Delayed access to bilateral input alters cortical organization in children with asymmetric hearing

Bilateral hearing in early development protects auditory cortices from reorganizing to prefer the better ear. Yet,
such protection could be disrupted by mismatched bilateral input in children with asymmetric hearing who
require electric stimulation of the auditory nerve from a cochlear implant in their deaf ear and amplified acoustic
sound from a hearing aid in their better ear (bimodal hearing). Cortical responses to bimodal stimulation were
measured by electroencephalography in 34 bimodal users and 16 age-matched peers with normal hearing, and
compared with the same measures previously reported for 28 age-matched bilateral implant users. Both auditory
cortices increasingly favoured the better ear with delay to implanting the deaf ear; the time course mirrored that
occurring with delay to bilateral implantation in unilateral implant users. Preference for the implanted ear
tended to occur with ongoing implant use when hearing was poor in the non-implanted ear. Speech perception
deteriorated with longer deprivation and poorer access to high-frequencies. Thus, cortical preference develops in
children with asymmetric hearing but can be avoided by early provision of balanced bimodal stimulation.
Although electric and acoustic stimulation differ, these inputs can work sympathetically when used bilaterally
given sufficient hearing in the non-implanted ear.