Binaural Hearing Using the ADHEAR Bone Conduction System in the Monaurally Occluded Ear

The study aimed to investigate binaural cues in the rehabilitation of unilateral occluded ears with a bone conduction hearing aid.The study sample consisted of 40 adult volunteers with normal hearing. Unilateral pseudo-conductive hearing loss was induced by inserting an earplug into the external auditory canal (EAC) and silicone material in the concha for ear impression. The adaptive speech-in-noise test (Italian Matrix test) was performed in three spatial orientations to assess binaural cues (summation, squelch, and head shadow effects). All evaluations were performed in the normal condition, after EAC occlusion, and after application of an adhesive bone conduction hearing aid. Binaural contrast differences were calculated in the three conditions.In the EAC occlusion condition, there was a significant increase in the signal-to-noise ratio (SNR) in both the S0N0 (2.4 dB) and the S90N-90 (7.7 dB) settings, and a slight albeit significant increase in the S0N90 setting (1.35 dB). After fitting the BC hearing aid, there was a reduction of -1.8 dB SNR (p < 0.001) in the S0N0 setting and -2 dB (p = 0.003) in the S90N-90 setting. There was no improvement in the SNR (p = 0.405) in evaluation of the squelch effect (S0N90). These data were corroborated by a better binaural contrast due to a reduction in the summation effect in the monaural occlusion condition and a subsequent reduction in binaural contrast after fitting the hearing aid due to an increase in the summation effect (-2.5 dB vs. 0.3 dB; p < 0.001).Application of a bone conduction hearing aid in unilateral pseudo-conductive hearing loss strengthens speech recognition of noise by improving the summation effect and impeding the shadow effect of the head; however, there appears to be no improvement in speech perception in noise due to spatial release from masking.