Synchronized Automatic Gain Control in Bilateral Cochlear Implant Recipients Yields Significant Benefit in Static and Dynamic Listening Conditions

Abstract

Individuals with bilateral cochlear implants (BiCIs) rely mostly on interaural level difference (ILD) cues to localize stationary sounds in the horizontal plane. Independent automatic gain control (AGC) in each device can distort this cue, resulting in poorer localization of stationary sound sources. However, little is known about how BiCI listeners perceive sound in motion. In this study, 12 BiCI listeners' spatial hearing abilities were assessed for both static and dynamic listening conditions when the sound processors were synchronized by applying the same compression gain to both devices as a means to better preserve the original ILD cues. Stimuli consisted of band-pass filtered (100-8000 Hz) Gaussian noise presented at various locations or panned over an array of loudspeakers. In the static listening condition, the distance between two sequentially presented stimuli was adaptively varied to arrive at the minimum audible angle, the smallest spatial separation at which the listener can correctly determine whether the second sound was to the left or right of the first. In the dynamic listening condition, participants identified if a single stimulus moved to the left or to the right. Velocity was held constant and the distance the stimulus traveled was adjusted using an adaptive procedure to determine the minimum audible movement angle. Median minimum audible angle decreased from 17.1° to 15.3° with the AGC synchronized. Median minimum audible movement angle decreased from 100° to 25.5°. These findings were statistically significant and support the hypothesis that synchronizing the AGC better preserves ILD cues and results in improved spatial hearing abilities. However, restoration of the ILD cue alone was not enough to bridge the large performance gap between BiCI listeners and normal-hearing listeners on these static and dynamic spatial hearing measures.

Keywords: automatic gain control; binaural hearing; interaural level differences; signal processing; spatial hearing; spatial resolution.

Figure 1.

The AGC processing of the sound processors. Flow chart of the AGC circuits used in this study (A) and magnitude response of the pre-emphasis filter applied to the audio inputs (B). The contralateral (in gray) audio input and pre-emphasis filter are exclusive to the experimental (synchronized) AGC. AGC = automatic gain control.

Figure 2.

Simulated Effects of Clinical Program and Experimental Program on Steady-State ILD as Function of Sound Source Azimuth for Different Frequency Channels for a 70 dBA White Noise Sound. ILDs (R – L, in dB) are plotted for low-frequency channels (Channels 1–5, blue lines), mid-frequency channels (Channels 6–10, red lines), and high-frequency channels (Channels 11–15, yellow lines). Panel A displays ILDs as a function of sound source azimuths for the natural cue due to the head-shadow (no effect of AGC), panel B for the clinical program (independent AGC), and panel C for the experiment program (synchronized AGC). Independent AGC (B) results in reduced ILD cues for high-frequency channels and inverted ILD cues due for low-frequency channels. Synchronized AGC (C) results in identical ILD as natural cue (A). AGC = automatic gain control; ILD = interaural level difference.

Figure 3.

Broadband Levels and ILDs of the Stimulus at the Left and Right Sound Processors as Function of Time for an Example MAA Trial. The first stimulus is presented at 0° and the second stimulus at +53° (rightward moving). The left column represents the actual physical cues for the tasks recorded from the clinical processors with the AB Listening Check device (before the AGC in the signal path). Middle and right columns show the simulated output with the clinical program (independent AGC) and the experiment program (synchronized AGC), respectively. The top and middle rows show the sound levels (dBA) of the left and right device, respectively. ILD (R-L) is shown in the bottom row. Vertical dashed lines represent stimulus onset and offset. ILD = interaural level difference; MAA = minimum audible angle.

Figure 4.

Results from the MAA task. Individual and median (gray filled circles) MAA thresholds with independent and synchronized AGC (panel A). Subjective difficulty of the MAA task for the independent and synchronized AGC programs are shown in panel B. A higher rating indicates the task being perceived as more difficult. Subjective report was not collected from participant 1. MAA = minimum audible angle.

Figure 5.

Broadband Levels and ILDs of the Stimulus at the Left and Right Sound Processors as a Function of Time for an Example MAMA Trial. The stimulus starts from 0° and sweeps +53° (rightward moving). The left column represents the actual physical cues for the task recorded from the clinical processors with the AB Listening Check device (before the AGC in the signal path). Middle and right columns show simulated output with the clinical program (independent AGC) and the experiment program (synchronized AGC), respectively. The top and middle rows show the sound levels (dBA) of the left and right device, respectively. ILD (R-L) is shown in the bottom row. Vertical dashed lines represent stimulus onset and offset. ILD = interaural level difference; MAMA = minimum audible movement angle.

Figure 6.

Results from the MAMA task. Individual and median (gray filled circles) MAMA thresholds for independent and synchronized AGC (panel A). Subjective difficulty of the MAMA task for the independent and synchronized AGC programs are shown in panel B. A higher rating indicates the task being perceived as more difficult. Note: Subjective report was not collected from participant 1. MAMA = minimum audible movement angle.