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. 2006 Feb;27(1):43-59.
doi: 10.1097/01.aud.0000194515.28023.4b.

Bilateral cochlear implants in children: localization acuity measured with minimum audible angle

Ruth Y Litovsky  1 , Patti M Johnstone, Shelly Godar, Smita Agrawal, Aaron Parkinson, Robert Peters, Jennifer Lake
Affiliations

Bilateral cochlear implants in children: localization acuity measured with minimum audible angle

Ruth Y Litovsky et al. Ear Hear. 2006 Feb.

Abstract

Objective: To evaluate sound localization acuity in a group of children who received bilateral (BI) cochlear implants in sequential procedures and to determine the extent to which BI auditory experience affects sound localization acuity. In addition, to investigate the extent to which a hearing aid in the nonimplanted ear can also provide benefits on this task.

Design: Two groups of children participated, 13 with BI cochlear implants (cochlear implant + cochlear implant), ranging in age from 3 to 16 yrs, and six with a hearing aid in the nonimplanted ear (cochlear implant + hearing aid), ages 4 to 14 yrs. Testing was conducted in large sound-treated booths with loudspeakers positioned on a horizontal arc with a radius of 1.5 m. Stimuli were spondaic words recorded with a male voice. Stimulus levels typically averaged 60 dB SPL and were randomly roved between 56 and 64 dB SPL (+/-4 dB rove); in a few instances, levels were held fixed (60 dB SPL). Testing was conducted by using a "listening game" platform via computerized interactive software, and the ability of each child to discriminate sounds presented to the right or left was measured for loudspeakers subtending various angular separations. Minimum audible angle thresholds were measured in the BI (cochlear implant + cochlear implant or cochlear implant + hearing aid) listening mode and under monaural conditions.

Results: Approximately 70% (9/13) of children in the cochlear implant + cochlear implant group discriminated left/right for source separations of <or=20 degrees , and, of those, 77% (7/9) performed better when listening bilaterally than with either cochlear implant alone. Several children were also able to perform the task when using a single cochlear implant, under some conditions. Minimum audible angle thresholds were better in the first cochlear implant than the second cochlear implant listening mode for nearly all (8/9) subjects. Repeated testing of a few individual subjects over a 2-yr period suggests that robust improvements in performance occurred with increased auditory experience. Children who wore hearing aids in the nonimplanted ear were at times also able to perform the task. Average group performance was worse than that of the children with BI cochlear implants when both ears were activated (cochlear implant + hearing aid versus cochlear implant + cochlear implant) but not significantly different when listening with a single cochlear implant.

Conclusions: Children with sequential BI cochlear implants represent a unique population of individuals who have undergone variable amounts of auditory deprivation in each ear. Our findings suggest that many but not all of these children perform better on measures of localization acuity with two cochlear implants compared with one and are better at the task than children using the cochlear implant + hearing aid. These results must be interpreted with caution, because benefits on other tasks as well as the long-term benefits of BI cochlear implants are yet to be fully understood. The factors that might contribute to such benefits must be carefully evaluated in large populations of children using a variety of measures.

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Figures

Fig. 1
Fig. 1
Schematic diagram of testing setup. An array of 15 loudspeakers mounted on an arc with a radius of 1.5 m at ear level, positioned every 10° (−70° to + 70°).
Fig. 2
Fig. 2
Data from five individual subjects are shown. Each panel contains results from three listening modes, bilateral (circles), first cochlear implant (triangles) and second cochlear implant (squares). Percent correct is plotted as a function of the loudspeaker positions. An angle of 20° indicates that the loudspeakers were positioned at 20° to the right and left, hence a total of 40° separation between the two positions. Dashed horizontal line in each panel crosses threshold criterion of 70.9%.
Fig. 3
Fig. 3
Minimum audible angle thresholds estimated from curves such as those plotted in Figure 2 are shown for the group of subjects who were able to perform the task at <60° on at least one condition. Each vertical set of thresholds represents data from a single subject’s performance on the three listening modes: bilateral (circles), first cochlear implant (triangles), and second cochlear implant (squares). Subject numbers are shown along the top of the graph, and results are plotted according their “bilateral age,” that is, number of months after activation of the second cochlear implant. On the vertical axis, MAA thresholds can range from 5 to 85°, and data points >85 denote conditions in which thresholds were not measurable because the subject could not perform the test (NM). In a few cases, there are absent data points for the second cochlear implant condition because data were not obtained. Finally, subject 4 is singled out (*) by way of reminder that she had more intensive training before final data collection.
Fig. 4
Fig. 4
Minimum audible angle threshold group means (±SD) are plotted for the group of nine children with bilateral implants whose individual data are shown in Figure 3. On the vertical axis, MAA thresholds can range from 5 to 85°, and data points >85 denote conditions in which thresholds were not measurable (NM). Performance is compared for measures obtained under the three listening modes: bilateral, first cochlear implant, and second cochlear implant. Within each panel, the subject population is divided into two subgroups, depending on the duration of experience with the second cochlear implant (<13 mos or >13 mos). One of the subjects in the <13 mo group, who was tested in the bilateral and first cochlear implant mode, was not tested in the second cochlear implant mode.
Fig. 5
Fig. 5
Individual results for three subjects (2, 3, and 4) are shown. Each child participated in the testing at a few different time intervals after activation of the second cochlear implant (3, 15, and 22 to 26 mos). Each panel contains data from a single subject, comparing performance on the three listening modes: bilateral, first cochlear implant and second cochlear implant. Within each panel, MAA thresholds are plotted as a function of the number of months after activation of the second cochlear implant. On the vertical axis, MAA thresholds can range from 5° to 85°, and data points >85 denote conditions in which thresholds were not measurable (NM). Filled symbols were taken from the fixed-speaker approach described in the Methods section; open symbols from the 3-mo visits represent MAA thresholds estimated using the adaptive method.
Fig. 6
Fig. 6
Minimum audible angle thresholds are plotted for all children with bilateral devices, including two cochlear implants, with <13 or >13 mos of bilateral experience, and one group with a cochlear implant in one ear and hearing aid in the opposite ear (cochlear implant + hearing aid). Left panel: Data collected while two devices were active (two cochlear implants or cochlear implant and hearing aid). Right panel: Data collected while subjects used only one cochlear implant, either first cochlear implant (cochlear implant + cochlear implant children) or only cochlear implant (cochlear implant + hearing aid children). On the vertical axis, MAA thresholds can range from 5 to 85°, and data points >85 denote conditions in which thresholds were not measurable (NM). Within each panel, data are clustered by group and include both individual data points as well as group means (±SD).
Fig. 7
Fig. 7
Minimum audible angle thresholds are plotted for the five children who found the task extremely difficult; four children had bilateral cochlear implants and one had a cochlear implant + hearing aid. Each child’s subject number and age are shown at the top of the graph, and for the four subjects with bilateral cochlear implants the number of months after activation of the second cochlear implant is indicated at the bottom of the graph. Results are compared for three listening modes: bilateral (circles), first cochlear implant (triangles), and second cochlear implant (squares); for two subjects, measures were not obtained in the second cochlear implant mode. On the vertical axis, MAA thresholds can range from 5 to 85°, and data points >85 denote conditions in which thresholds were not measurable (NM).
Fig. 8
Fig. 8
Results from repeated testing/training during a 2-day period are shown for one subject (subject 4). Each panel includes data from 1 day of testing, separated by morning (am) and afternoon (pm), including three listening modes: bilateral (circles), first cochlear implant (triangles) and second cochlear implant (squares). On the vertical axis, MAA thresholds can range from 20 to 70°, and data points >65 denote conditions in which thresholds were not measurable at 70°, the largest angle tested during those sessions (NM). Above each data set, the text indicates whether overall sound level was fixed (60 dB SPL) or roved (60 ± 4 dB SPL).
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