Cortical Processing of Level Cues for Spatial Hearing is Impaired in Children with Prelingual Deafness Despite Early Bilateral Access to Sound

Vijayalakshmi Easwar, Hiroshi Yamazaki, Michael Deighton, Blake Papsin, Karen Gordon

Bilateral cochlear implantation aims to restore binaural hearing, important for spatial hearing, to children who are deaf. Improvements over unilateral implant use are attributed largely to the detection of interaural level differences (ILDs) but emerging evidence of impaired sound localization and binaural fusion suggest that these binaural cues are abnormally coded by the auditory system. We used multichannel electroencephalography (EEG) to assess cortical responses to ILDs in two groups: 13 children who received early bilateral cochlear implants (CIs) simultaneously, known to protect the developing auditory cortices from unilaterally driven reorganization, and 15 age matched peers with normal hearing. EEG source analyses indicated a dominance of right auditory cortex in both groups. Expected reductions in activity to ipsilaterally weighted ILDs were evident in the right hemisphere of children with normal hearing. By contrast, cortical activity in children with CIs showed: (1) limited ILD sensitivity in either cortical hemisphere, (2) limited correlation with reliable behavioral right-left lateralization of ILDs (in 10/12 CI users), and (3) deficits in parieto-occipital areas and the cerebellum. Thus, expected cortical ILD coding develops with normal hearing but is affected by developmental deafness despite early and simultaneous bilateral implantation. Findings suggest that impoverished fidelity of ILDs in independently functioning CIs may be impeding development of cortical ILD sensitivity in children who are deaf but do not altogether limit benefits of listening with bilateral CIs. Future efforts to provide consistent/accurate ILDs through auditory prostheses including CIs could improve binaural hearing for children with hearing loss.