss with two different, but appropriate auditory prostheses promotes bilateral auditory development and bilateral hearing abilities. Unilateral deafness during development drives extensive reorganization in bilateral auditory pathways, limiting spatial hearing and putting children at risk for social and educational challenges. Providing electric cochlear implant hearing to the deaf ear and acoustic sound through a hearing aid or normal hearing in the other ear (bimodal hearing) restores bilateral access to sound. But it remains unclear how the two very different auditory signals can be processed by the auditory system to prevent abnormal neurophysiological changes from occurring. Hearing development in bimodal users was assessed by measuring evoked electrophysiological changes in the brainstem and cortex, as well as behavioural perception of important differences in sounds between the two ears. Even though electrical and acoustic hearing are unique and stimulate the auditory system in different ways, symmetric neural conduction in bilateral brainstem pathways and the expected representation of each ear in the auditory cortex were achieved in children who had sufficient residual hearing in their non-implanted ear. Moreover, most children experienced a bilateral advantage for speech perception over wearing either device alone, benefited from spatial hearing and could detect changes in binaural inter-aural level cues. These findings suggest that bilateral hearing does not need to be restricted to one modality and that the auditory system can integrate bimodal hearing to some extent. However, prolonged experience with asymmetric hearing during development extensively and persistently reorganizes the auditory system, limiting future efforts to restore bilateral and spatial hearing with bimodal devices. These findings are highly relevant to the clinical treatment of asymmetric hearing loss and support the recommendation to provide the most appropriate bilateral auditory prostheses as soon as possible to children who have significant hearing loss.Ph.D