Bilateral implantation and binaural cues

Two ears let normal listeners localize sound and unmask speech in noise using interaural time differences (ITDs, the microsecond-scale delay between ears) and interaural level differences (ILDs). Bilateral cochlear implantation restores substantial ILD-based localization and gives head-shadow and some squelch benefits for speech in noise, and is now common practice, especially in children. However bilateral users largely lack access to the fine ITD cues that dominate low-frequency localization, because independent, free-running sound processors are not synchronized and envelope coding discards the low-frequency fine structure that carries ITD.

Restoring binaural hearing is an active research target: bilaterally synchronized processors that preserve interaural timing, explicit ITD-preserving coding on matched low-frequency electrode pairs, and interaural place matching (aligning the two arrays' frequency maps, since mismatched insertion depths degrade fusion). Bimodal hearing (a CI in one ear, an acoustic hearing aid in the other) is a common real-world configuration and raises its own fusion and loudness-balancing problems. Truly normal binaural hearing through two implants has not been achieved.