Binaural Fusion and Listening Effort in Children Who Use Bilateral Cochlear Implants: A Psychoacoustic and Pupillometric Study

Abstract

Bilateral cochlear implants aim to provide hearing to both ears for children who are deaf and promote binaural/spatial hearing. Benefits are limited by mismatched devices and unilaterally-driven development which could compromise the normal integration of left and right ear input. We thus asked whether children hear a fused image (ie. 1 vs 2 sounds) from their bilateral implants and if this “binaural fusion” reduces listening effort. Binaural fusion was assessed by asking 25 deaf children with cochlear implants and 24 peers with normal hearing whether they heard one or two sounds when listening to bilaterally presented acoustic click-trains/electric pulses (250 Hz trains of 36 ms presented at 1 Hz). Reaction times and pupillary changes were recorded simultaneously to measure listening effort. Bilaterally implanted children heard one image of bilateral input less frequently than normal hearing peers, particularly when intensity levels on each side were balanced. Binaural fusion declined as brainstem asymmetries increased and age at implantation decreased. Children implanted later had access to acoustic input prior to implantation due to progressive deterioration of hearing. Increases in both pupil diameter and reaction time occurred as perception of binaural fusion decreased. Results indicate that, without binaural level cues, children have difficulty fusing input from their bilateral implants to perceive one sound which costs them increased listening effort. Brainstem asymmetries exacerbate this issue. By contrast, later implantation, reflecting longer access to bilateral acoustic hearing, may have supported development of auditory pathways underlying binaural fusion. Improved integration of bilateral cochlear implant signals for children is required to improve their binaural hearing.

Bilateral cochlear implants in children: Effects of auditory experience and deprivation on auditory perception

“…An example of unique binaural listening of direct electrical stimulation comes from a study from Gordon and colleagues [36], in which bilaterally implanted children were reported to inconsistently hear a fused bilateral image in experimental conditions. As shown in Figure 6A, children frequently indicated that they heard 1 rather than 2 sounds when listening to simultaneous pulse trains from paired apical implant electrodes containing ILDs.…”

“…However, when bilateral input was presented with ITDs at balanced levels (ie. no ILDs), children with bilateral implants responded at chance levels when asked if they heard 1 or 2 sounds [36] (Figure 6B). This means that, without ILD cues, direct bilateral cochlear implant input is difficult for children to fuse into one image.…”

“…Such strategies may not be useful for shorter ITDs more typically needed for spatial listening. In the same study, reaction time and pupilometry were used to gauge listening effort involved during the binaural fusion task across groups [36]. Clear increases in both of these measures occurred with decreases in perception of a single fused binaural image.…”

“…Perception of these inputs as one sound deteriorated with increasing asymmetry of brainstem function as measured by differences in in brainstem response latencies [36]. These asymmetries arise from sequential stimulation of one ear before the other [28, 49–52].…”

“…Asymmetric timing in the brainstem is in agreement with functional findings; although children’s sensitivity to small ILDs [7, 34] emerges very quickly after bilateral implantation [34], detection of timing cues are relatively poor [7, 25, 34, 35], take many years to emerge [34], and appear to be weighted to the first implanted ear [34] as discussed above. Some of these listening challenges may contribute to or even result from poor binaural fusion of direct electrical stimulation [36] as also discussed above.…”