Studies on bilateral cochlear implants at the University of Wisconsin's Binaural Hearing and Speech Laboratory

Abstract

This report highlights research projects relevant to binaural and spatial hearing in adults and children. In the past decade we have made progress in understanding the impact of bilateral cochlear implants (BiCIs) on performance in adults and children. However, BiCI users typically do not perform as well as normal hearing (NH) listeners. In this article we describe the benefits from BiCIs compared with a single cochlear implant (CI), focusing on measures of spatial hearing and speech understanding in noise. We highlight the fact that in BiCI listening the devices in the two ears are not coordinated; thus binaural spatial cues that are available to NH listeners are not available to BiCI users. Through the use of research processors that carefully control the stimulus delivered to each electrode in each ear, we are able to preserve binaural cues and deliver them with fidelity to BiCI users. Results from those studies are discussed as well, with a focus on the effect of age at onset of deafness and plasticity of binaural sensitivity. Our work with children has expanded both in number of subjects tested and age range included. We have now tested dozens of children ranging in age from 2 to 14 yr. Our findings suggest that spatial hearing abilities emerge with bilateral experience. While we originally focused on studying performance in free field, where real world listening experiments are conducted, more recently we have begun to conduct studies under carefully controlled binaural stimulation conditions with children as well. We have also studied language acquisition and speech perception and production in young CI users. Finally, a running theme of this research program is the systematic investigation of the numerous factors that contribute to spatial and binaural hearing in BiCI users. By using CI simulations (with vocoders) and studying NH listeners under degraded listening conditions, we are able to tease apart limitations due to the hardware/software of the CI systems from limitations due to neural pathology.

American Academy of Audiology.

Figures

1)

Schematic diagram of the 9 possible conditions (3 masker configurations × 3 listening modes). Binaural listening conditions (1,2,3) are in the top row, Left-ear conditions (4,5,6) in the middle row, and Right-ear listening conditions (7,8,9) in the bottom row. Target-Masker in front occurs in the three conditions in the left column (1,4,7). Masker on the left is shown in the conditions in the middle column (2,5,8) and masker on the right is shown in the three conditions in the right column (3,6,9).

2)

Panel A shows the experimental setup, with 11 loudspeakers positioned in the horizontal plane at 10° intervals (spanning ±50°). In addition, loudspeakers positioned at +35°, +55°, −35° and −55° were used to present the competing/ masking speech. Panel B shows data from the word identification task, for BiCI users and NH listeners; percent correct is plotted as a function of SNR. Panel C shows data for the sound localization measure, with RMS errors plotted as a function of SNR.

3)

In panel A, pitch magnitude estimation scores for each electrode are shown for the electrodes in the left ear (open squares) and right ear (closed circles). In panel B, direct pitch comparison scores (μ) are shown for a range of electrodes in the right ear for 5 electrodes in the left ear (19, 16, 14, 10, and 6). The pitch-matched electrode in the right ear is the one closest to μ = 0 and shown by a closed symbol.

4)

ITD just-noticeable difference thresholds in a left-right discrimination task are shown for 34 BiCI users, using a 100-pps constant-amplitude pulse train. Each listener’s threshold represents the value of the ITD relative to 0 μ that could be reliably discriminated. Data from three subject groups with unequal N size are shown: prelingual onset of deafness (leftmost), childhood onset of deafness (middle) and adult onset of deafness (rightmost). For each subject, up to three data points are shown in different symbols, for the three possible places of stimulation along the cochlear array (base, middle, apex).

5)

Results from 10 children whose SRT thresholds were tested at three intervals. Group Means and Standard Errors are shown relative to the 60dB SPL competitor level for Baseline (CI+HA or UniCI), BiCI and 1st CI Only listening modes at 3-mo and 12-mo test Intervals.

6)

Bilateral Advantage group average is shown for the group of 10 children who were tested at three intervals. We subtracted the SRTs obtained in the BiCI listening mode from those obtained in the UniCI listening mode. Thus, positive values denote better performance in the BiCI condition while negative values suggest a disadvantage in the BiCI listening mode. These values are compared for the 3- and 12-month visits.

7)

Results from published studies in which sound localization was measured in children and errors were quantified as root mean square (RMS) error are summarized here. On the left, data from two studies in which children normal hearing were tested (Grieco-Calub & Litovsky; Litovsky & Godar (2010). On the right, data from Grieco-Calub & Litovsky are shown for children with BiCIs, tested either when using both CIs or when using a single CI (see also Litovsky, 2011 for review).

8)

MAA thresholds from several studies, with children ranging in age from 3.5-16 years (Litovsky et al., 2006) or 5-10 years (Godar and Litovsky, 2010), and a group of 2-year old toddlers (Grieco-Calub et al., 2008). In the two groups of children who use BiCIs (left panel), performance was significantly better (lower MAAs) when tested in the bilateral listening mode (filled circles) than in the unilateral listening mode (open circles). For these same children, MAA thresholds were nonetheless higher than those measured in children with normal hearing (triangles).