Fundamental frequency processing in Mandarin-speaking children with cochlear implants : evidence from behavioral and electrophysiological measurements

Abstract

Cochlear implantation has been a common intervention for Mandarin-speaking preschool children with sever to profound hearing impairment. So far, the very limited understanding on the processing of fundamental frequency (F0), i.e., the primary acoustic cue for lexical tone perception, in Mandarin-speaking children with cochlear implants (CIs) has prevented improvement in speech processing strategies for lexical tone perception. Therefore, the aim of this thesis is to investigate F0 processing in Mandarin-speaking preschool children with CIs as compared to that in normal-hearing (NH) peers. Three studies were conducted. The first study investigated the effects of F0 contours on sentence recognition in Mandarin-speaking children with either CIs or normal hearing. Sentence recognition with normal F0 contours was compared to that with flattened F0 contours in both quiet and noise. Results showed that while the F0 contour-induced reduction was only seen in noise condition among NH children, it was observed in both test conditions among children with CIs. Moreover, the F0 contour-caused reductions in both test conditions were significantly greater in CI group. These findings suggest that despite the limitation of F0 extraction in CIs, F0 contour still have significant impacts on sentence recognition in children with CIs, and its role is even more salient than that in NH children. F0 level is the other acoustic dimension of F0 except for F0 contour. In the second study, F0 contour and F0 level perception in Mandarin-speaking children with CIs was investigated by measuring just-noticeable differences (JNDs) of F0 contour and F0 level changes. Age-matched NH children were recruited as controls. The CI group demonstrated significantly larger JNDs of F0 contour change (JND-C) and F0 level change (JND-L) compared to the NH group. Further within-group comparison revealed that JND-C was significantly smaller than JND-L among children with CIs. This pattern contradicts that was observed among NH children, suggesting different mechanisms of F0 processing in these two groups as a result of hearing experience. The third study explored mechanisms of F0 processing in children with CIs at the electrophysiological level. Positive mismatch responses (p-MMRs) to F0 contour and F0 level change were compared between children with CIs and NH children. Results showed that while NH children showed significantly longer peak latency of p-MMR to F0 contour change compared to that of F0 level change, opposite pattern was seen in children with CIs, and p-MMR to F0 contour change in children with CIs showed the shortest peak latency among all. Results in this study supported different mechanisms of F0 processing between children with CIs and NH children. This thesis investigated for the first time F0 contour and F0 level perception in Mandarin-speaking children with CIs at both behavioral and electrophysiological level. Findings in this thesis consistently suggest that F0 processing in these children is probably different from that in NH children, which may serve as references for designing tonal language-oriented speech processing strategies.