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.

Conflict of interest statement

Competing Interests: The authors have declared that no competing interests exist.

Figures

Fig 1. Age at CI-1 and pre-implant hearing.

Children who were implanted at later ages (n = 15) had better residual hearing (ie. lower aided thresholds) at 250 Hz when assessed with standard audiometric testing prior to implantation (R = -0.50, p = 0.026). This relationship is was also present at 500 Hz (R = -0.50, p = 0.013), but not 1000 Hz (R = -0.28, p = 0.23), 2000 Hz (R = -0.01, p = 0.97), or 4000 Hz (R = 0.15, p = 0.57).

Fig 2. Schematic diagrams of experimental conditions.

a: Bilateral level cues were presented by either holding stimulus levels constant in the left normal ear/CI-2 and increasing level in the right normal ear/CI-1 or the reverse. Increases in stimulus level are represented by the size of the circle. The circles are fairly small as levels were presented at threshold levels (T) or slightly above (T+10 or 20 dB or CU). Unilateral control conditions were also presented (T+10 in either right ear/CI-1 or left ear/CI-2 with 0 in the opposite side). Mean (±1 SD) cochlear implant unilateral stimulation levels (T+10 in each ear) and presented ILDs are shown. b) Bilateral input containing interaural/implant timing differences are shown. These were presented from e20 on both implants at levels which were comfortably loud and behaviorally balanced as shown by the larger circles at the apical end of the schematic cochlear implant array. c) Bilateral input presented at different places along the electrode array (for CI users only). Levels were comfortably loud and behaviorally balanced. The presentations were simultaneous (ITD = 0).

Fig 3. Mean (± 1SE) proportions of 1 response across all conditions are shown by the black bars.

Overall CI listeners (n = 25 perceived 1 sound less frequency than NH peers (n = 24; p

Fig 4. Fusion with interaural level differences.

a) Group performance for conditions containing ILDs (ITD = 0 ms). Biphasic pulses were delivered from electrode 20 in the CI group (n = 25). CI listeners consistently perceived one image when there were level differences, albeit less frequently than NH peers (n = 24; p 0.05). For CI users, the majority of curves tend to decrease as a function of increasing ILD. Significant slopes (n = 3) are represented by dark grey solid lines.

Fig 5. Fusion with interaural timing differences.

a) Mean responses in children with normal hearing (n = 24) and cochlear implants (n = 25) for conditions with varying ITDs (interaural/implant timing differences). Balanced stimuli presented for ITD-varying trials in the CI group contained a small mean ILD of-0.34 ± 0.90 dB. Children with normal hearing were more likely to hear two separate sounds as the ITD increased to ±2 ms (p 0.05). Negative values denote R/CI-1 leading ITDs. b) Individual regression functions are plotted across ITDs ranging from-2 to 2 ms. As shown in the bottom plot, ITDs did not affect fusion in 24/25 CI users (p > 0.05).

Fig 6. Fusion with interaural place of stimulation differences.

a) Mean responses in the CI group (n = 25) are displayed as a function of increasing difference in the place of stimulation (IPlD) between sides. Place of stimulation was held constant at electrode 20 on one side while pulses were delivered from more basal electrodes on the contralateral side (electrode 16 for IPlD = 4 and electrode 9 for IPlD = 11). b) IPlDs did not affect fusion in 16/25 children with CIs (p > 0.05).

Fig 7. Predicting binaural fusion.

Multiple regression analysis revealed that mean proportion of 1 response for individual CI users (n = 24) when level and place differences were absent can be predicted (p 0.05).

Fig 8. Fusion and reaction time.

a) Mean overall RTs (reaction times) and RTs for each subset of conditions (ILDs = interaural/implant level differences; ITDs = interaural/implant timing differences; IPlDs = interaural/implant place of stimulation differences) are displayed for both groups. CI users (n = 24) had longer RTs than their peers with NH (n = 24; p 0.05).

Fig 9. Fusion and pupil diameter.

a) Mean PCPD (percent of change in pupillary diameter) is shown for each group. Children with bilateral CIs (n = 21) had greater changes in pupillary diameter than their NH peers (n = 19; p 0.05).

Fig 10. Reaction time and pupil diameter.

Mean overall pupillary responses were positively correlated with RTs (R = 0.69, p