Spectral-Temporal Modulated Ripple Discrimination by Children With Cochlear Implants

Abstract

Objectives: A postlingually implanted adult typically develops hearing with an intact auditory system, followed by periods of deafness (or near deafness) and adaptation to the implant. For an early implanted child whose brain is highly plastic, the auditory system matures with consistent input from a cochlear implant. It is likely that the auditory system of early implanted cochlear implant users is fundamentally different than postlingually implanted adults. The purpose of this study is to compare the basic psychophysical capabilities and limitations of these two populations on a spectral resolution task to determine potential effects of early deprivation and plasticity.

Design: Performance on a spectral resolution task (Spectral-temporally Modulated Ripple Test [SMRT]) was measured for 20 bilaterally implanted, prelingually deafened children (between 5 and 13 years of age) and 20 hearing children within the same age range. Additionally, 15 bilaterally implanted, postlingually deafened adults, and 10 hearing adults were tested on the same task. Cochlear implant users (adults and children) were tested bilaterally, and with each ear alone. Hearing listeners (adults and children) were tested with the unprocessed SMRT and with a vocoded version that simulates an 8-channel cochlear implant.

Results: For children with normal hearing, a positive correlation was found between age and SMRT score for both the unprocessed and vocoded versions. Older hearing children performed similarly to hearing adults in both the unprocessed and vocoded test conditions. However, for children with cochlear implants, no significant relationship was found between SMRT score and chronological age, age at implantation, or years of implant experience. Performance by children with cochlear implants was poorer than performance by cochlear implanted adults. It was also found that children implanted sequentially tended to have better scores with the first implant compared with the second implant. This difference was not observed for adults. An additional finding was that SMRT score was negatively correlated with age for adults with implants.

Conclusions: Results from this study suggest that basic psychophysical capabilities of early implanted children and postlingually implanted adults differ when assessed in the sound field using their personal implant processors. Because spectral resolution does not improve with age for early implanted children, it seems likely that the sparse representation of the signal provided by a cochlear implant limits spectral resolution development. These results are supported by the finding that postlingually implanted adults, whose auditory systems matured before the onset of hearing loss, perform significantly better than early implanted children on the spectral resolution test.

Figures

Figure 1

Bilateral performance on the SMRT is presented for children (left panel) and adults (right panel). In the left (pediatric) panel, SMRT scores (higher is better) are presented as a function of age. Green circles represent scores for normal hearing children, Red triangles represent scores for the same normal hearing children with the vocoded SMRT. Blue squares represent scores for children with cochlear implants. Adult SMRT scores are presented with boxplots (right panel) such that the green box represents the range of scores for the normal hearing adults, the red box represents the scores for normal hearing adults performing the task with vocoded stimuli, and the blue box represents the scores for adults with cochlear implants. Note that the vertical scales for the two panels match, allowing for direct comparison of results between adult and pediatric populations.

Figure 2

Bilateral performance on the SMRT is presented for adults with cochlear implants as a function of age at testing.

Figure 3

Bilateral SMRT performance for children with cochlear implants as a function of years of experience with a cochlear implant (top panel) and age at implantation (bottom panel). The correlations explain no more than 1% of the variability and none are significant.

Figure 4

Performance on the SMRT task with a single cochlear implant relative to bilateral performance are plotted as a function of age for children (left panel) and adults (right panel). Each plotted point represents the deviation between the bilateral SMRT and one of the unilateral SMRT conditions. Baseline performance (i.e. the bilateral SMRT score for a given user) is set to 0 and is represented by the dashed blue line. Positive values indicate a unilateral SMRT condition that provides a higher score than the bilateral baseline. Negative values indicate a unilateral SMRT condition that provides a lower score than the bilateral baseline. For sequentially implanted users, the first implant is represented by a solid triangle and the second implant is represented by a hollow triangle. For simultaneously implanted users, scores with each implant alone are represented by a grey circle. Note that for one subject (CA06), no SMRT score was measurable for the second implant and therefore no second data implant data point is presented for this subject.