Objectives: Cochlear implants (CIs) help many deaf children achieve near-normal speech and language (S/L) milestones. Nevertheless, high levels of unexplained variability in S/L outcomes are limiting factors in improving the effectiveness of CIs in deaf children. The objective of this study was to longitudinally assess the role of verbal short-term memory (STM) and working memory (WM) capacity as a progress-limiting source of variability in S/L outcomes after CI in children.
Design: Longitudinal study of 66 children with CIs for prelingual severe-to-profound hearing loss. Outcome measures included performance on digit span forward (DSF), digit span backward (DSB), and four conventional S/L measures that examined spoken-word recognition (Phonetically Balanced Kindergarten word test), receptive vocabulary (Peabody Picture Vocabulary Test ), sentence-recognition skills (Hearing in Noise Test), and receptive and expressive language functioning (Clinical Evaluation of Language Fundamentals Fourth Edition Core Language Score; CELF).
Results: Growth curves for DSF and DSB in the CI sample over time were comparable in slope, but consistently lagged in magnitude relative to norms for normal-hearing peers of the same age. For DSF and DSB, 50.5% and 44.0%, respectively, of the CI sample scored more than 1 SD below the normative mean for raw scores across all ages. The first (baseline) DSF score significantly predicted all endpoint scores for the four S/L measures, and DSF slope (growth) over time predicted CELF scores. DSF baseline and slope accounted for an additional 13 to 31% of variance in S/L scores after controlling for conventional predictor variables such as: chronological age at time of testing, age at time of implantation, communication mode (auditory-oral communication versus total communication), and maternal education. Only DSB baseline scores predicted endpoint language scores on Peabody Picture Vocabulary Test and CELF. DSB slopes were not significantly related to any endpoint S/L measures. DSB baseline scores and slopes taken together accounted for an additional 4 to 19% of variance in S/L endpoint measures after controlling for the conventional predictor variables.
Conclusions: Verbal STM/WM scores, process measures of information capacity, develop at an average rate in the years after cochlear implantation, but were found to consistently lag in absolute magnitude behind those reported for normal-hearing peers. Baseline verbal STM/WM predicted long-term endpoint S/L outcomes, but verbal STM slopes predicted only endpoint language outcomes. Verbal STM/WM processing skills reflect important underlying core elementary neurocognitive functions and represent potential intervention targets for improving endpoint S/L outcomes in pediatric CI users.