Expansion of Prosodic Abilities at the Transition From Babble to Words: A Comparison Between Children With Cochlear Implants and Normally Hearing Children

This longitudinal study examined the effect of emerging
vocabulary production on the ability to produce the phonetic cues to
prosodic prominence in babbled and lexical disyllables of infants with
cochlear implants (CI) and normally hearing (NH) infants. Current
research on typical language acquisition emphasizes the importance
of vocabulary development for phonological and phonetic acquisition.
Children with CI experience significant difficulties with the perception
and production of prosody, and the role of possible top-down effects is,
therefore, particularly relevant for this population.
Design: Isolated disyllabic babble and first words were identified and segmented
in longitudinal audio–video recordings and transcriptions for nine
NH infants and nine infants with CI interacting with their parents. Monthly
recordings were included from the onset of babbling until children had
reached a cumulative vocabulary of 200 words. Three cues to prosodic
prominence, fundamental frequency (f0), intensity, and duration, were measured
in the vocalic portions of stand-alone disyllables. To represent the
degree of prosodic differentiation between two syllables in an utterance, the
raw values for intensity and duration were transformed to ratios, and for f0,
a measure of the perceptual distance in semitones was derived. The degree
of prosodic differentiation for disyllabic babble and words for each cue was
compared between groups. In addition, group and individual tendencies on
the types of stress patterns for babble and words were also examined.
Results: The CI group had overall smaller pitch and intensity distances
than the NH group. For the NH group, words had greater pitch and
intensity distances than babbled disyllables. Especially for pitch distance,
this was accompanied by a shift toward a more clearly expressed
stress pattern that reflected the influence of the ambient language. For
the CI group, the same expansion in words did not take place for pitch.
For intensity, the CI group gave evidence of some increase of prosodic
differentiation. The results for the duration measure showed evidence
of utterance final lengthening in both groups. In words, the CI group
significantly reduced durational differences between syllables so that a
more even-timed, less differentiated pattern emerged.
Conclusions: The onset of vocabulary production did not have the same
facilitatory effect for the CI infants on the production of phonetic cues for
prosody, especially for pitch. It was argued that the results for duration
may reflect greater articulatory difficulties in words for the CI group than
the NH group. It was suggested that the lack of clear top-down effects of
the vocabulary in the CI group may be because of a lag in development
caused by an initial lack of auditory stimulation, possibly compounded
by the absence of auditory feedback during the babble phase.