Rationale and Implementation of a Bimodal Fitting Formula

Introduction
The effectiveness of cochlear implants has increased markedly over the past years. As a consequence, candidacy for implantation has been relaxed and more CI users have functionally useful contralateral hearing. Such bimodal users often report improved speech understanding, particularly in noisy situations, better tonal quality and improved lateralization of sound sources. The individual bimodal benefit, however, varies considerably. A potential reason for this is that conventional hearing aid fitting does not account for certain specific characteristics of bimodal listening. Of particular importance are: low-frequency audibility, spectral overlap of electric and acoustic stimulation, loudness balance, inter-aural tonotopy and (dynamic) synchronization of adaptive signal processing. While many of these requisites are highly individual and thus, may have to be addressed by individual fine-tuning, others might be accounted for, at least partially, by a prescriptive fitting formula specifically designed for bimodal listeners.

Method
In order to improve the efficiency of bimodal fitting, a bimodal fitting formula is being developed that aims at accounting for the specific characteristics of bimodal listening.

This formula can use any established fitting formula (e.g. DSL v5, NAL-RP) as a starting point. In the current implementation, Phonak’s proprietary formula (Adaptive Phonak Digital) is used. The importance of low-frequency audibility is addressed by maximizing effective audibility [1] based on the individual audiogram. Edge frequencies of Dead Regions can be either entered manually or are estimated automatically from the audiogram. As a second step, the static and dynamic behavior of the CI’s AGC has been approximated with the hearing aid’s compression system to account for loudness balance and dynamic synchronization between electric and acoustic hearing. This is accomplished by a special parameterization of channel-coupling, time constants, compression knee-points and compression ratios in the hearing aid.

Results
Compared to conventional hearing fitting aid formulae, the novel bimodal fitting formula typically results in decreased gain at high frequencies and increased gain at low frequencies. Technical measurements using percentile analysis show that the dynamic behavior of the CI’s AGC can be approximated with the HA’s AGC nicely for speech signals. First results from studies using the bimodal fitting formula indicate that this formula can improve speech understanding in noise and provides a good starting point for individual fine-tuning.