Objective Binaural Loudness Balancing Based on 40-Hz Auditory Steady-State Responses. Part II: Asymmetric and Bimodal Hearing.

In Part I, we investigated 40-Hz auditory steady-state response (ASSR) amplitudes for the use of objective loudness balancing across the ears for normal-hearing participants and found median across-ear ratios in ASSR amplitudes close to 1. In this part, we further investigated whether the ASSR can be used to estimate binaural loudness balance for listeners with asymmetric hearing, for whom binaural loudness balancing is of particular interest. We tested participants with asymmetric hearing and participants with bimodal hearing, who hear with electrical stimulation through a cochlear implant (CI) in one ear and with acoustical stimulation in the other ear. Behavioral loudness balancing was performed at different percentages of the dynamic range. Acoustical carrier frequencies were 500, 1000, or 2000 Hz, and CI channels were stimulated in apical or middle regions in the cochlea. For both groups, the ASSR amplitudes at balanced loudness levels were similar for the two ears, with median ratios between left and right ear stimulation close to 1. However, individual variability was observed. For participants with asymmetric hearing loss, the difference between the behavioral balanced levels and the ASSR-predicted balanced levels was smaller than 10 dB in 50% and 56% of cases, for 500 Hz and 2000 Hz, respectively. For bimodal listeners, these percentages were 89% and 60%. Apical CI channels yielded significantly better results (median difference near 0 dB) than middle CI channels, which had a median difference of -7.25 dB.

Comparison between adaptive and adjustment procedures for binaural loudness balancing.

Binaural loudness balancing is performed in research and clinical practice when fitting bilateral hearing devices, and is particularly important for bimodal listeners, who have a bilateral combination of a hearing aid and a cochlear implant. In this study, two psychophysical binaural loudness balancing procedures were compared. Two experiments were carried out. In the first experiment, the effect of procedure (adaptive or adjustment) on the balanced loudness levels was investigated using noise band stimuli, of which some had a frequency shift to simulate bimodal hearing. In the second experiment, the adjustment procedure was extended. The effect of the starting level of the adjustment procedure was investigated and the two procedures were again compared for different reference levels and carrier frequencies. Fourteen normal hearing volunteers participated in the first experiment, and 38 in the second experiment. Although the final averaged loudness balanced levels of both procedures were similar, the adjustment procedure yielded smaller standard deviations across four test sessions. The results of experiment 2 demonstrated that in order to avoid bias, the adjustment procedure should be conducted twice, once starting from below and once from above the expected balanced loudness level.

Real-time loudness normalisation with combined cochlear implant and hearing aid stimulation.

BACKGROUND: People who use a cochlear implant together with a contralateral hearing aid-so-called bimodal listeners-have poor localisation abilities and sounds are often not balanced in loudness across ears. In order to address the latter, a loudness balancing algorithm was created, which equalises the loudness growth functions for the two ears. The algorithm uses loudness models in order to continuously adjust the two signals to loudness targets. Previous tests demonstrated improved binaural balance, improved localisation, and better speech intelligibility in quiet for soft phonemes. In those studies, however, all stimuli were preprocessed so spontaneous head movements and individual head-related transfer functions were not taken into account. Furthermore, the hearing aid processing was linear. STUDY DESIGN: In the present study, we simplified the acoustical loudness model and implemented the algorithm in a real-time system. We tested bimodal listeners on speech perception and on sound localisation, both in normal loudness growth configuration and in a configuration with a modified loudness growth function. We also used linear and compressive hearing aids. RESULTS: The comparison between the original acoustical loudness model and the new simplified model showed loudness differences below 3% for almost all tested speech-like stimuli and levels. We found no effect of balancing the loudness growth across ears for speech perception ability in quiet and in noise. We found some small improvements in localisation performance. Further investigation with a larger sample size is required.

Optimal gain control step sizes for bimodal stimulation.

OBJECTIVE: Cochlear implants (CI) and hearing aids (HA) have a gain control that allows the bimodal user to change the loudness. Due to differences in dynamic range between CI and HA, an equal change of the gains of the two devices results in different changes in loudness. The objective was to relate and individualise the step sizes of the loudness controls to obtain a similar perceptual effect in the two ears. DESIGN: We used loudness models parametrised for individual users to find a relation between the controls of the CI and the HA such that each step resulted in an equal change in loudness. We conducted loudness balancing experiments to validate the results. STUDY SAMPLE: Eleven bimodal users of whom six were tested in a prior study. RESULTS: The difference between the optimal gain from the loudness balancing procedure and actual gain was 3.3 dB when the new relation was applied. In contrast, the difference was 8 dB if equal step sized were applied at both sides. CONCLUSION: We can relate the controls such that each step results in a similar loudness difference.