12Making Two Different Ears Work Together: Bimodal Fitting
Two ears hearing by two different mechanisms will not automatically fuse into one percept. Bimodal fitting is the clinical craft of balancing an electric ear against an acoustic ear so that loudness, timing and frequency content line up well enough for the brain to combine them. Done badly, the worse ear can drag the better one down.
FWhy bimodal fitting is hard
The implant and the hearing aid are independent devices with different processing delays, different loudness growth, and different compression behavior, so they are not naturally matched. If the two ears differ too much in loudness or timing, the brain cannot fuse them into a single image and the benefit of binaural combination collapses. The goal of fitting is not to make the two ears identical, which is impossible, but to align them well enough that the listener perceives a single, balanced auditory scene. Each device must first be optimized on its own (a well-mapped implant and a well-fit hearing aid) before any cross-device balancing is attempted.[2016][2007]
TLoudness balancing and matching the hearing aid to the map
Loudness balancing aligns the loudness growth of the hearing-aid ear to that of the implant ear so that a sound presented to both is perceived as centered rather than pulling to one side. Frequency-dependent balancing, rather than a single broadband adjustment, better matches the two ears because their loudness mismatch differs across frequency bands. The hearing aid is fit to complement, not compete with, the implant map: it concentrates on amplifying the residual low frequencies the implant codes poorly, while the implant covers the highs. Aligning the hearing aid's compression and automatic gain control timing to the implant's slow-acting gain control reduces moment-to-moment loudness mismatches that otherwise disrupt fusion.[2016][2019][2004]
TCoordinated devices and dedicated formulas
Using a hearing aid and implant from the same manufacturer allows linked, coordinated devices that share settings, synchronize volume and program changes, and stream audio to both sides. Dedicated bimodal fitting formulas (for example coordinated manufacturer formulas) prescribe the hearing-aid gain and align loudness growth and AGC to the partner implant, rather than fitting the hearing aid as if the implant were not there. Such coordinated fitting has been shown to improve balance and the perceived sound quality of the combined percept, beyond intelligibility alone. Even without same-brand hardware, the principle holds: prescribe the hearing aid deliberately for its bimodal role rather than reusing an old unilateral fitting.[2019][2017][2016]
CVerifying benefit, troubleshooting, and knowing when to stop
Verify that the second ear actually helps by comparing performance with the implant alone versus the bimodal combination; the bimodal configuration should be at least as good as, and ideally better than, the implant alone. If the hearing-aid ear interferes (bimodal worse than implant alone), revisit loudness balance, frequency response and hearing-aid candidacy before abandoning the configuration; interference often reflects a fitting problem, not a hopeless ear. Persistent interference despite good fitting, or a steadily declining residual ear that no longer contributes useful low-frequency information, signals that the hearing aid has reached its limit. When the aided ear stops adding measurable or subjective benefit, the conversation shifts to a second cochlear implant for that ear to restore meaningful binaural input.[2011][2007][2017]
What is the most appropriate first step?
The primary goal of loudness balancing in bimodal fitting is to:
An advantage of using a coordinated hearing aid and implant from the same manufacturer is that:
If a well-fit, well-balanced bimodal configuration still performs worse than the implant alone and the residual ear has clearly declined, the appropriate next step is to: