13Bimodal and Bilateral Fitting

FWhy two ears are the goal

Binaural input restores interaural level and timing differences that drive sound localisation and the ability to separate a talker from noise, advantages a single device cannot recreate. Bimodal listening preserves low-frequency acoustic fine structure in the aided ear, which carries voice pitch, melody and prosody that current electric coding conveys poorly. For children with bilateral profound loss, two implants generally yield better speech-in-noise scores, localisation and spoken-language outcomes than one implant or bimodal use. The unifying aim of every binaural fitting is integration: the brain should fuse the two signals into one percept rather than attend to whichever ear happens to be louder.[2020][2009]

CBalancing a CI with a contralateral hearing aid

The contralateral hearing aid is prescribed to a verified target so that low-level inputs near 55 dB SPL remain audible while conversational (65 dB SPL) and loud inputs stay comfortable. Bimodal-specific fittings try to align loudness growth across ears using matched compression, with a kneepoint near 63 dB SPL and a high compression ratio of about 12:1. In sloping losses the aid emphasises low-frequency gain and attenuates the high frequencies, where electric stimulation and likely cochlear dead regions make acoustic amplification unhelpful. Matched slow-acting automatic gain control time constants between aid and processor prevent one ear's level from leaping ahead of the other when the overall sound level changes. Subjective loudness balancing across the two ears, adjusting hearing-aid gain until a running-speech signal sounds centred and equally loud, is the practical end point.[2020][2018]

CBalancing two implants

With bilateral implants the fitter creates two independently optimised maps and then balances upper stimulation levels across devices so a centred sound is heard in the midline rather than pulled to one side. Where the two ears tolerate it, similar coding strategy, stimulation rate and frequency allocation are used so the brain receives comparable spectral and temporal cues from each side. Simultaneous implantation gives both ears matched experience from day one; sequential implantation risks the second ear underperforming early and being abandoned if counselling does not set realistic expectations. A long interval and a long duration of deafness in the second ear predict slower progress, so the second implant is activated and balanced with patience over months, not judged at switch-on.[2020][2009][2014]

TSingle-sided deafness and binaural integration

Single-sided deafness means severe-to-profound loss in one ear with normal or near-normal hearing in the other; it degrades speech-in-noise, localisation and often brings tinnitus in the deaf ear. Unlike CROS aids or bone-conduction devices that merely reroute sound to the good ear, an implant is the only option that delivers true binaural input by reactivating the deaf side. The implant is fitted to match the loudness and timing of the acoustically normal ear, and many recipients report substantial reduction or elimination of tinnitus after activation. Benefit is most robust when the signal of interest arrives from the implanted side, overcoming the head-shadow that previously cost roughly 6 to 7 dB of signal level. Integration is trained, not automatic: structured listening practice helps the cortex weight and fuse a brand-new electric ear with a lifelong acoustic one.[2020][2017]