4.2 Sound localisation
The results of this study showed evident declines in directional hearing in both children with UMA and stimulated UHL; however, the group of UMA children performed better than those with stimulated UHL. Consistent with previous reports, good performers among children with congenital UCHL may have learned to localize sound sources using monaural cues and residual binaural difference cues after a long period of adaption. Vogt et al. 11 confirmed that patients with congenital UCHL rely on monaural spectral cues to detect high-frequency sound sources by comparing localisation accuracy with and without moulding the normal hearing ear pinna. Van et al.10 evaluated nine listeners with chronic unilateral hearing loss through a group of broadband sound stimuli fixed at 60 dB; the results indicated a strong reliance on the ambiguous HSE in familiar acoustic environments.
Our study revealed no significant improvement in sound localisation accuracy between the BCD-unaided and aided conditions. Similar results have also been obtained in previous studies 8, 9regarding the application of bone-anchored hearing aids and Bonebridge (Med-EL, Innsbruck, Austria) in congenital UCHL. The inability to achieve binaural hearing may be a consequence of two factors. First, hearing symmetry still exists, as the BCD is not able to provide the same hearing threshold as that of the normal hearing ear, and the insufficient intensity input also disrupts binaural hearing. Second, the processing time delay and inconsistent stimulation are inherent characteristics of the BCD signals; Additionally, the bone conduction signals with less reliable and constant cues may also prevent children with congenital UCHL from having a restored binaural hearing17. However, other studies showed improvement in sound localisation accuracy when BCDs were used in patients with congenital UCHL 6, 7, 18. Potential explanations for these conflicting observations may be the age gap of the enrolled patients and methodological differences in the procedure. Besides, it is favourable that the sound localisation abilities of the intact ear did not deteriorate owing to cross-hearing induced by the BCD, likely due to insufficient high-frequency sound transmission of the BCD19, which did not interfere with the use of spectral cues by the healthy ear. Furthermore, the original sound localisation performance was a good predictor of sound localisation accuracy under the BCD-aided conditions. Moreover, there is a high need for early hearing intervention in poor performers who cannot make good use of asymmetric binaural cues to localize sound sources.
CONCLUSIONS
Some children with UMA were able to compensate using the remaining distorted binaural cues to detect sound sources, unlike the children with acutely stimulated UHL; however, this compensating ability was still far worse than children with NH and varied across individuals. As the application of BCD provided a definite benefit on speech recognition abilities and high participant satisfaction, it is recommended that children, particularly those with poor sound localisation performance, should be fitted with non-surgical BCDs at an early age.