Examining Constraints on Speech Growth in Children with

Cochlear Implants

J. Bruce TomblinThe 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 Periodsa 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

OpenPartiallyClosed

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 1.7-6.46

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 ( Years)

3 4 5 6 7 8 9 10 11 12 13

Pe

rce

nt P

hone

mes

Cor

rect

0.0

0.2

0.4

0.6

0.8

1.0

21 25 32 37 33 37 28 27 22 21 14

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 plateauN= 13 AOI 29 mo.

Individuals who do not plateauN= 28 AOI 38 mo.

Group without Plateau

Does growth continue past middle childhood (6-9 years)?

Months

0 20 40 60 80 100 120 140 160 180 200

Pe

rcen

t P

hone

me

s C

orre

ct

0.0

0.2

0.4

0.6

0.8

1.0

1.2

9 years of age

N=29

Chronological Age (Months)

20 40 60 80 100 120 140 160 180

Pro

po

rtio

n P

ho

ne

me

s C

orr

ect

0.0

0.2

0.4

0.6

0.8

1.0

Slope= 0.002758 p<.0002

N=13

Children Followed Beyond 10 Years of Age

13.3

Months Post Implanation

0 20 40 60 80 100 120 140

Pro

port

ion

Pho

nem

es C

orre

ct

0.0

0.2

0.4

0.6

0.8

1.0

Slope=0.00414 p<0.0001

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 PlateauDoes the point of plateau

appear to be linked to chronological age or length of

hearing?

Months

60 80 100 120 140 160 180 200

Months

0 20 40 60 80 100 120

0.0

0.2

0.4

0.6

0.8

1.0

Years

0 2 4 6 8 10

Years

5 6 7 8 9 10 11 12 13 14 15

Average Age at Asymptote

Growth in Children who ReachPlateau

N=13

Average (90%)

Mon

ths

to A

sym

ptot

e

0

20

40

60

80

100

120

Co

eff

icie

nt o

f V

aria

tion

0.00

0.05

0.10

0.15

0.20

0.25

0.30

0.35

HearingAge

ChronologicalAge

HearingAge

ChronologicalAge

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

96.2

66.5

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.