Kathleen Currie Hall
9 May 2005

Notes on: Kuhl, Patricia K., & Meltzoff, Andrew N. (1996). Infant vocalizations in response to speech: Vocal imitation and developmental change. Journal of the Acoustical Society of America, 100(4), 2425-2438.

Summary:
In this paper, Kuhl and Meltzoff present the results from a tightly controlled infant vowel production study.  They show that infant vowel productions vary along the same dimensions that adult productions do; that even between 12 and 20 weeks, infants tighten up their vowel spaces; and that infants show imitation of ambient input at these ages.

Details:
1. While it is generally accepted that there are basically 5 universal stages of child language development (reflexive phonation, cooing, expansion, canonical babbling, and meaningful speech), the progression from one to the next has not been very well studied and is not fully understood.  This paper tries to explore at least some of these issues.  The two main hypotheses are: anatomical change and vocal learning.

2. Infants of three ages, 12-, 16-, and 20-week-olds, participated in the experiment.  Each child heard and saw 5 minutes of a woman's face producing either [a], [i], or [u].  The child was recorded and transcribed, and acoustic measurements were taken.

3. Unlike other imitation studies, the stimuli were tightly controlled -- the same for all infants, presented at the same level and for the same duration, etc.

4. The three questions: 
a. How do infant productions compare to adult? -- infants produce vowels that do not use the full range of formant values that adults' do, but which are still concomitant with the adult vowel range.  They differentiate vowels along the same dimensions (F1, F2, etc.) that adults do. 
b. Are there developmental changes between 12 and 20 weeks? -- yes, there are; these seem to be along the lines of tightening up the vowel space encompassed by each vowel.  Kuhl and Meltzoff propose that this is due to the formation of memory representations of clusters of vowels encountered in the ambient language, which in turn helps shape the motor gestures and the output produced by the child.
c. Do infants imitate? -- Yes, there was a significant amount of infant vowel productions that matched adult stimuli, which was above that expected if the infants were just randomly cooing.  This imitation presumably helps with the memory representations needed to produce speech closer and closer to that of the ambient language.  The question of normalization (how the child knows that his [a] is [a]-like when the frequencies are in fact different from those of the adult, etc.) is still unresolved.

Questions:
1. The infants each came in for 3 sessions.  Did they hear the same vowel each time?  Why were there 3 sessions?  Was there any effect of session?  E.g., did children imitate more during the third session?  (Especially c.f. comments on p. 2435 about 15 minutes of exposure being "sufficient to influence vocalizations" -- how do we know that it was the 15 minutes that was sufficient?  Some infants did not imitate the stimuli.  Maybe those who did were predisposed to anyway, regardless of the 15 minutes . . . .

2. What was done with the 14% inter-transcriber disagreement?  Did they come to a consensus or throw out the data?  (The latter seemed to be the case in one of the analyses, but not all?)

3. Why do they say that grave/acute and compact/diffuse values are "not frequency specific"?  I would think that without some sort of standardization, it still wouldn't be fair to simply compare infant and child productions directly (especially with the grave/acute measure).