Head Shadow, Summation, and Squelch in Bilateral Cochlear-Implant Users With Linked Automatic Gain Controls

Abstract

Speech understanding in noise is poorer in bilateral cochlear-implant (BICI) users compared to normal-hearing counterparts. Independent automatic gain controls (AGCs) may contribute to this because adjusting processor gain independently can reduce interaural level differences that BICI listeners rely on for bilateral benefits. Bilaterally linked AGCs may improve bilateral benefits by increasing the magnitude of interaural level differences. The effects of linked AGCs on bilateral benefits (summation, head shadow, and squelch) were measured in nine BICI users. Speech understanding for a target talker at 0° masked by a single talker at 0°, 90°, or -90° azimuth was assessed under headphones with sentences at five target-to-masker ratios. Research processors were used to manipulate AGC type (independent or linked) and test ear (left, right, or both). Sentence recall was measured in quiet to quantify individual interaural asymmetry in functional performance. The results showed that AGC type did not significantly change performance or bilateral benefits. Interaural functional asymmetries, however, interacted with ear such that greater summation and squelch benefit occurred when there was larger functional asymmetry, and interacted with interferer location such that smaller head shadow benefit occurred when there was larger functional asymmetry. The larger benefits for those with larger asymmetry were driven by improvements from adding a better-performing ear, rather than a true binaural-hearing benefit. In summary, linked AGCs did not significantly change bilateral benefits in cases of speech-on-speech masking with a single-talker masker, but there was also no strong detriment across a range of target-to-masker ratios, within a small and diverse BICI listener population.

Keywords: bilateral cochlear implants; binaural technology; spatial release from masking.

Figure 1.

Individual and Average Speech Understanding in Quiet for Listeners Grouped by Degree of Asymmetry. An interaural difference of ≥ 20% characterized a listener as asymmetrical. Five listeners were characterized as symmetrical and four were characterized as asymmetrical. Group averages based on this designation were also calculated. Error bars represent ± 1 standard deviation. BE = functionally better ear; PE = functionally poorer ear.

Figure 2.

Average Percent Correct on CRM Sentences in Noise. PE and BE were determined by listeners’ IEEE scores in quiet. The target was in the front for all test conditions. Error bars represent ± 1 standard deviation. AGC = automatic gain control; BE = functionally better ear; PC = percent correct; PE = functionally poorer ear; TMR = target-to-masker ratio.

Figure 3.

Average Percent Correct Values for Listeners Separated into Symmetric and Asymmetric Groups Based on IEEE Scores. Conventions are the same as in Figure 2. AGC = automatic gain control; BE = functionally better ear; PC = percent correct; PE = functionally poorer ear; TMR = target-to-masker ratio.

Figure 4.

Average Value of Each Bilateral Benefit (Summation, Head Shadow, and Squelch) for Linked and Independent AGC Conditions. These benefits are shown for symmetrical listeners (≤20% difference in monaural IEEE scores), asymmetrical listeners (≥20% difference in monaural IEEE scores), and all listeners together. Filled bars represent benefits with linked AGCs and open bars represent benefits with independent AGCs. Error bars represent ± 1 standard deviation. AGC = automatic gain control.

Figure 5.

Summation and Squelch Values Averaged Across TMR. Summation (top row) and squelch (bottom row) values for each participant are shown for independent (left column) and linked (right column) AGC conditions. Participants are arranged in ascending order based on the difference in their monaural IEEE scores, with participants in the symmetric group plotted in green and participants in the asymmetric group plotted in blue. Open and filled symbols for each plot represent the values for the bilateral benefit calculated in reference to the monaural ear indicated. BE = functionally better ear; PE = functionally poorer ear; AGC = automatic gain control.