babbling, reflecting the human left hemi-
sphere control of natural language. If bab-
bling were simply a way for the baby to flexthe motor control system for the mouth,tongue, and throat—no different from the
system used in chewing—then symmetry inmouth opening would have been observed.
Instead, we witnessed an asymmetrical pat-tern of mouth opening for babbling, whichsupports the fundamentally linguistic view
Siobhan Holowka1 and Laura Ann Petitto1,2*
that babbling reflects babies’ sensitivity toand production of patterns in the linguistic
Baby babbling is the universal developmental
opening, two “blind” independent coders scored
input (1). We thus conclude that babbling
milestone before the onset of language pro-
150 randomly selected segments of babbles,
represents the onset of the productive lan-
duction in humans, yet little is known about
nonbabbles, and smiles according to whether
whether the neural determinants of this be-
greater right, left, or equal mouth opening was
havior are fundamentally linguistic (1, 2) or
observed. A standard Laterality Index (LI) (5)
reflect only oral-motor developments (3, 4 ).
was computed for each baby for their production
sphere cerebral specialization for babies
In adults, the presence of right asymmetry in
of babbles, nonbabbles, and smiles: LI ϭ (R Ϫ
while babbling, which, in turn, suggests that
mouth aperture during linguistic tasks as
L/R ϩ L ϩ E), and mean LI scores were calcu-
language functions in humans are lateralized
contrasted with left or equal mouth opening
lated for each group of babies (English and
from a very early point in development.
during nonlinguistic tasks has been widely
French). Thus, a mean positive LI score indicat-
Moreover, the smile results illustrate the
ed more instances of right mouth opening, and a
specificity of the right-sided mouth advan-
cerebral specialization for language (5).
mean negative LI score indicated more instances
tage of babbling behavior in babies, corrob-
that, like adults (7 ), babies’ emo-
Fig. 1. Consecutive frames from video recordings showing a baby’s left
mouth opening while smiling (left) and right mouth opening while babbling
(right). Mean LI scores for all of the babies were as follows: babble ϭ ϩ0.88
(red), nonbabble ϭ Ϫ0.08 (yellow), and smile ϭ Ϫ0.82 (blue).
vide insight into the neural basisof babbling and hence into the origins of
of left mouth opening for the given production. References and Notes
The mean LI scores indicated that all babies
1. L. A. Petitto, S. Holowka, L. E. Sergio, D. Ostry, Nature413, 35 (2001).
To control for any language-specific effects
had right mouth asymmetry while babbling,
2. S. Pinker, P. Bloom, in The Adapted Mind: Evolutionary
of mouth asymmetry, we videotaped 10 babies
equal mouth opening while nonbabbles were
Psychology and the Generation of Culture, J. H.
acquiring either English (n ϭ 5) or French (n ϭ
Barkow et al., Eds. (Oxford Univ. Press, New York,
5). The babies were studied between the ages of
smiling (Fig. 1). Statistical analysis was per-
3. P. F. MacNeilage, B. L. Davis, Science 288, 527 (2000).
5 and12 months, according to the age at which
formed with a two-way mixed analysis of vari-
4. J. L. Locke, Science 288, 449 (2000).
each baby first entered the syllabic babbling
ance: group (English and French) ϫ production
5. R. Graves, T. Landis, Int. J Psych. 25, 179 (1990).
6. M. S. Gazzaniga, C. Smylie, Neurology 13, 536 (1983).
stage. Once this developmental milestone was
(babble, nonbabble, and smile). No significant
7. J. C. Borod et al., Neuropsychologia 26, 759 (1988).
achieved, we examined three types of oral ac-
effect of group was detected (F ϭ 0.09, NS),
8. We thank D. W. Holowka, K. Dunbar, U. Harasymo-
tivity produced by the babies: babbles, non-
indicating that no significant differences were
wycz, and K. Byers-Heinlein. L.A.P. was funded by the
babbles, and smiles (table S1). Babbles were
found between the English and French babies
Social Sciences and Humanities Research Council of
Canada, the Spencer Foundation, and Dartmouth
defined as vocalizations that contained a re-
(table S1). A significant main effect was dis-
duced subset of possible sounds ( phonetic
covered for production (F ϭ 236.91, P Ͻ
Supporting Online Material
units) found in spoken language, had redupli-
0.001), and all pair-wise comparisons were sig-
cated (repeated) syllabic organization (conso-
nificant (P Ͻ 0.001), indicating that the babies’
nant-vowel alternations), and were produced
mouth opening differed depending on whether
without apparent meaning or reference; all vo-
a babble, nonbabble, or smile was produced
calizations lacking any of these three criteria
1Department of Psychology, McGill University, 1205
were coded as nonbabbles. Spontaneous smiles
Dr. Penﬁeld Avenue, Montre´al, Que´bec, H3A 1B1,
were coded as an additional control of babies’
remained elusive as a result of controversy
Canada. 2Department of Psychological and Brain Sci-
ences and Department of Education, Dartmouth Col-
specificity of mouth opening for distinctive
over the neural basis of babbling. Like adults,
lege, 302 Silsby Hall, Hanover, NH 03755, USA.
the right mouth asymmetry observed in ba-
*To whomcorrespondence should be addressed.
At 50 ms (three video frames) from initial lip
bies suggests left hemisphere asymmetry for
www.sciencemag.org SCIENCE VOL 297 30 AUGUST 2002
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