Examining Constraints on Speech Growth in Children with Cochlear Implants J. Bruce Tomblin The University of Iowa

Research Opportunities with Pediatric Cochlear Implantation  Most of the research on implants has been concerned with evaluating the effectiveness of this clinical intervention with regard to –Improved auditory function –Spoken communication development and function –Improved quality of life

Research Opportunities with Pediatric Cochlear Implantation  The CI population also provides opportunities to test theoretical issues –Speech and language development using an alternate neural representation of the acoustic signal. –Effects of differential timing of onset of auditory-linguistic experience relative to biological and cognitive development.

Viewpoints on Developmental Constraints  Constrained Periods a time in development in which the organism is particularly responsive to experience –Critical Period  A period with sharp onset and offset  Response to experience limited to period and is irreversible. –Sensitive Period  A period with gradual onset and incomplete offset open close Critical Period Sensitive Period Open Partially Closed Bruer, 2001

Critical Periods and Language  Lenneberg (1967) brain lateralization at puberty closes down the brain's ability to acquire language. (critical period)  Pinker (1994) ”acquisition of a normal language is guaranteed for children up to the age of six, is steadily compromised from then until shortly after puberty, and is rare thereafter” (sensitive period)  Note that in both cases the interest is in the nature of the closing point.

Mechanisms for CP/SP  Maturational Processes –Biologically driven events that cause the brain to become open or closed to experience. –Timing is closely linked to physical maturation and thus to chronological age.  Experience Dependent Processes –The brain becomes more or less responsive to experience as a function of prior learning.  Entrenchment results in biases toward some learning and against other learning – accented speech in L2 adults.

Rationale  Speech development in the hearing child is typically mastered by 5 or 6 and those who fail to reach mastery by 8 or 9 years are unlikely to do so without help (Shriberg, Gruber, & Kwiatkowski; 1994). –To what extent is this constraint on speech development determined by maturational or experience?  Children receiving CIs in childhood will be delayed in speech development. –Is speech sound development constrained by the child’s chronological age?  Does speech growth extend beyond the developmental period of 6 to 9 years of age?  Is the attainment of a plateau in speech associated with chronological age or hearing age? –Is speech sound development constrained by the child’s length of hearing?

Participants  41 Prelingually Deaf Children with 4 or more years of implant experience – Average First Observation  CA = 3.28 (1.1) years  Hearing Age = 0.32 (0.53) –Average Last Observation  CA = 9.72 (2.83)  Hearing Age = 6.76 (2.66) years  Mean Age of Implantation –2.97 Years (SD 1.2) range

Methods  Speech elicited via a story retell  Analysis –Utterance is phonemically glossed using child’s signed and spoken utterances in conjunction with story context. –Child’s spoken utterance transcribed phonetically –Phoneme accuracy assigned according to correspondence between phonetic transcript and gloss standard. –Percent phonemes correct determined by ratio of number of accurate phonemes/number of phonemes in gloss.

N=41 Chronological Age Development of Phoneme Accuracy

Characterizing Growth Maximal Level of Performance Plateau or Asymptotic Level Linear Growth Phase

Growth Heterogeneity Individuals who reach a plateau N= 13 AOI 29 mo. Individuals who do not plateau N= 28 AOI 38 mo.

Group without Plateau Does growth continue past middle childhood (6-9 years)?

9 years of age N=29

N=13 Children Followed Beyond 10 Years of Age 13.3

Growth in Group without Plateau  Growth does appear to persist after 9 years of age.  In the 13 children followed into early adolescence there is little evidence of a clear closing of speech growth.  Growth appears more closely related to amount of hearing experience than chronological age, thus a simple maturational account does not fit.  No support for a critical period. Perhaps room for a sensitive period that is influenced by experience rather than maturation.

Group with a Plateau Does the point of plateau appear to be linked to chronological age or length of hearing?

Growth in Children who Reach Plateau N=13 Average (90%)

Variation in Point of Asymptote as a Function of CA or Hearing Age Coefficient of Variation

Features of Growth in Children who Do Reach Plateau  These children had more rapid rates of growth than the non-plateau children. –Is this a sign of a sensitive period? –Is this evidence that fast learners will reach plateau faster?  The age at plateau was within the age range of normal hearing children.  The level of performance at plateau was below that of normals, but near ceiling.  The point where plateau was reached was more consistently related to CA than Hearing Age.  These results leave room for a type of maturational influence on speech development.

Conclusions  A strict critical period determined by maturational processes linked to chronological age does not operate for speech sound development.  Growth in speech is linked to the amount of hearing and does not appear to be constrained by chronological age.  Growth rates in older children may be slower, than younger children which may represent a sensitive period, however this may simply be a product of selection where only the slow learners remain in the non-plateau group.