Cochlear implantation restores cortical processing for spatial hearing in asymmetric hearing loss

In asymmetric hearing loss (AHL), the normal pattern of contralateral hemispheric dominance for monaural stimulation is modified, with a shift towards the hemisphere ipsilateral to the better ear. The extent of this shift has been shown to relate to sound localisation deficits. In this study, we examined whether cochlear implantation to treat AHL can restore the normal functional pattern of auditory cortical activity and whether this relates to improved sound localisation. We recruited 10 subjects with a cochlear implant for AHL (AHL-CI) and 10 normally-hearing controls. The participants performed a voice/non-voice discrimination task with binaural and monaural presentation of the sounds, and the cortical activity was measured using positron emission tomography (PET) brain imaging with a H215O tracer. The auditory cortical activity was found to be lower in the AHL-CI participants for all of the conditions. A cortical asymmetry index was calculated and showed that a normal contralateral dominance was restored in the AHL-CI patients for the non-implanted ear, but not for the ear with the cochlear implant. It was found that the contralateral dominance for the non-implanted ear strongly correlated with sound localisation performance (rho = 0.8, p < 0.05). We conclude that the restoration of binaural mechanisms in AHL-CI subjects reverses the abnormal lateralisation pattern induced by the deafness, and that this leads to improved spatial hearing. Our results suggest that cochlear implantation fosters the rehabilitation of binaural excitatory/inhibitory cortical interactions, which could enable the reconstruction of the auditory spatial selectivity needed for sound localisation.