Determining the Mechanisms of Spoken Language Processing Delay for Children with Cochlear Implants

Abstract

The long-term objective of this project was to better understand how shorter auditory experience and spectral degradation of the cochlear implant (CI) signal impact spoken language processing in deaf children with CIs. The specific objective of this research was to utilize psycholinguistic methods to investigate the mechanisms underlying observed delays in spoken word recognition and the access of networks of semantically related words in the lexicon, which are both vital components for efficient spoken language comprehension. The first experiment used eye-tracking to investigate the contributions of early auditory deprivation and the degraded CI signal to spoken word recognition delays in children with CIs. Performance of children with CIs was compared to various typical hearing (TH) control groups matched for either chronological age or hearing age, and who heard either clear or vocoded speech. The second experiment investigated semantic processing in the face of a spectrally degraded signal (TH adult listeners presented with vocoded speech) by recording event-related potentials, specifically the N400. Results children with CIs show slower lexical access and less immediate lexical competition, and while early hearing experience supports more efficient recognition, much of these observed delays can be attributed to listening to a degraded signal in the moment, as children with TH demonstrate similar patterns of processing when presented with vocoded speech. However, some group differences remain, specifically children with CIs show slower speed of lexical access and longer-lasting competition, suggesting potential effects of learning from a degraded speech signal. With regards to higher-level semantic processing, TH adult listeners demonstrate more limited access of semantic networks when presented with a degraded speech signal. This finding suggests that uncertainty due the degraded speech signal may lead to less immediate cascading processing at both the word-level and higher-level semantic processing. Clinically, these results highlight the importance of early cochlear implantation and maximizing access to spectral detail in the speech signal for children with CIs. Additionally, it is possible that some of the delays in spoken language processing are the result of an alternative listening strategy that may be engaged to reduce the chance of incorrect predictions, thus preventing costly revision processes.