Differing Bilateral Benefits for Spatial Release From Masking and Sound Localization Accuracy Using Bone Conduction Devices

Abstract

Objectives: Normal binaural hearing facilitates spatial hearing and therefore many everyday listening tasks, such as understanding speech against a backdrop of competing sounds originating from various locations, and localization of sounds. For stimulation with bone conduction hearing devices (BCD), used to alleviate conductive hearing losses, limited transcranial attenuation results in cross-stimulation so that both cochleae are stimulated from the position of the bone conduction transducer. As such, interaural time and level differences, hallmarks of binaural hearing, are unpredictable at the level of the inner ears. The aim of this study was to compare spatial hearing by unilateral and bilateral BCD stimulation in normal-hearing listeners with simulated bilateral conductive hearing loss.

Design: Bilateral conductive hearing loss was reversibly induced in 25 subjects (mean age = 28.5 years) with air conduction and bone conduction (BC) pure-tone averages across 0.5, 1, 2, and 4 kHz (PTA 4 ) <5 dB HL. The mean (SD) PTA 4 for the simulated conductive hearing loss was 48.2 dB (3.8 dB). Subjects participated in a speech-in-speech task and a horizontal sound localization task in a within-subject repeated measures design (unilateral and bilateral bone conduction stimulation) using Baha 5 clinical sound processors on a softband. For the speech-in-speech task, the main outcome measure was the threshold for 40% correct speech recognition when masking speech and target speech were both colocated (0°) and spatially and symmetrically separated (target 0°, maskers ±30° and ±150°). Spatial release from masking was quantified as the difference between colocated and separated masking and target speech thresholds. For the localization task, the main outcome measure was the overall variance in localization accuracy quantified as an error index (0.0 = perfect performance; 1.0 = random performance). Four stimuli providing various spatial cues were used in the sound localization task.

Results: The bilateral BCD benefit for recognition thresholds of speech in competing speech was statistically significant but small regardless if the masking speech signals were colocated with, or spatially and symmetrically separated from, the target speech. Spatial release from masking was identical for unilateral and bilateral conditions, and significantly different from zero. A distinct bilateral BCD sound localization benefit existed but varied in magnitude across stimuli. The smallest benefit occurred for a low-frequency stimulus (octave-filtered noise, CF = 0.5 kHz), and the largest benefit occurred for unmodulated broadband and narrowband (octave-filtered noise, CF = 4.0 kHz) stimuli. Sound localization by unilateral BCD was poor across stimuli.

Conclusions: Results suggest that the well-known transcranial transmission of BC sound affects bilateral BCD benefits for spatial processing of sound in differing ways. Results further suggest that patients with bilateral conductive hearing loss and BC thresholds within the normal range may benefit from a bilateral fitting of BCD, particularly for horizontal localization of sounds.

Fig. 1.

Schematic illustration of setup for speech recognition in competing speech. Filled square denotes active BCD. A, Target speech and masking speech colocated at 0° azimuth, with unilateral BCD. B, Target speech and masking speech colocated at 0° azimuth, with bilateral BCD. C, Target speech at 0° azimuth and masking speech at ±30° and ±150°, with unilateral BCD. D, Target speech at 0° azimuth and masking speech at ±30° and ±150°, with bilateral BCD. BCD indicates bone conduction hearing devices.

Fig. 2.

Schematic illustration of setup for sound localization. Twelve loudspeaker/display pairs arranged equidistantly in an 110° arc in the frontal horizontal plane. A display is placed above each loudspeaker (not visualized). Target signal switching randomly to different loudspeakers. Filled square denotes active BCD. Left panel: Unilateral BCD. Right panel: Bilateral BCD. BCD indicates bone conduction hearing devices.

Fig. 3.

Mean (black) and individual (gray) thresholds for detecting frequency-modulated tones in sound field (n = 25). Solid, dashed, and dotted lines denote bilateral BC stimulation, unilateral BC stimulation, and unaided simulated conductive hearing loss conditions, respectively. BC indicates bone conduction.

Fig. 4.

Recognition thresholds for speech in competing speech. Symbols denote means, and error bars denote 95% confidence intervals. Right panels: Symbols denote individual data for unilateral SRT as a function of bilateral SRT in colocated and separated target and masking speech condition. Symbols above the line of equality reflects subjects with a benefit in SRT with bilateral BCD. Filled symbols in (B) and (C) corresponds to subjects who had a bilateral disbenefit for both spatial conditions. BCD indicates bone conduction hearing devices; SRT, speech recognition threshold.

Fig. 5.

Simple linear regression analysis between the bilateral BCD benefit in SRT and the bilateral BCD benefit in hearing thresholds for bone conduction stimulation (y = 0.28 + 0.30 × x, r = 0.41, p = 0.04). BCD indicates bone conduction hearing devices; SRT, speech recognition threshold.

Fig. 6.

Spatial release from masking for unilateral and bilateral BC conditions. Left panel: Mean SRM for unilateral (gray) and bilateral (black) BC conditions. Error bars denote 95% confidence intervals. Right panel: individual SRM values for bilateral BC as a function of unilateral BC. Symbols above the line of equality reflect subjects with a bilateral SRM benefit, whereas symbols below the line of equality reflect subjects with a bilateral SRM disbenefit. BC indicates bone conduction; SRM, spatial release from masking.

Fig. 7.

Horizontal sound localization accuracy with unilateral (gray) and bilateral (black) bone conduction stimulation. A, Symbols denote means, and error bars denote 95% confidence intervals. The gray shaded area reflects random performance. B–E, Symbols denote individual sound localization accuracy data. Symbols above the line of equality reflect subjects with a bilateral sound localization benefit, whereas symbols below the line of equality reflect subjects with a bilateral sound localization disbenefit. CF indicates center frequencies.

Fig. 8.

Scatterplot of horizontal sound localization accuracy for unilateral (left column) and bilateral (right column) BC stimulation across five different stimuli (one stimulus per row). The size of the circles reflects the proportion of perceived azimuths across presented azimuths. Each panel illustrates data for 600 presentations of the corresponding stimulus (25 subjects, 24 presentations). For the BC-only condition (top row), the same musical melody was used as stimulus as shown in row 2, but presented at an individual level (mean = 43.9 dB SPL, n = 25) inaudible by air conduction.