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.

Interaural asymmetry of dynamic range: Abnormal fusion, bilateral interference, and shifts in attention

“…An attentional basis of contralateral interference is suggested by the finding that performance remains intact if the target is in the better ear. This has been demonstrated for listeners with BiCIs (Goupell et al, 2016(Goupell et al, , 2018Bakal et al, 2021), listeners with one CI and one NH ear (Bernstein et al, 2016(Bernstein et al, , 2020, and simulations of BiCIs in listeners with NH (Goupell et al, 2021). If contralateral interference results purely from an inability to segregate target from masker, then it should not matter whether the target is in the better or poorer ear.…”

“…The target speech is instead perceived on the side of the ear it is presented, resulting in unmasking. In contrast, patients with BiCIs who have marked asymmetry in speech understanding between the ears demonstrate contralateral interference when target speech is presented to their poorer ear (Bernstein et al, 2016;Goupell et al, 2016Goupell et al, , 2018Bakal et al, 2021). Listeners with a CI in one ear and NH in the other ear show the same pattern of 10.3389/fnins.2022.1018190 performance (Bernstein et al, 2020).…”

“…Because of the inherent connection between sound source segregation and attention, it is difficult to disentangle both processes from one another and determine how they might affect patients. While there is a right-ear advantage noted in the literature for listeners with NH, listeners with BiCIs can have considerably different speech outcomes between the ears due to many underlying factors (e.g., Litovsky et al, 2006;Mosnier et al, 2009;Goupell et al, 2018;Bakal et al, 2021). One of the major challenges of studies of interaural asymmetry in listeners with BiCIs is that large sample sizes are required to account for differences between patients.…”

“…Bilateral CIs (BiCIs) improve sound source localization performance and speech understanding in noise relative to unilateral implantation (e.g., Litovsky et al, 2006 ). However, the extent of this benefit varies highly across patients ( Litovsky et al, 2006 ; Mosnier et al, 2009 ; Yoon et al, 2011 ; Reeder et al, 2014 ; Goupell et al, 2016 , 2018 ; Bakal et al, 2021 ).…”

“…Studies of patients with BiCIs and simulations in NH suggest that interaurally asymmetric hearing outcomes may limit performance in binaural tasks ( Mosnier et al, 2009 ; Yoon et al, 2011 ; Ihlefeld et al, 2015 ; Goupell et al, 2016 , 2018 ; Anderson et al, 2019b , 2022 ; Bakal et al, 2021 ). These studies assessed sensitivity to binaural cues, sound source localization, and speech understanding in background noise, and related them to asymmetry in sensitivity to temporal cues or monaural speech understanding.…”