Improved horizontal directional hearing in bone conduction device users with acquired unilateral conductive hearing loss

Abstract

We examined horizontal directional hearing in patients with acquired severe unilateral conductive hearing loss (UCHL). All patients (n = 12) had been fitted with a bone conduction device (BCD) to restore bilateral hearing. The patients were tested in the unaided (monaural) and aided (binaural) hearing condition. Five listeners without hearing loss were tested as a control group while listening with a monaural plug and earmuff, or with both ears (binaural). We randomly varied stimulus presentation levels to assess whether listeners relied on the acoustic head-shadow effect (HSE) for horizontal (azimuth) localization. Moreover, to prevent sound localization on the basis of monaural spectral shape cues from head and pinna, subjects were exposed to narrow band (1/3 octave) noises. We demonstrate that the BCD significantly improved sound localization in 8/12 of the UCHL patients. Interestingly, under monaural hearing (BCD off), we observed fairly good unaided azimuth localization performance in 4/12 of the patients. Our multiple regression analysis shows that all patients relied on the ambiguous HSE for localization. In contrast, acutely plugged control listeners did not employ the HSE. Our data confirm and further extend results of recent studies on the use of sound localization cues in chronic and acute monaural listening.

FIG. 1

Head movement localization responses to narrow band noises (NBN) in the azimuth and elevation directions. A Typical example of a head saccade to a long (1,000 ms) stimulus (500 Hz, 70 dB SPL at location Azi = −60°, Ele = 0°) from patient P2. Azimuth (black line) and elevation (gray line) are plotted as a function of time. The stimulus event (1,000 ms) is indicated by the bold black line. B Two-dimensional trajectory of the same head movement. C Example of a head saccade to a short (150 ms) stimulus (3 kHz, 55 dB SPL at location Azi = −33°, Ele = 0°) from patient P1. D Two-dimensional trajectory.

FIG. 2

Spatial trajectories of patient P5, listening with his intact left ear only (A) or with the BCD on (B), and of control listener C5, listening with the right ear plugged (C) or with both ears (D). Note that for both listeners monaural listening (A and C) shifts responses towards the hearing side, while BCD and binaural listening yields more symmetric response distributions in the horizontal plane (B and D). Stimuli (1,000 ms, four per location) were presented at seven different azimuth positions ranging from −90° (far left) to +90° (far right) at 30° intervals. X indicates the hearing-impaired (A) and plugged (C) sides.

FIG. 3

Sound localization responses for patients P1–P5. Responses are plotted for the 500-Hz and 3-kHz stimuli in the unaided (BCD-off) and aided (BCD-on) conditions. Note that two patients (P3 and P4) did not benefit from the BCD system when they had to localize 500-Hz stimuli, as they had already fairly good localization of 500-Hz stimuli in the unaided condition. Two other patients (P1 and P2) demonstrated a surprisingly good unaided localization of 3-kHz stimuli. r ² = coefficient of determination, b = response gain. X indicates the hearing-impaired side. Empty circle indicates the side of the BCD.

FIG. 4

Sound localization responses for a representative control listener (C2) with the left ear plugged. A, C Responses toward 500 Hz were less affected by the earplug than responses towards the 3-kHz noise, as indicated by the higher r ² value. All 3-kHz noises (C) were perceived at the hearing side (large positive bias +49° vs. +14° for 500 Hz). B, D Binaural hearing performance is good for both stimulus types (r ² > 0.9, b small). Data pooled across presentation levels. The dashed gray line denotes the linear regression fit. r ² coefficient of determination, g response gain, b bias, X indicates the plugged side.

FIG. 5

Comparison of the response gain (A) and r ² values (B) of the unaided condition (BCD off) against the aided condition (BCD on). Filled square indicates response gains and r ² for responses to long 500-Hz narrow band noise stimuli. Empty square indicates response gains and r ² for the response to long 3-kHz narrow band noise stimuli. Filled circle indicates response gains and r ² for responses to short 500-Hz narrow band noise stimuli. Empty circle indicates response gains and r ² for the response to short 3-kHz narrow band noise stimuli.

FIG. 6

Multiple linear regression analysis of monaural and binaural azimuth localization performance for 500-Hz (circles, left) and 3-kHz (squares, right) noises of patients P1–P5 and control listeners. The coefficients for proximal sound level (q in Eq. 2) and azimuth (p in Eq. 2) are plotted against one another for each patient (A and B) and control listener (C and D). For clarity, data points with a proximal sound level coefficient of zero are slightly shifted. Note that most patients have an azimuth coefficient close to one in the aided condition, except patient P4 who demonstrates a high sound level coefficient for 3-kHz stimuli.